QuestionQUESTION: So, here's a hard question for you. My '97 A4 recently began to have issues starting after being driven for some time (after coolant reaches operating temp). The engine cranks, catches, but then fails to reach a stable idle and shuts off. After a signficant period of time, at least an hour or so, the car will restart fine. While running, idle is erratic (varies by about 50-75 rpm) and low(~800 rpm after cold-start). Now, for the complications. Whatever the problem is throws the code for the idle stablization valve. However, after replacing that, the problem has failed to disappear. O2 sensors have also been replaced to no effect.
Interestingly when the car fails to start, if one removes the cap from the coolant tank under the hood and then tries to start the car with the cap off, the car runs fine, albeit with the aforementioned slightly lower idle speed. Any thoughts would be apperciated, and thanks in advance.
ANSWER: Wow, One thing I would like to know is how you found out it starts better with the coolant cap removed? This is wild...! If the code shows idle stabilization valve, then this means an issue with the circuit, so even though you have replaced the unit, it's possible there is corrosion in the plug or a problem with one of the wires to it. So check the connections first. I am trying to think of why removing the coolant pressure would affect any of this, but cannot. Is there any bubbling in the coolant tank when you remove the cap, or white smoke coming out of the exhaust? If so there could be a small head gasket problem or cylinder head crack which is affecting things when the engine is hot. Let me know, Jan
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QUESTION: No bubbling when I remove the cap under pressure, just the hiss of escaping gas. A modicum of white smoke comes out of the exhaust while driving, but no more than one would expect, and it doesn't seem to precipitate or follow the issue. Also, new symptoms. It now sometimes stalls while idling for extended periods, is grossly underpowered and when it fails to start, a gurgling noise comes from the rear of the car. This was present before, but you never know, it may be significant.
ANSWER: It sounds like you may have a problem with the rear fuel pump or the fuel pickup. Gurgling means it's grabbing air, and air does not work well in injection systems... Is there a chance there is a foreign object in the fuel tank? Let me know, Jan
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QUESTION: No, I don't think that's possible, no. However, shouldn't a fuel pressure test find any flaws that could introduce air into the system? If so, I'll add that to the list of things my mechanic needs to check when I take the car in on Wednesday. This is in addition to the idle air control valve and the coolant sensors/surrounding system. Aside from that, can you think of any other system that would result in a failure to idle when lukewarm and/or intermittent stalling at idle (only twice in two weeks for this). Thanks for your help by the way, you're saving me the scads of money my dealer would charge to work through all of this :).
AnswerI would certainly do a fuel pressure test first, and instead of diving straight into other components I would read out the DTC codes again to find if there is something else that the computer has identified for us :-) I would also check the engine coolant temp sensor ECT (high failure item) which could give the computer wrong information, giving the wrong mixture and possibly contribute to poor idling. Does the engine temperature gauge seem to work properly? let me know, Jan
