1990 Audi 100 fuel delivery problem

Going crazy here. My Audi 100 will not start after sitting a day or so in damp weather. Cold weather is okay - it seems to be the moisture that's killing me. Weird thing is, the ignition spark is fine, but fuel
is not reaching the cylinders. If I put a heat lamp under the hood for an hour or so, it starts fine. This makes me think it's an electrical problem related to fuel delivery - but not the fuel pump. I wanted to try changing out the Engine Control Unit, but don't know where it is located. Any advice? Thanks!
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My first thought would be that you have a faulty contact in the fuel pump relay.
You might also have a sticky air metering plate. It is located just above the air filter in the intake manifold. Reach in above the air filter and gently push the plate upwards if it is sticky it might not move when cold and that will keep the fuel from flowing to the injectors.
Faulty fuel frequency valve. You may want to look that up elsewhere. I only suspect it but have no personal experience.
Also search on the differential fuel pressure switch. Located on the front of the fuel distributor.
Also be sure you are not loosing vacuum from the decel valve located in the fender beside the air cleaner.
T44s can be fun to find a problem like this but a bit of searching should solve the problem. Test parts before throwing money at the problem.
You may want to search the history on AudiWorld.com T44 forum: http://forums.audiworld.com/v8 /
TonyJ Former four T44s (last one '91 100QM) current '04 A4 1.8TQM
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I have only worked on one Audi 100 for a crank sprocket problem but here are my other experiences..... I have been replacing some faulty ign. coils in 94-98 Jettas. The original ign. coils fail due to moisture allowing the spark to short at the coil and the aftermarket coils fail (quickly too) allowing a weak spark at the plug. I would want to either see the spark your coil is producing or examine the spark plugs to see if they are wet or dry? A wide spark plug electrode gap along with a weak spark might create problems in either cold or moist conditions. AFAIK The spark will change under compression too! Of course the spark plug wires are also affected by moisture!
Have you replaced anything related to the ign. system recently? Engine tune or service? Spark plugs, wires, distributor cap and rotor should get changed periodically. ;-)
Also check in the Bentley for proper testing of the fuel delivery system. I have seen bad or missing fuses cause the fuel not to flow. ;-)
Moisture might affect the wiring terminals at the Coolant Temperature Sensor for the Fuel Injection, the Fuel Pressure Regulator or maybe some ground connections
Does your Audi have blink codes?
later, dave (One out of many daves)
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The '90 100 doesn't have coil packs. Main ign coil and plug wires.
He might want to try to test the cold start injector but routing 12V directly to it while cranking to start.
Tony
dave AKA vwdoc1 wrote:

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WHAT??? Coil packs??? Who mentioned coil packs? Get a rope! lol
I should have indicated that these are NOT multicylinder coil packs nor individual coil packs but are the almost regular ign coils you will find on some of the '94-98 VW 2.0 engines. These ign. coils should be similar to what the 1990 Audi 100 is running. I was trying to say that the spark has to be strong enough to fire the plugs under compression. ;-)
Good point on the cold start injector. Is it powered by the starter or the ECM/ECU? This injector should have 2 wires going to it and you should make sure it also has a ground connection if you are going to "hot" wire it. ;-)
later, dave (One out of many daves)

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I think it is fired by the ECU if the MFTS indicates that the engine is in a cold start condition.

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