Questions and Answers
Audi Running Rough
Q. Hi! I bought a 1990 Audi 100 4-door sedan, with a 5 cylinder automatic, that doesn't want to run. The first thing I noticed was that the fuel pump runs all the time when the key is turned forward to the start position. It will start, but runs very rough when it does and I have to feather the gas pedal to keep it running.
For the most part though, it doesn't want to run. I tried to find a spot to tap into the fuel line to check the pressure but had no luck due to the individual lines coming from the fuel distribution (for lack of a better term) block. I also added another 5 gallons of gas to be sure that it wasn't an incorrect fuel level reading. Any ideas on where to start?
Thanks for any help you can give me,
PhilA. Okay, this is how the fuel pump is supposed to work, the pump operates only when the fuel pump relay is energized. The ECU energizes the fuel pump relay for approximately 5 seconds when the ignition is turned ON. After that, the relay is de-energized and the pump turns "OFF". The ECU then waits for a "cranking" signal before it will energize the relay again. Once the engine is started, the relay will stay energized as long as the ECU is getting an "engine running" signal from the ignition system. If the engine dies, the ECU will turn the relay OFF after 5 seconds so the pump doesn't run when the engine is not running.
To check the fuel pressure disconnect fuel inlet hose from fuel filter and connect the fuel pressure gauge. You'll probably need an adapter to make the connection. Run the fuel pump for about 15 seconds then turn pump "OFF". Wait 10 minutes then observe residual pressure on gauge. The pressure should be 51 psi (3.5 bar) minimum.
Now if the fuel pump is running constantly, you have either a bad fuel pump relay or the ECU is bad. Since there is a running problem as well, I would tend to think the ECU has a problem. I would certainly check the fuel pump relay first, but I think with the pump running and the idle problem, we're looking at a bad ECU.
Additional Information provided courtesy of ALLDATA
