Question about 1999 Audi A4
Hi, it sounds like your throttle valve is having trouble keeping the idle. First check for a clogged PCV valve, which may not allow sufficient auxiliary flow for the throttle controller to manage the idle. The valve is near the throttle as shown in the pic below. Loosen the clamps, take out the valve, and shake it to make sure it isn't stuck closed. If the PCV valve is OK, the problem may be with the feedback (throttle position circuit) circuit in the throttle valve controller. Testing the throttle valve controller requires special test equipment that your Audi/Volkswagen service center should have on hand. Please let me know if you have more questions, and thanks for using FixYa.
Posted on May 29, 2011
As a car gets over 10 to 12 years old, it tends to like to be warmed up before being driven or anything like that. With age, the engine wears in to running at a normal temperature -- and as metal heats to this temperature it expands a bit. This happens more with age. Be patient with your car and let it heat up to normal running temperature before you do anything (like driving or turning on the AC unit, etc.). When it gets up to normal temp, is it still having the revving issue at idle (without the AC)? If not, then your engine parts have stretched over time -- it happens with all older vehicles -- just give it some time to warm up before you drive it and you will extend the life of your car by quite a bit.
Hopefully, that will resolve the issue of the jumping RPMs at startup ... so lets move on to the AC RPM jump. The AC unit takes a lot of power from the engine (generally, it uses the full HP and torque of one cylinder of your engine). This means that when you have the system turned on, the compressor motor will eat about 1/4 of your car's running power. Due to this fact, smaller engines (like a 1.8 L, 4 cylinder engine) will need to work a lot harder to run the AC than a V6 or a V8. If you listen to your engine closely, is there a click just before the engine starts to increase RPMs when you turn on the AC? This click should be the AC compressor motor turning on to cool the air. Just before the RPMs drop back to normal, there should be a similar clicking sound -- the compressor turning off. If you hear these clicks at the beginning and end of the compressor's run cycle, then your RPM jump is actually normal. You wouldn't notice it while driving because the engine is already running fast enough to run the AC ... but at idle you will notice the difference because the AC unit takes SO much power. The engine should not jump more than 750 to 1000 RPMs to run the AC unit, and should only do so when the compressor is running. In order to save fuel and make your AC unit more efficient, the compressor does not run constantly, so when you turn it on you should have a 15 to 45 second cycle -- click REV .... click idle ... click REV ... click idle.
This rev cycle should not be a problem (and is actually normal for small engines) unless it is not consistent or if it revs way too high. If you still believe that it is a problem, have a mechanic look at the car (especially the AC system) to give you a proper diagnostic (without actually seeing and hearing the car it is difficult to be sure that this is normal ... but I have owned a lot of 1.8L 4 cylinder cars -- mostly Toyotas -- and they ALL do this rev cycle with the AC on at idle). Best of luck!
Posted on May 29, 2011
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Posted on Jan 02, 2017
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Engine stalls cold or RPM
fluctuates at idle up and down or dies near idle.
This problem is commonly caused by a dirty automatic idle speed control valve and throttle valve but always run a OBD2 fault code as well as the cleaning procedure. Buy a can of throttle valve cleaner (do not use carburetor spray cleaner!) from NAPA or Carquest (made by CRC chemicals) and spray it into the air intake while the engine is running, use up about 1/2 the can, engine will try to stall hold the speed up, shut it down and let it soak for 30 minutes, restart and blow out the remaining fluid, shut it down and disconnect the negative battery cable for 5 Min's to reset the base idle control. If the above cleaning doesn't cure your issues you need to have the computer controlled automatic idle speed control system diagnosed and repaired by a dealer or electronic engine control repair shop.
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