Car: 1991 Audi 80. 2.0L, 4 cylinder, KE-Motronic fuel injection.

The car was running fine until it started this behavior suddenly.

Two symptoms, each happens sporadically, sometimes 2 seconds apart, sometimes 2 minutes.

1) vroom at idle,
2) miss, most noticeable under load, but seems to occur at any speed or load

The "vroom" is a momentary increase in RPM from idle by 300-400 rpm. The idle then goes back to normal immediately.

Extensive investigation has already been conducted to no avail.

 

Idle: Cleaning the Idle Stabilizing Valve sometimes helps. I use carb and choke cleaner. The valve inside should turn smoothly and freely. Don’t use any sharp objects as you can damage it. Also check the wiring to the microswitch that engages when the throttle is closed. Do a cylinder balance test, not a leakage test, to see if it is not isolated to one cylinder due to a sticking valve, broken spring, clogged injector, etc.

Under load: More difficult to trouble shoot, but I would start with checking the fuel pressure, sticking air flow sensor (CIS-E), timing. Check the grounds and electrical connections, I have seen too many times good parts replaced with new ones and the derivability issues still not resolved. Also automatic transmission problems can sometimes make you think that you have engine problems.

I doubt that it is a vacuum leak. Small leaks are not noticeable under driving conditions and if it were big enough your car would most likely stall at idle.

Make a test light and do a self-test using the ECU's built in diagnostic. On CIS-E cars I find this self-test rather useless but worth a try.

Remember no mater how complex the machine is it still needs 3 things to work: fuel, air, and a spark!

 

Thanks Offramp,

Some of these I haven't tried yet. Will do so.

Also, using a multimeter have noticed that the voltage output from the potentiometer which is part of the air flow sensor, is 'noisy', indicating wear. Would you know if the ECU can handle fluctuating signals?

The pot is impossible to get at for cleaning or replacing. It's inside a sealed plastic surround. A complete new air flow assembly would be required which is expensive.

 

I once read that the Motronic and the CIS-E3 ECU are adaptive and self compensates for wear and tear unless the part is too far gone. The slight erratic idle is sometimes normal as the engine management tries to compensate for the normal fluctuations (rich, lean, rich, lean,...etc. is normal). Slow drifting or hunting is not normal and might indicate clogged injectors, vacuum leak, bad O2 sensor, etc.

Are you reading the potentiometer while the engine is running? If so you will not get a good reading, at idle the sensor plate actually pulsates. If you ever adjusted the CO screw with a 3mm Hex wrench you will know what I am talking about. You should remove the sensor and test it on a bench. You want to measure the resistance at rest and at full opening. Next you want to check for any open or dead spots by slowly going form rest to open and see if the resistance drops somewhere along the way. Most of the wear will be at the spot where the engine idles. How bad is OK? Don’t really know you would need to check your readings against factory specs.

I have taken airflow sensors apart. If you dig past that impossible to remove sealer you will find four screws. Once you remove them the potentiometer just lifts right out. Believe me there is nothing to clean as it is rather well sealed from the environment and there is no grease to gum up. The potentiometer is rather simple ceramic substrate with a carbon film. The carbon film wears out way before wiper blades do. Don't quote me on it but I believe they are all the same regardless of car make and engine size. Getting one at a junkyard is pointless, as the cars tend to be all high mileage.