ecu swap - boost problems

Charles Baer charlie at
Fri Jun 14 10:28:43 EDT 2002

This is a multi-part message in MIME format.
> -----Original Message-----
> From: Chewy4000 at [mailto:Chewy4000 at]
> Sent: Friday, June 14, 2002 7:54 AM
> To: 200q20v at
> Subject: Re: ecu swap - boost problems
> I had a hole that I could fit 3 fingers in the crankcase
> breather hose, but this had no effect whatsoever on the boost
> highs or smoothness.
> I changed it hoping this may help something in the car, but
> nothing changed.
> The car ran the same with our without the new hose.
> Chewy

That would be on the other side of the pressure valve where it's
ability to cause loss of boost would be limited by the valve.

Scott describes it here

The short hose is on the intake side of the valve and makes a BIG
whoosh when it opens at +10lbs relative pressure.

[ Converted text/html to text/plain ]

SJM Auto-Technik

The ABC's of Running High Boost for the
1991 200TQ 20V Turbo and the
1992-96 Audi S4

Last Update May 21, 2002

Stock Boost Levels-How does the ECU control Boost[3]
Boost Graphs, Stock and Modified[4]
Stock Boost Gauge, Aftermarket Boost Gauges, Pressure Conversion Info.[5]
Boost Problem Check List[6]
ECU Self Diagnostic System-Fault Codes, Output Tests[7]
Overboost Fault Code? 2224? Check the boost hoses? Check the MAF electrical
Throttle Cable Adjustment[9]
Are you using the correct Spark Plugs?[10]
Are you using the correct Distributor Rotor on the 1991 200TQ 20V with 3B
Coolant Pipe O-Rings, Seal and Turbo coolant hose replacement[12]
Ignition misfire on S4/S6 with coil over direct ignition system?[13]
Engine Control Unit (ECU) Vacuum/Boost Line[14]
Vacuum Leaks-Breather Hoses![15]
Vacuum/Boost Leaks-Turbo and Intercooler, Turbo Bypass Valve[16]
Thermostat and other Cooling system checks[17]
Throttle Switch/Potentiometer - Defective?[18]
Multi-Function Temp. Sensor-Defective?[19]
Waste Gate Controls[20]
Idle Stabilizer Valve (ISV)[21]
Knock Sensor Intermittent? Hesitation? Low Power?[22]
Air Temp Sensor[23]
Fuel Injectors[24]
O2 Sensor Replacement[25]
Idle Speed/Mixture Adjustment?[26]
Exhaust System Issues[27]
Drive Train Issues[28]
Important Safety Information[29]

It should be your responsibility (or your mechanics) to check out and verify
all the ECU and Fuel Injection system components mounted on your engine are
working correctly BEFORE you do any type of modification that will raise the
boost on your vehicle. Raising the boost on a high mileage car isn't a great
idea UNTIL you get the stock system working correctly as it was originally
designed when it left the factory.
If you install a stiffer Waste Gate spring or use a valve connected to boost
pressure to add pressure on top of the Waste Gate Diaphragm, you may be
unaware if the engine ECU/Fuel system components are not working correctly,
because the boost may be raised regardless of the condition of the stock
system components.
Whenever I purchase a high mileage Turbocharged Audi, I check the compression
to verify the engine valves, rings, and head gasket are in good shape. I also
recommend having a cylinder leak down test done by a local repair shop. This
test is better than a basic compression test and involves pressurizing the
cylinder with air and measuring the amount of leakage out of the cylinder.
This can pin point cylinder leakage out of the intake or exhaust valves, past
the rings, or through the head gasket.
I replace the plugs, air filter, and fuel filter and change the engine oil. I
check/replace the O2 sensor. I replace the valve cover gasket and any suspect
breather or vacuum hoses. I make sure the timing belt has been replaced at the
suggested 60-90K interval. I remove the lower Intercooler/turbo hose(s) and
inspect them, I check/replace the accordion style hose at the Intercooler
connection. It is also very important to check/replace the turbo bypass valve.
On the 1991 200TQ, it is important to Check/Replace the small Bypass valve
hose (vacuum/boost hose) that connects from the bypass valve to the small
fitting at the back of the intake manifold.
You can also pressure test the intake/boost hoses, by making an adapter that
is inserted into the hose between the MAF and the turbo, and then apply
regulated compressed air (~15psi) and pressure test the hoses from the turbo
inlet, out the turbo exit hose, through the Intercooler, and into the
I check the operation of the idle/potentiometer throttle switch, and I remove
and clean the idle stabilizer valve. I check the ECU measurement block values
(10 values) to ensure the ECU is operating the engine as expected. If a
problem with the mixture adjustment shows up, the intake vacuum hoses, or the
fuel pressure may need to be checked.
The cooling system integrity should be accessed. Replacing the thermostat, and
all the coolant hoses, the heater control valve, the heater core to engine
hoses, the radiator fan temperature sensor, the after-run temp sensor, and the
Multi-Function temperature sensor is also a good idea on vehicles with over
100k miles. You don't want a $10 hose ruining your expensive 20V engine when
the hose blows and causes overheating and a possible blown headgasket!
Once I complete the replacement of these many high mileage components, and
ensure the engine/ECU is operating correctly, I can rest assured, and I don't
need to "guess" if they are ok if the car starts running poorly. Don't get
lazy and "assume" these components are ok, just bite the bullet and inspect or
replace them as necessary.
NOTE: Increasing the boost on your vehicle may change the emissions output
which may not be legal in your area.


