Audi 4000 S Quattro fuel system problem write-up.

Tigran Varosyan tigran at
Tue Dec 3 18:36:51 EST 2002

Bottom line: Problem caused by the Diaphragm Pressure Regulator.

Car: 1985 Audi 4000S Quattro CIS-E Fuel Injection 5 Cyl.

Quick Problem Symptoms:
Car stalled while taking off from a light. When working in idle the car
would run for about a minute. When running with alternating throttle
(revving) car would die within a few seconds. Under load the car would die
almost immediately. The car would need to sit for a few minutes before it
would run again.

Detailed Symptoms:
Fuel sensor air plate was very stiff when the system was pressurized. It
became easy and soft when pressure was released from the fuel distributor.
Taking injectors out and testing flow produced a nice cone shape at idle but
when the air sensor plate was moved the flow sputtered and eventually nearly
cut out even when the plate was returned back to the idle position. The
reason the engine eventually dies is that there is not enough fuel going to
it -- the mixture is too lean. Pressure would have to be released from the
distributor (or be allowed to go down on it's own over the cource of a few
minutes) before the engine would run again.

Fuel delivery outflows the fuel return rate. The plunger was becoming
pressure-locked and stopped fuel delivery.

Steps to check the system:
1 Check system pressure. Pressure should be around 75PSI. If pressure is too
low, this write-up is not your problem.
2 Disconnect fuel return line from the fuel distributor and place it in a
bottle. Perform the same test. If pressure is back to normal, your return
line is clogged. If pressure is still too high (in my case well over
110PSI), Diaphragm Pressure Regulator is at fault. In my case it was still
letting quite a bit of fuel into the return line but obviously was keeping
the pressure much too high.

Part description:
Diaphragm Pressure regulator is located right next to the fuel distributor.
It has 2 lines coming out of the distributor going to it and one bigger line
going out which becomes the fuel return line. It also has a plastic vacuum
hose coming out of it and going into the side of the air box.

Possible problem after replacement:
In my case it seemed that the regulator began to fail before I bought the
car. The previous mechanic must have just raised the fuel ratio to bandaid
the problem. As a result, when I  replaced the part, my car run WAY too
rich. Resolve the problem by adjusting the 3mm allenhead Air ratio screw and
the plunger position nut.

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