The engine will start and run fine when it is cold but after it reaches operating temp it will idle down to 500 rpm and start dipping below that then die when stopped at redlight. Then most of the time it will not restart. I changed all of the vacuum hoses because one had a leak. I also found that the vacuum hose to the idle diaphram mechanizm does not have any vacuum pulling on it. I rigged up a vacuum hose to it and created vacuum by mouth and it increased the idle significantly. I am wondering if the main vacuum canister is funtioning properly. Where does the main can get its vacuum from? Is it possible to re-rig the vacuum source from another spot directly to the idle gizmo on the throttle linkage. After the car cools down it will start most of the time but not all the time. You may have to wait a day or two before it will start again. I just got it inspected and the only thing that showed up as faulty was the O2 sensor. But it did pass inspection.
Engine will not idle after operating temperature has been reached. on 1998 Audi A6 Quattro
by Marklgaines in Spring, TX on June 02, 2010
1 answer 1 comment
ANSWER by dandd , June 04, 2010
A defective Oxygen Sensor can cause the car to not idle or even run correctly if it is over reporting. If the Oxygen Sensor is telling the Engine that too much fuel is going into the engine, the the engine and can/ will shut off the fuel to the engine, so don't dismiss a bad oxygen sensor. As for the vacuum issue, check the engine codes to see if there are any intake manifold runner control codes, because those too can act like a huge vacuum leak and will cause the engine to run poorly when it warms up . When the engine is cold, it runs very much richer and ignores the oxygen sensor until it gets up to at least 160 degrees, so an engine with a bad vacuum leak, a bad Mass Air Flow Sensor or a bad Oxygen Sensor can run pretty well when it is cold, because the richer cold running mixture covers up a multitude of sins.
COMMENT by Visitor , June 06, 2010
Thanks man but I have put the car into the shop and they said it was the mass air flow sensor, cracked thottle body, and ignition coil along with plug wires and plugs. The mechanic called back yesterday and said that he was putting off ordering the throttle body and was going to check the coil and wires plus MAF first. The total bill is going to be around $1000 bucks. I sure wish I had my old shop back where I could fix the thing myself. I hope it is not the O2 sensors. I hear that there are 4 of them on this car at $200 bucks a piece. I have not been able to crawl under it to see yet.