Stock Boost Levels-How does the ECU control Boost?
The 1991 200TQ with 20V Turbo Engine was designed to run a maximum of 1.825
bar boost (absolute) or ~12 psi gauge pressure, as set by the Motronic Engine
Control Unit (ECU). The 1992-96 Audi S4 in overboost mode will have boost set
to about 2.15 bar absolute. The actual boost produced will vary depending on
engine RPM, air temperature, fuel quality, and altitude.
The 20V boost control system is different than what was used on the earlier
10V Audi Turbo engines. The ECU uses the Waste Gate solenoid to send boost to
the lower chamber in the waste gate to open up the waste gate at the
appropriate boost level. The Waste Gate solenoid valve is designed to be open
with no electrical signal applied, which will send turbo boost pressure
directly to the lower chamber in the waste gate and open the waste gate at a
pressure only determined by the stiffness of the waste gate spring. When the
waste gate solenoid is energized with a high duty cycle, (more on time) the
waste gate solenoid will prevent boost pressure from reaching the waste gate
lower chamber, and allow the boost pressure to climb higher. The boost will
climb, until the required boost level set in the ECU is reached, at which
point, the duty cycle percentage (%) to the waste gate solenoid will be
reduced, so the boost pressure will once again be sent to the lower chamber in
the waste gate diaphragm and the boost pressure will be controlled.
In the stock 20V engines, this corresponds to about 1.4-1.5 bar of boost as
controlled by the waste gate spring.
If you completely remove the lower waste gate hose, or block it off, the waste
gate will only open when "exhaust" pressure acting on the waste gate valve
face, exceeds the waste gate spring pressure. This is why a modified 20V ECU
can make 2.25 bar absolute boost pressure, even with the stock waste gate
spring installed.
The ECU uses a barometric sensor to monitor the altitude the vehicle is at,
and will trim back the boost slightly as the altitude increases above 3900
feet. For example, in the 1991 200TQ 20V, the boost will be reduced to about
1.715 bar absolute, when the altitude is above 8500 feet [1].

Stock Digital Boost Gauge, Aftermarket Boost Gauge, Pressure Conversion Info.
The stock digital boost gauge located in the instrument cluster displays the
intake manifold "absolute" boost pressure, from ~0.0 bar to 2.0 bar. The S4
will read up to 2.5 bar on the 1992 models with the digital boost gauge. Some
of the later Canadian S4 models also have the boost gauge. For more details,
go to the  Stock Boost Gauge[30] page

Boost Problem Checklist
Here is a checklist you can go through when you have low or high boost
Check for any stored ECU fault codes, are the knock sensors working ok?
Check Throttle Cable Adjustment
Check Air Filter, is it really clogged up?
Check/Replace the Turbo Bypass Valve
Replace small vacuum hose from Bypass valve to Intake Manifold
Check the Operation of the Multi-Function Temp Sensor
Temporarily disconnect Multi-Function Temp Sensor 4 pin connector and recheck
boost output
Remove and inspect the Intercooler exit hose (Ribbed Hose) and the lower turbo
exit pressure hose for leaks.
Check the rubber hose between the intake pipe and the throttle body.
Check the hoses on the Idle Stabilizer for leaks
Check Vacuum hoses and their connections at the back of the intake manifold
Check/Replace the coolant thermostat, is the engine running too cold? or too
Check the operation of the Waste Gate Solenoid using the ECU Output Test
Check Waste Gate Solenoid hoses
Check Air Temp sensor resistance at the ECU, and Repair the Air Temp Sensor
wiring connections if needed.
Plug or pinch off hose to lower waste gate and check for maximum boost and
overboost cutout
Check Waste Gate for sticking open, defective spring?
Check the Exhaust System Back pressure, possible exhaust restriction
Inspect the Turbo inlet vanes and end play, has the turbo gone south?
Check Engine Coolant Temperature Sensor Resistance
Check Idle Stabilizer Hoses, and other small vacuum/boost hoses
Check Lower Waste Gate Hose from Intake Manifold for leaks/restrictions
Check the operation of the Waste Gate Solenoid using the ECU Output Test
Check Vacuum/Boost Hose from Intake Manifold to Engine Control Unit (ECU) for
Pressure test the intake/boost hoses using hose adapter and compressed air
regulator adjusted to 15psi.
Check and clean the electrical connections at the MAF sensor
Check Waste Gate Diaphragm for Leaks
Check Waste Gate Valve for sticking or restricted movement

Self Diagnostic System-Fault Codes
The self-diagnostic system test should be run before you have your stock ECU
modified, to make sure that there are no faults in any of the components
(knock sensors, coolant/air temp sensors, throttle switch etc.) that the ECU
relies on for correct engine operation. You need to do a 3rd gear full
throttle run to exercise this diagnostic system correctly. The 1991 200TQ's
with the Bosch Motronic ECU, have a non volatile memory for the fault codes
that can be read later even if the engine has been turned off after the test
The ECU system OUTPUT tests should also be done on the stock ECU before doing
any modifications to verify that the Wastegate Solenoid, the Carbon Canister
valve, the idle stabilizer valve and the fuel injectors are working correctly.
In some cases the ECU internal drive transistors for these solenoids can be
defective and will prevent correct operation of the engines fuel and boost
control systems. If there is a problem with any of the solenoids or the
internal operation of the ECU, this needs to be known BEFORE you send in a ECU
for modification.
Go to the 20V ECU Fault Code[31] page at this web site to run these Output

