SJ Audi 100 Won't Start (Vacuum Leak?)

Marc Boucher mboucher70 at
Tue Feb 3 21:37:19 EST 2004

Thanks for the analysis SJ.  I did a crude flow test at the return line last
week that passed and thus the focus moved from the fuel pump to the items up

However, possibly as a result of something I did yesterday, there is now NO
flow in the return line.  Previously I could hear the fuel pump running
continuously while energized, with essentially all the fuel being pumped
back to the tank.  As of yesterday, the pump runs until its built up more
pressure than it can pump against (like its clogged) and slows to a point
where you don't hear it.

What did I do yesterday?  I blocked off the flow to/from the DPR.  I wanted
to see if fuel was getting to the return any way other than through the DPR.
However when I removed the temporary blockage, there was still no return
flow !!! Possibly as a result of what I did, or possibly sheer coincidence
(something clogged since last week, fuel pump went bad since last week)
there is now no return flow.  There was last week.  It gushed...I swear !!!

Looks like I've located an Audi owner in my city with a CIS meter and that
will clear up any questions about the fuel pump.  Going on the assumption of
a blockage up front, I hope we can still measure the pressure before the
distributor since there is no flow on the return.

----- Original Message ----- 
From: "SJ" <syljay at>
To: "Marc Boucher" <mboucher70 at>
Cc: <audi at>; <quattro at>
Sent: Tuesday, February 03, 2004 4:59 PM
Subject: Re: SJ Audi 100 Won't Start (Vacuum Leak?)


The sensor plate potentiometer is only used for better driveability during
cold engine operation.

During cold engine operation there is reduced control pressure(more fuel) on
the plunger. This causes an overswing of the air sensor plate, during
cold-engine, part-throttle operation.

The potentiometer provides input to the control unit(ECU). The ECU sends a
compensating output to the DPR. For about one second, pressure actuator
current increases, depending on sensor plate movement and on engine
temperature and rpm. The greatest enrichment is when the engine is cold:
middle enrichment when the engine is cool: no enrichment when the engine is
above 175 F.

I doubt that the potentiometer is your no-start problem.

>From what you have described so far . .symptoms and tests . .I think you
have a fuel delivery problem. Bad pump, or clogged fuel distributor, or some
blockage in-between the pump and the fuel distributor.

An easy overall check for the fuel pump:
1. Disconnet fuel Return line. There is a union coupling located just behind
the fuel filter.
2. Put line into container.
3. Bypass Fuel Pump Relay, activate fuel pump for 30 seconds.
4. Quantity of fuel depends on the voltage at the pump. This voltage should
be about 2 volts less than battery voltage. I would put a voltmeter on the
pump and take a reading with pump actuated.
9 v - 335 ml
10v - 450 ml
11v - 600ml
12v - 760 ml

If you get the quantify of fuel specified, your pump is working ok, and you
have no obstructions in the main fuel delivery system.

The cold start valve is connected to the System Pressure line in the fuel
distributor. It will spray when you have system pressure(any pressure) in
the fuel system and you energise the Cold Start Valve.
You said you passed this test. You can start the car with CSV energized

The main injectors will spray only when system pressure is higher than
injector opening pressure.
This test you fail.
I would suspect a problem with system pressure.

System Pressure = 88.5 - 94 PSI
Injector opening Pressure = 62 - 67 PSI

If your system pressure is below 62 PSI, the cold start valve will work, but
the main injectors will not work.

Two things determine System Pressure;
1. Fuel pump
2. Fuel pressure regulator.

The Fuel Pressure Regulator is located just behind the Fuel Distributor. The
fuel tank return line is connected to one end of the Fuel Pressure
Regulator. The fuel pump works against a spring inside the regulator to
maintain system pressure.

You could have a problem with the regulator. But, the regulator is a simple
device and rarely fails. Of the two possibilities I would bet on the fuel
pump being bad.

- - - - - - - - - - - - - - -
Pressure testing.
If you had a pressure tester, it would be easy to isolate the problem. The
gauge itself is simple. The guage is connected to two places at the same
time. There is a reason for this.

The adapter hoses are not so simple. You need banjo fittings. A male banjo
and banjo bolt to fit the lower chamber Test Port on the fuel distributor. A
female banjo fitting is required for the CSV injector line. Also you need a
shut off valve on the line going to the CSV.

With the valve open, the guage will read System Pressure (88.5 - 94 PSI) . .
recall that the CSV valve operates on System Pressure.

With the valve closed(System Pressure shut off), the guage reads the lower
chamber pressure. This pressure is determined by the Differential Pressure
With a cold engine, or a 15K resistor attached to the temperature sensor
connector(simulate a cold engine), the lower chamber pressure should be 17
to 21 PSI lower than system pressure.

If you disconnect the DPR connector( limp home mode), the Lower chamber
pressure will be 4.3 to 7.3 PSI lower than System Pressure.

Because these low readings are within system pressure variations, the
pressure gauge is connected to the two pressure sources so you can flip back
and forth and determine the correct PSI values.

In conclusion, to get your "no spray" of injector symptom:
1. Fuel pump not producing enough pressure to open injectors or,
2. Fuel pressure regulator defective and not providing the required system
pressure or,
3. Internal blockage in the fuel regulator, or
5. Blocked Differential Pressure Regulator. If you get no fuel flow thru the
DPR, the lower chamber pressure is very low .. . that shuts off fuel flow to
the injectors. This actually happens when you decelerate.

Out of the 5 possibilities, and assuming you dont have the fuel pressure
guage, I would guess at 1 and 5. #1 is the cheapest and easiest to test for
fuel flow. #5 is relatively easy to take apart to check for clogging.


> I unplugged the lead to the potentiometer during cranking. . . . <snip>. .
..but it
> could also be a worn potentiometer?
> Marc

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