[200q20v] Lack of boost very puzzling... long winded question

Forhan, Thomas Thomas.Forhan at mail.house.gov
Wed Mar 21 08:38:58 EST 2001

I just replaced my idle switch/throttle pot which restored full boost and
eliminated a stalling problem that a turbo bypass valve replacement had
helped but not cured. 

Testing following Scotts instructions the idle switch seemed to be OK but I
did not have a scope and could not really see the curve of the

The part was about $160 from Mac at Clair. I never got into the old one to
see if it had any broken solder joints, but its on my workbench. Someday...

Tom F.

-----Original Message-----
From: C1J1Miller at aol.com [mailto:C1J1Miller at aol.com]
Sent: Wednesday, March 21, 2001 7:42 AM
To: Alan.Cordeiro at mts.com
Cc: 200q20v at audifans.com
Subject: Re: [200q20v] Lack of boost very puzzling... long winded

Hi Alan,
Looks like you're doing the right diagnosis path.
If you haven't reviewed Scott Mockry's "The ABC's of Running High Boost for
the 1991 200TQ 20V Turbo"
it's a good place to start.

The idle switch may be dropping out and your meter may be sampling too
slowly to catch it.  An ocilliscope would be better.  Might want to change
it out; don't know the cost, but the part number is:
Bosch 0 280 120 410, Audi part number 034 133 154F
see: http://www.sjmautotechnik.com/ecu20v.html
Scott says "I have found that the idle switch can have cracked solder joints
inside this throttle switch and cause an intermittent 2121 fault code. The
photo shows the inside connections that can crack and cause intermittent
problems. If you are careful, you can use a razor knife to cut the plastic
top along the seam and pop the top off the throttle switch and then
re-solder the cracked idle connections."
That page also mentions the potentiometer that measures throttle angle; that
could also be your problem.

You should probably also buy an analog boost gauge for diagnostic purposes.
HTH< Chris Miller, c1j1miller at aol.com, Windham NH

In a message dated Tue, 20 Mar 2001 11:28:30 PM Eastern Standard Time,
"Cordeiro, Alan" <Alan.Cordeiro at mts.com> writes:

Hi everyone,

I have been trying to figure out a lack of boost for the last many
months, not urgent, since the car has considerable performance even
at 1.5 bar... however I have run out of ideas.

The patient is a 200q20v, which seems to refuse to allow me more than
1.5 bar boost. When I purchased the car it only had 1.3-1.4, which
was cured by replacing the wastegate solenoid (open circuit failure).
Boost improved to 1.5 immediately, which caused failure of the accordion
hose shortly thereafter (weak with age, low boost prolonged its life).

In trying to cure an occasional stalling problem, I discovered a leak
in the bypass valve, replaced that, but still only see 1.6 bar on rare 

The only code that occasionally reappears is the idle switch, sometimes
it takes two or three days for this code to re-appear. I have put a 
meter across the throttle position potentiometer, and watch it run smoothly
from 0.3 to 4.7 volts. I also checked the engine temp sensor and the 
multifunction sensor was replaced by me to fix a "too low" temperature
reading after I replaced the thermostat with no noticeable improvement
in running closer to normal engine temperatures.

I have measured the output of the altimeter, it sits just over 4 volts,
seems about right for the Michigan area. By temporarily putting in a much
stronger spring, the gauge goes to 1.9 almost right away and the boost
shuts me down, so the turbo is just fine. (It spools right up to 1.3 almost
as soon as the rpm gets over 1500, another good indication).

I have a 1.2 watt lamp across the wastegate solenoid, and watch the lamp
glow solid when I floor the loud pedal. Around 1.3 it begins to flicker
and by 1.5 it is pretty much totally off, eliminating all further boost.
So it would appear the CPU feels that for some reason it should limit the
boost to 1.5. BTW the gauge seems right, it reads 0.9 or 1.0 when I first
turn on the ignition, and runs 0.5 to 0.6 at idle, 0.7-0.8 cruising at
"typical freeway speeds".

I am using 93 octane, but I have experimented with lowering it to 87
just to see if the boost control point goes down some more, but it has no
effect. And the dash (added) check engine light never comes on after
the engine starts, and there is no evidence of knocking (even when I 
put in 87 octane).

Any ideas, does this ring a bell with someone???


'91 200q20v, 150k

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