Overboost Cutout? ECU fault code 2224?
If you are encountering an overboost condition which is causing the ECU to cut
the fuel pump, and you have stored the 2224 ECU fault code, here are some
items to check out.
You may have a leak in the boost hoses between the turbo exit connection and
the throttle body. If you have a hand pump or an air compressor, you can make
up an adapter to fit into the hose that connects to the turbo inlet, and this
will allow you to pressurize the air from the turbo inlet, through the turbo
exit hose, through the intercooler, from the intercooler exit hose, and from
the metal intake pipe to the throttle body and check for any leaks. Use a
pressure regulator set to about 15psi if you are using an air compressor.
You can also do a visual check of the turbo exit hose underneath the car and
look for any signs of oil leaking out of the turbo exit hose, indicating a
leak. Normally a small amount of oil vapors and mist are sucked in by the
turbo from the crankcase breather system, and this oil mist will typically
show up outside the hoses, or intercooler connections if a pressure leak is
occurring. Check and or replace the ribbed intercooler exit hose as well,
these often split open on the bottom side, in-between the wire re-enforced rib
areas, due to the crankcase breather oil mist in the system. Removing the
hoses and inspecting them is also a good idea on high mileage engines.
If you don't have any leaks in these hoses, check the vacuum/boost hose that
connects from the back of the intake manifold to the ECU inside the car. See
below for details. Engine Control Unit (ECU) Vacuum/Boost Line[32]
I have run across a couple of 20V Turbo cars which had a 2224 code stored
inside the ECU and were getting the overboost fuel pump cutout, even though
the boost was only going up to 1.0-1.2 bar. In the first case this would occur
only when the throttle was quickly floored, and would not happen if the
throttle was gradually opened all the way. In the first case, it ended up
being a defective MAF sensor. In the other case it was only a poor connection
at the MAF sensor. Try using some contact cleaner on the MAF connector and
terminals, and see if the overboost problem is eliminated.

Throttle Cable Adjustment
It is a good idea to check the throttle cable adjustment to ensure the
throttle is being opened all the way when the gas pedal is pushed all the way
down to the floor.
With the engine off, have a friend push down on the gas pedal inside the car,
while you check the throttle lever under the hood. The throttle lever should
be pulled all the way open with the gas pedal pushed down all the way to the

Are you using the correct Spark Plugs?
The 20V Turbo engines as found in the 1991 200TQ and in the later 1992-95
S4/S6 should have installed the Original Equipment F5DPOR BOSCH PLATIN
Electrode Spark Plugs. Be aware, that they are expensive, about $14 each.
These F5DPOR plugs have a single thick platinum electrode.

Bosch F5DPOR PLATIN Spark Plug
These thick electrode Bosch PLATIN plugs should not be confused with the
cheaper Bosch platinum spark plugs that have a thin center electrode.
There are Bosch 3 electrode copper core plugs (FR5 DTC) that cross reference
to the F5DPOR, but I don't recommend using these 3 electrode copper core
plugs, use the correct single thick electrode platinum F5DPOR plugs.
It is also a good idea to check the torque of the spark plugs, every 5k miles
if you are running high boost levels, as many have found the plugs to come
loose on the 20V Turbo engines. The Bosch F5DPOR plugs are spec'd to be
tightened to 22 lb-ft.
It is also a good idea to replace the valve cover gasket on the 20V Turbo
engines, if you have higher mileage on your engine, and still have the
original gasket. The center gaskets around the spark plug holes can leak and
mess up the plug area and the spark plug wire ends.

Are you using the correct Distributor Rotor for the 3B 20V Turbo Engine in the
1991 200TQ ?
The 3B designated 20V Turbo engine in the 1991 200TQ used a unique Distributor
Rotor with a narrow tip that can be difficult to find in the US. This narrow
tip rotor is required to provide the precise ignition timing at the higher
RPM's and to prevent any ignition cross firing inside the distributor cap.
Some people have reported engine damage (bent rod) that may have been caused
when the incorrect cylinder was fired under very high boost.
The Bosch catalog shows the INCORRECT distributor rotor (Bosch 04170 or the 1
234 332 350 R1) for this 20V Turbo application. This 04170 (1 234 332 350 R1)
rotor has a wider electrode tip measuring 0.710 inches (~18mm).
The correct Distributor Rotor is the Bosch 1 234 332 414 R1 (Audi 054 905 225)
which has a narrow electrode tip measuring only 0.432 inches, (~11.0mm).

The original 3B distributor rotor with narrow tip.
The other important thing to note about this rotor, is that it is glued onto
the distributor shaft, and it needs to be broken or crushed when removing it
from the shaft using a pair of channel lock pliers. You can also use a dremel
cutting wheel to cut off the old rotor. Be careful not to damage the shaft
when crushing this rotor and removing the left over pieces. You need to
thoroughly clean the distributor shaft with some sand paper and some non
residue cleaning solvent, (loctite primer solution is also recommended) and
then use Loctite 640 (or equivalent) to secure the new rotor in place. The
Loctite needs at least 4 hours to dry before you can start the engine.
This operation is not easy given the restricted access to the distributor on
the 3B 20V Engine. You can remove the intake manifold to allow easier access
to the distributor.
NOTE: Even though it is possible to pry off the anti-tamper plate from over
the distributor hold down nut with the intake manifold installed, I recommend
taking the extra time and remove the intake manifold to allow easy access to
the ignition distributor. With the intake manifold removed, you may want to
replace the coolant pipe O-Rings, seal and Turbo coolant hose while you have
the intake manifold removed. See section below for details.
When you unbolt the intake manifold, you can leave the fuel lines attached to
the intake fuel rail, and just use some heavy wire to tie up the intake
manifold out of the way as shown in the Bentley repair manual. A long (over 8
inch) 6mm allen wrench tool is usually required to remove the intake manifold
bolts. (VAG1669 or equivalent). You can also use a long 1/4 inch drive
extension, with a 6mm socket and a 6mm allen wrench socket shaft in
combination to access these intake manifold bolts but this is a little tricky.
The intake manifold gasket should be replaced and the bolts should be torqued
to 16 ft-lbs upon reassembly, start in the middle and work your way outward
when torquing them down.
Once you have the intake manifold tied up out of the way, you can cut off the
anti tamper cover (a Dremel cutting wheel works here as well). Before you
loosen the distributor hold down nut, you should first set the engine to be at
TDC with the flywheel O mark lined up, and the distributor rotor pointed at
the line mark on the rim of the distributor. Once you get the engine rotated
to the correct position, you can remove the hold down clamp and pull out the
Removing the distributor gives you better access to the distributor so you can
either crush the old rotor with some pliers, or cut the old rotor off
"carefully" using a dremel cutting wheel. As mentioned, you should remove all
traces of the old rotor with some 200-300 grit sandpaper, and use some
non-residue solvent (loctite primer solution is also recommended) to clean the
distributor shaft before you apply some Loctite 640 to the inside area on the
new distributor rotor. Insert the rotor over the distributor shaft with the
key way lined up. Allow to dry for at least 4 hours before starting the
The Bentley Repair manual also indicates that you should use the special tool
#3233 to accurately line up the distributor rotor tip when setting the basic
distributor position in relation to the flywheel O TDC mark, and the cam gear
dot mark. The Bentley also states that you should turn the distributor body
counter-clockwise when you line up the rotor with the 3233 tool and then hold
the distributor body with your hand while you tighten down the hold down nut.
NOTE: The R1 in the distributor rotor part number refers to the fact that this
rotor has a 1K ohm resistor between the center electrode and the outer tip
electrode. This rotor resistor can sometimes burn up after many years and
cause a no start, and prevent the engine from running.
You may also want to replace the valve cover gasket, the main breather hose,
any vacuum hoses and other hard to reach coolant hoses and coolant temperature
sensors while the intake manifold is moved out of the way.
See section below for replacing the Coolant Pipe O-Rings, seal and turbo
coolant hose while you have the intake manifold off.

Coolant pipe O-Rings, seal and Turbo coolant hose
If you have the intake manifold removed to replace the distributor or rotor, I
also recommend that you remove the cast aluminum coolant distribution pipe
(water manifold) that bolts to the cylinder head and replace the 3 O-Rings and
the triangular shaped rubber seal. You will need to order these seals from the
Audi dealer.
Replace the short piece of turbo coolant hose while you are at it, you may
need to move the power steering pump out of the way or remove it completely.
Some generic auto parts store carry coolant bypass hoses for other makes that
have the correct diameter that will work to replace this hose on the 20V
turbocharged Audis.

Ignition Misfire on 1992-96 S4 AAN engine with coil over ignition?
The 1992-96 US made S4's 5 cylinder 20V engine uses individual ignition coils
for each cylinder and two ignition coil modules (Ignition coil drivers). This
system also uses a short ignition wire connector lead between the ignition
coil and the spark plug which normally should have approximately 5000 ohm
resistance when measured across each end with an Digital Multi-Meter (DMM) in
Ohms setting.
One ignition module controls the coils on cylinders 1 and 2, and the other
module controls the ignition coils on cylinders 3, 4, 5. The original Audi
part number for the module is 4A0 905 351A, the Bosch part number is 0 227 100
209. The system uses two of these modules. Several folks have run across
intermittent misfire problems that have been traced to a defective ignition
coils and in some cases to a defective ignition module. Trouble-shooting these
ignition misfires can be tough as it is difficult to locate the one ignition
coil or module that is causing the problem.
The original Audi part number for the ignition coil is 034 905 105, these
coils are manufactured by Beru, the original Beru number shown on the coil is
0 040 100 009. The replacement ignition coil I purchased on Nov. 1, 2001 had
the Beru # 0 040 100 043.
Test Case
I worked on one 1994 S4 recently with an intermittent misfire that only
occurred when the boost level was above ~1.6 bar when the ignition system is
under the most strain. I first tried connecting a known good used ignition
module in place for the module for cylinders 1, 2, but the misfire was still
there. Then I changed the module for cylinders 3, 4 and 5 with the used good
module. This did not affect the misfire either, so I pulled the coil pack from
the top of the engine and disconnected the connectors at the firewall. I
installed a known good coil pack from another S4 and found that the
intermittent misfire was gone. Then I went back to the suspect coil pack and
tested each of the 5 short ignition wire connectors to make sure they each had
approximately 5k ohms of resistance. Each of the 5 ignition coil wire
connector leads tested ok.
Then on the suspect coil pack I tested the resistance from the center of the
coil high voltage output to the metal case of the coil.
When using a Fluke 87 DMM set to the 40 Mega-ohm resistance test range, and
with the Positive lead connected to the center coil output and the DMM common
lead connected to the coil metal case, I saw very high resistance readings.
The DMM showed "OL" which means the measurement is over 40 Mega-ohms and is
too large to correctly display or measure correctly. One of the ignition coils
looked to have been replaced recently on Cylinder #4 which may explain why it
measured only 5.4 Mega-ohms, when all of the other cylinders ignition coils
measured over 40Mega-ohms, and gave the OL reading on the DMM.
The Fluke 87 DMM has a unique range when measuring very high resistance, up to
100,000 Mega-ohms, it is called the conductance range that reads in
Nano-Siemens (nS). One Nano-Siemen is 1 X10-9 Siemens, or 0.0000000009
Siemens. The range for this setting is up to 40nS. You can convert a
Nano-Siemens conductance measurement to its equivalent resistance in ohms, by
dividing 1000 by the nano-Siemens reading.
For example: 2.00 nano-Siemens equals 1000/2.0X10-9 or 500 Mega-Ohms.
I measured across the ignition coil center output using the Fluke 87 DMM in
the regular Mega-ohm resistance range and using the special Conductance range
setting with the common test lead on the coil metal case, and the positive
test lead on the coil High Voltage output center conductor.
AAN Engine Resistance in Ohms Conductance in nano-Siemens (nS) Cylinder 5 OL
(above 40.0M) 16.5nS Cylinder 4 5.4 M OL (above 40.0nS) Cylinder 3 OL 23.85nS
Cylinder 2 OL 0.02nS Cylinder 1 OL 16.80nS
Notice that cylinder #2 ignition coil has a very low conductance reading 0.02
nS, which equates to a very high resistance reading, almost up to 100,000
Mega-Ohms which is near the limit of the DMM.
I suspected that this Cylinder #2 coil was the one causing the intermittent
misfire. Replacing coil #2 fixed the problem, my assumption was correct!
The known good S4 coil pack I used for test purposes had the following
readings when I measured across the ignition coil center output using the
Fluke 87 DMM in the Mega-ohm resistance range and also when in the special
Conductance range setting with the common test lead on the coil metal case,
and the positive test lead on the coil High Voltage output center conductor.
AAN Engine Resistance in Ohms Conductance in nano-Siemens (nS) Cylinder 5
5.05M OL (above 40nS) Cylinder 4 3.84M OL (above 40nS) Cylinder 3 6.60M OL
(above 40nS) Cylinder 2 6.07M OL (above 40nS) Cylinder 1 6.30M OL (above 40nS)

Engine Control Unit (ECU) Vacuum/Boost line
In order for the ECU to work correctly, it must receive an accurate pressure
signal (vacuum or boost) from the intake manifold. There is a plastic line
with rubber hoses on each end connected from the ECU to the intake manifold.
The vacuum/boost line (Green plastic line and rubber hose) from the intake
manifold to the ECU should be checked for leaks if you are getting too much
boost or a fault code 2221, 2222 or 2224. You can connect a hand vacuum pump
to the line after removing it from the back of the intake manifold and see if
it holds vacuum. If you have access to compressed air and have an accurate
pressure regulator, you can apply 10-15psi to the vacuum/boost line and listen
for any leaks. If you turn on the ignition, (Don't start the engine) you can
read the boost pressure off the digital instrument panel boost pressure
display, and see if it corresponds to the actual pressure applied to the ECU
line from the air pressure regulator.
Be careful when applying pressure to this line, and avoid exceeding 20 psi or
you could damage the internal ECU pressure sensor.
The 1991 200TQ 20V Motronic ECU has the moisture trap located next to the ECU
underneath the kick panel cover that can leak.
The S4/S6 has the moisture trap located underneath the plastic cover on the
firewall at the back of the engine compartment. It is very common for the hose
to get blown off the moisture trap on the S4/S6 vehicles and cause the vehicle
to have low boost and run poorly in the "Limp Home" mode. I recommend
replacing the crimped on clamps with conventional screw type hose clamps at
this moisture trap location.
Vacuum Leaks-Breather Hoses!
You need to make sure your engine’s various breather/vacuum hoses and turbo
intake and pressure hoses are in good shape. Leaks anywhere in these hoses can
cause lean or rich running and poor starting. Vacuum leaks would typically
cause lean running, pressure leaks after the turbo could cause slightly rich
running. It is not sufficient to just visually look at these hoses without
first removing them!
On the 1991 200TQ 20V with the 3B designated engine, the large breather hose
coming from the side of the engine that is routed around the back of the
engine typically gets rotten and mushy on the inside part of the hose facing
the engine block. Often times the hose can collapse on itself or will split
open on the back side. This hose does not have hard vacuum applied inside like
the 10V Audi Turbo MC engine but leaks could allow un-metered air into the
engine. This 1991 200TQ 20V breather hose has the Audi part number
Here is a photo of the 1991 200TQ 20V hose, flow regulating valve, check valve
and the small hose that connects to the intake manifold.

This 1991 200TQ 20V breather hose is routed around the back of the engine and
one end connects to the round black crankcase pressure regulator valve near
the back of the valve cover. This black regulator valve, controls the flow of
crankcase fumes to the turbo intake boot [1]. The valve has two ports inside,
one is approximately 0.400 diameter and a smaller one that is approximately
0.180 inches. The larger port and the smaller port are open during low boost
operation but when the turbo makes enough boost and flows enough air to
produce a vacuum on the suction side of the turbo, the regulator valve
diaphragm sucks closed and blocks off the larger port. This leaves only the
smaller port open to flow crankcase breather fumes.
Here are two diagrams showing the valve in operation under low boost and low
intact tract flow with both the 0.180 and the 0.400 ports open, the second
diagram shows the operation under high boost, high intact tract flow, high
intact tract vacuum conditions when the internal diaphragm is pulled against
the 0.400 opening to close it off, leaving only the smaller 0.180 port open
for reduced flow.


Diagram courtesy of Audi of America
Diagram courtesy of Audi of America
NOTE: On the 1991 200TQ 20V there is also a flame arrestor (spiral wire brush)
inside this breather hose on the end near the black flow regulating valve, so
be sure to remove this flame arrestor and install it in the replacement hose.
This flame arrestor prevents a back fire in the intake boot from traveling
into the engine crankcase. Audi refers to this wire brush as the "flame
deflector plate" and the part number in the parts fiche is 035-103-477A. I
have not verified if the S4 uses this same flame arrestor.
This 1991 200TQ 20V breather hose also has a Tee in the center of the hose,
which connects to the intake manifold via a flow regulating one way check
valve and a very expensive (~$95 list) molded rubber hose. The check valve
will close under positive manifold "boost" pressure.
On the 1991 200TQ 20V the carbon canister solenoid valve also connects at this
tee location. The carbon canister solenoid allows gasoline fumes from the fuel
tank that are stored in the carbon canister, to flow into the intake manifold
when the engine is running. The carbon canister solenoid valve is closed when
the engine is off to prevent fuel tank fumes from entering the engine and
causing a rich mixture when starting. If the carbon canister solenoid or
electrical connection should fail, this valve also has an internal diaphragm
which will open under engine vacuum to allow fuel tank fumes to flow into the
engine. It is very difficult to see this connection to the intake manifold
hose, as there is very little room behind the engine.
NOTE: On the 1991 200TQ 20V if you are working around the back of the engine
and replacing this breather hose, make sure you mark and identify any hoses or
electrical connectors before removing them. In one instance, I ran across a
car that had the electrical connectors from the heater valve temp sensor, the
carbon canister valve, and the engine coolant temp sensor mixed up and
connected to the wrong item. The color coding on the connector, may not match
up with the item it plugs into.
The Bentley manual shows that the Engine Coolant temp sensor connector should
have two wires, one Gray with brown stripe, one Green with black stripe. The
Carbon Canister solenoid connector has two wires, a Red colored wire and a
White with red stripe wire. The heater valve coolant temp sensor electrical
connector should have two wires, a Yellow and red stripe wire and a Brown with
black stripe wire.
The two small idle stabilizer hoses should be removed and checked also as they
can get blown off from the pressure. Any of the smaller hoses connected from
the intake manifold to the heater/AC system should be checked for cracks or

Vacuum/Boost Leaks-Turbo and Intercooler
Often times on high mileage (100k>) cars, the turbo hoses begin to leak boost
pressure from loose hose clamps and in other cases when running higher boost
above the stock 12psi (1.8 bar absolute) levels, the original boost hoses can
tear and split open.
You can make a pressure test cap to pressurize the intake/boost hose system to
15-20psi and look for any leaks from the turbo intake hose, to the turbo exit
hose, from the intercooler to intake ribbed hose, into the throttle valve and
finally to the intake manifold. This test cap can be made using a 3 inch black
plastic sewer pipe cap, and a bolt in tire valve stem. You insert this test
plug into the turbo intake hose after removing the hose from the MAF sensor.
You may need to use a pressure regulator with your air compressor to limit the
pressure of the system to 15-20psi.
Common boost leaks: 1991 200TQ: The accordion-style hose that connects the
intercooler exit to the intake manifold normally rots (from oil) over time and
can split at the bottom between the ribs while under high boost. On the later
1992-95 S4, the ribbed hose from the boost pipe into the intercooler
underneath the engine will often split in the ribbed area for the same reason.
Remove and check them. If your car has over 100,000 miles on it and this hose
has never been replaced, save yourself some grief and buy a new hose. Samco
has some heavy duty silicone replacement boost hoses for the Audi 20V's. The
turbo exit hose can also split on the 1991 200TQ 20V and on the S4's where the
bypass valve hose is melded into this turbo exit hose.
Many folks are running over 20psi with the stock K24 Turbo, and if you get a
severe boost hose leak, you can over-spin the K24 into destruction. The 1991
200TQ 20V accordion hose has the Audi part number 035-145-720A and is
available. See my Audi parts section[33] for availability and pricing details.
The large turbo intake hose with connections to the bypass valve, breather
pipe and Mass Air Flow sensor should be removed and checked for any cracks as
this will allow un-metered air inside the engine.
While you have the intake hose off the turbo, you may also want to check the
cold side turbo shaft for excessive play. The earlier turbo Audis spec the max
end play to be 0.008 inches, and the side play to be no more than 0.022
inches. Normally you need a dial indicator to accurately measure this. You
should also look for any metal shrapnel caused by worn turbo bearings which
may have allowed the turbo compressor blade to rub against the turbo housing.
The turbo exit hose (turbo pressure) going to the intercooler should be
removed and checked as well. The 1991 200TQ 20V has a metal boost pipe that is
inserted into the intercooler inlet uses an O-Ring for sealing this
connection, see section below for details if you see evidence of a leak at
this point.

Turbo Bypass Valve
This valve should be removed and checked as this can cause a loss in boost
pressure if the internal diaphragm is leaking. Use a handheld vacuum pump
connected to the small hose fitting to check this valve operation. It should
hold vacuum and open and close freely. This bypass valve relies on boost
pressure acting on this internal diaphragm to hold the valve closed when the
engine is making positive manifold pressure (above 1.0 bar absolute boost
pressure) This bypass valve opens during manifold vacuum conditions (below 1.0
bar manifold pressure) to allow intake air to circulate around the turbo cold
side impeller. This bypass valve helps to reduce turbo lag and keep the turbo
spinning during transmission shifting when the engine is producing boost. It
also helps to prevent boost pressure spikes from causing leaks or damage to
the intercooler or connecting hoses.
Audi Bypass Valve Part Number Info:
The 1991 200TQ 20V with 3B engine originally used a bypass valve with the Audi
part number 034 145 710 which had the metal reinforcement plate on the
diaphragm, this bypass valve may have had the Bosch 0 280 142 106 part number
on it. The Audi dealer replacement part number for this bypass valve at one
point was changed to the 034-145-710A which had the Bosch -108 suffix.
An Audi Service bulletin: Group 21 Number 00-01 Dec. 8, 2000 was issued that
described a problem with the Audi TT's that were equipped with the ATC, AMU
and AWP code 1.8T engines. A groaning, howling or rattling type noise may be
heard to come from the bypass valve. A new Bosch bypass valve (Audi part
number 06A 145 710N) was introduced to address these complaints and is
reported by other Audi tuners to be a good replacement valve for other Audi
turbocharged engines utilizing a bypass valve.
This new 06A 145 710N Audi part number valve is reported to have the Bosch 0
280 142 114 part number. This Audi -710N valve is also reported to have the
same internal components and stronger diaphragm material as the -110 suffix
designated Bosch bypass valve.
See my Audi parts section[34], to check availability for this Bosch bypass
Note: The normal failure with these plastic Bosch bypass valves, is the rubber
diaphragm develops a tear, either near the outside edge of the diaphragm, or
at the sealing surface.
A few aftermarket performance companies have a new all metal, version of the
bypass valves, one is called the "Bailey" valve, and one I believe is called
the Forge valve. I have not tested these valves yet, so I can't comment on
their effectiveness.
The intercooler end cap seals can leak under high boost and you should install
some large straps or one or two large hose clamps around the intercooler to
hold it together under high boost. Many hydraulic supply houses carry the
screw type hose clamps that are 4 ft long which fit around the intercooler
Because there normally is an oil mist flowing through the intercooler, any
leaks around the intercooler seals will show up as oil seepage out of the
intercooler end caps. In some cases you can carefully re-crimp the aluminum
tabs that hold the plastic end tanks on to tighten up the end cap seal. The
internal intercooler rubber seal on the inlet side that seals the internal
upper and lower sections of the intercooler should be checked when you remove
the hoses from the intercooler.
Here is a picture of the larger single pass intercooler from the 1991 200TQ
20V. There is an O-Ring on the inside of the intercooler inlet on these
intercoolers that seals this pipe to the intercooler. This O-ring should be
checked for leakage and replaced if necessary.

Cooling System checks
The engines cooling system needs to be working correctly before you decide to
up the boost on your car. The Engine Control Unit uses a coolant temp sensor
that should be tested if cold or hot start problems or problems during engine
warmup are occurring.
Go to 20V Engine ECU System[35] page for details on checking this sensor.
All the coolant hoses should be checked/replaced and the cooling system should
be flushed out and refilled with the correct mix of coolant. When you have the
upper radiator hose removed, check the condition of the plastic upper radiator
hose fitting, as these plastic fittings can crystallize and break off while
driving. The plastic heater valve should be replaced if it has high mileage,
they have a bad habit of breaking, causing a loss of coolant and some rapid
The thermostat should be replaced with the factory specified 87 degree Celsius
version if you still have the original one installed, or if it is several
years old. Often times a malfunctioning thermostat will cause engine running
problems, and poor gas mileage if the engine is running too cool.
You may also want to replace the short piece of coolant hose located
underneath the intake manifold, as this hose can blow and make your life
miserable if you have to change it when the engine is hot. Audi sells the hose
in bulk lengths, or you can look for similar sized coolant hose from an
aftermarket parts supplier, Gates etc. I don't recommend using fuel line or
other hose not designed for high temperatures.
The after run cooling fan and turbo pump should be checked for correct
operation. Go to the Cooling system[36] page in the Troubleshooting section
for additional details.
Idle Switch or Throttle Potentiometer Defective?
A defective Throttle Potentiometer will prevent the correct operation of the
Waste Gate (WG) solenoid system that the ECU uses to adjust the turbo boost.
Note: On the 3B 20V Engine, a Potentiometer is a device that outputs a varying
resistance that the ECU uses to measure how much the throttle is opened. The
ECU uses the Throttle Potentiometer readings to adjust the boost and mixture
depending on how fast and how much you push the throttle down [1].
Go to  20V Engine ECU System [37]for details on checking the Throttle
Note: Using a stiffer Waste Gate spring is a great idea on the MC engines and
possibly on some 20V engines with non-stock Turbos as it will improve the rate
at which boost is produced, but...... it can mask problems with the ECU system
components (Throttle switch/throttle potentiometer etc.) as the boost will be
mostly controlled by the stiffer Waste Gate Spring.
Multi-Function Temp. Sensor-Defective?
One additional sensor to check when having low boost problems, is the
Multi-function Temperature sensor. This sensor is mounted underneath the
intake manifold, on the coolant pipe on the 20V Turbo Engine.
This Multi-function temp sensor uses a 4 terminal connector which is normally
is covered with a protective rubber boot. The newer style replacement Sensors
will have only 3 terminals as they simplified the internal design of this
sensor and eliminated the +12V supply to this sensor. Go to  20V Engine
Multifunction Sensor[38] section for more details on this sensor.
Waste Gate Controls
The Waste Gate solenoid connecting hoses should be checked for leaks and the
ECU Waste Gate Solenoid "Output Test" should be run on the Waste Gate
Solenoid, as these solenoids are often found to be defective on the 20V
engine. Go to  20V ECU Output Tests [39]for more details.

NOTE: Normally, there is not supposed to be any hose connected to the top of
the waste gate cap fitting. It should be left open to the atmosphere.
Idle Stabilizer Valve (ISV)
The idle stabilizer valve (ISV) can also stick from oil/crude built up inside
and this can cause strange high idle conditions. Cleaning the ISV with carb
cleaner may help.
The hoses on the ISV valve to the intake manifold can blow off under boost, or
can be cracked and cause a loss in boost pressure.
This engine uses the ECU to control the idle stabilizer valve, which is a
rotary type valve. Go to  20V ECU System [40]for details on checking the
operation of this rotary valve

Knock Sensor Intermittent? Hesitation? Low Power? Check Engine light On?
If your 1991 200TQ 20V is running poorly or the "Check Engine" light has come
on, it is possible that the ECU is detecting a fault in the knock sensor
system caused by a loose, defective or intermittent knock sensor. There may
also be a problem with the knock sensor wiring or connectors.
Go to  20V Engine Knock Sensors [41]for details on the 20V Turbo Engine knock
The newer style "replacement" knock sensors have gold plated terminals and the
wiring harness connector terminal pins should be replaced with these gold
plated versions as well. Some of the 1991 200T/Q's have the "Check Engine"
light bulb removed from the dash, so if the car is having a problem with one
or both of the knock sensors, you may not know it. The "Check Engine" light
should come on with the ignition key turned on to the first position.
Air Temp Sensor
The air temp sensor that the ECU monitors to regulate boost and ignition
timing can have poor external connections or can have intermittent connections
on the small thermistor at the tip of the sensor. This sensor is located in
the intake manifold near the throttle valve. The wire terminals are normally
soldered or spot welded to the air temp sensor. Factory replacement air temp
sensors use a separate connector with gold plated terminals instead of the
soldered/welded terminals on the original sensor.
Go to  20V ECU System [42]for details on checking the operation of this air
temp sensor.

Fuel Injectors
Leaky injectors can increase the hot/cold starting times. Dirty/clogged
injectors can cause poor low end performance and uneven performance when cold.
Intake valve carbon deposits can also cause cold running problems as the
carbon acts like a sponge, soaking up the fuel at first. With severe carbon
buildup some repair shops can use a walnut shell blasting device to clean off
the carbon buildup. Lighter buildup can be removed with the use of a fuel
additive or a fuel with Techron.
 O2 Sensor Replacement
The O2 sensor should be replaced if it has over 60k miles on it, or if you are
having lean surging during light throttle conditions, poor cold running
problems during engine warm up, poor gas mileage or emissions related
failures. Older O2 sensors can get contaminated and will respond too slowly to
correctly adjust the mixture and can cause poor running when transitioning
from light throttle to full throttle conditions. See the 20V ECU System [43]section
for details on checking/replacing the O2 sensor.

Idle Speed/Mixture Adjustment/Check:
NOTE: The Idle speed and Idle mixture on the 20V engine are not adjustable.
The 20V Motronic Engine Management system has the capability to adjust the
fuel mixture over time (adaptive system) so periodic adjustments have been
eliminated [1].
Exhaust System Issues
You should have the exhaust back pressure checked by a muffler shop if you
have low boost conditions and suspect a plugged catalytic converter or
muffler/exhaust system. They can use the pre-catalytic converter sniffer pipe
bolted to the intake manifold to do this, or they can drill a small hole in
the exhaust pipe to insert the pressure gauge pipe. A low pressure (0-10 psi)
fuel pump pressure gauge can be used for this. Typically with the car idling
in neutral, as you slowly rev the engine to 4000 RPM and hold it at 4000 RPM,
the exhaust back pressure should not increase above 1-2 psi. If you run a long
hose from the sniffer pipe to a gauge inside the car, an assistant can check
the exhaust back pressure while you drive the car.
Test Example: When the car is driven and under load in 3rd gear at WOT and
near redline, it is not unusual to see over 4-7psi of back pressure when
running 1.8 bar of boost pressure. The older catalytic convertors can create
some of this back pressure and may need to be replaced as the car gets over
75K miles.

The RS2 type exhaust manifold is recommended for improving the exhaust flow
out of the head and to prevent burned exhaust valves when more extensive ECU
and Turbocharger mods are performed. As mentioned, the catalytic converter and
muffler system can get plugged or restricted over the life of a car which may
cause poor running with low power. Catalytic Converter meltdown can occur if
extremely rich mixture levels occur in the combustion chamber which are not
thoroughly burned which allows high levels of raw gasoline (Hydrocarbons) to
pass into the catalytic convertor. If this occurs under engine load conditions
and is followed by normal lean running or idling the catalytic convertor will
get very hot and damage may occur. Ignition misfires can also dump raw fuel or
hydrocarbons (HC) into the catalytic convertor and this will also cause
meltdown of the ceramic honeycomb material if the engine is driven under load.
If you ever need a reminder to let your engine run after a high boost run,
just open up your hood at night and see the red hot exhaust manifold and
turbo! Even with the water cooled turbo you should let the car idle for
several minutes. It may take 10-15 minutes of normal low boost driving before
the exhaust manifold/turbo cool down. It is a good idea to avoid high boost
initially after starting a cold engine until the oil temp gets up to 60C (this
is recommended in the factory operators manual).
Drive Train Issues
Driving with more boost/horsepower places more stress on the transmission,
clutch, suspension system and tires. The Audi I5 turbo engines are very robust
motors but running Higher Boost levels obviously places some additional stress
on the internal engine components.

The two hydraulic engine mounts and the rear transmission mounts should be
checked, the exhaust side (passenger side) engine mount is subjected to a lot
of heat. A defective hydraulic engine mount on this side tends to place more
stress on the exhaust manifold/system. Of course the tires, CV joints, wheel
bearings, shocks, sub frame bushings and other suspension bushings/parts
subject to wear and tear should be replaced where necessary. Having more power
will make the car a handful to drive if you have a worn out suspension as you
will discover in short order. Better to fix it now than to crash and burn it

References: [1] Audi of America, Technical service training publication:
"The New 20V Turbo Engine for the Audi 200 Quattro-publication #
WSP-521-209-00"All rights reserved.
© Copyright 1998-2002 SJM Auto-Technik, all rights reserved.
Return to Troubleshooting Tips page.[44]
Return to SJM Auto-Technik main page.[45]

  1. support.htm
  2. #intro
  3. #level
  4. graphbst.html#200t
  5. 10vgauge.html
  6. #chklist
  7. #diag
  8. #oboost
  9. #cable
 10. #plugs
 11. #rotor
 12. #coolpipe
 13. #s4coil
 14. #ecuvac
 15. #hose
 16. #hoset
 17. #cool
 18. #switch
 19. #mfsense
 20. #waste
 21. #idle
 22. #knock
 23. #air
 24. #inject
 25. #O2sense
 26. #mixture
 27. #exhaust
 28. #drive
 29. disclaim.html
 30. 10vgauge.html
 31. ecuf20v.html
 32. #ecuvac
 33. parts.htm
 34. parts.htm
 35. ecu20v.html
 36. cooling.html
 37. ecu20v.html
 38. ecu20v.html
 39. ecuf20v.html
 40. ecu20v.html
 41. ecu20v.html
 42. ecu20v.html
 43. ecu20v.html
 44. trouble.html
 45. index.html


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