Questions and Answers
Audi GT Coupe Idle Speed
Q. Hello Mr. Ciulla,
My car:
1984 Audi GT Coupe w/ 1990 Audi 100 engine
2.2 liter engine
5-speed transmission
185,667 miles
Bosch Motronic EFI
No ABS
P/S - yes
No A/C, no C/C
Rack n pinion steering - yesMy question:
Low RPM (often revving between 600 - 700) during cold start. RPM remains between 600 - 700 after a warmed engine. I think the recommended idle speed is between 850 - 900. The mechanic's answer was that the electrical might be supplying too low of a voltage. I asked to have the idle speed tweaked a bit higher to match the recommendation but the mechanic declined the request.Here are some of the recent parts in the car:
* All new fuel injectors (seven months ago)
* New Catalytic converter (seven months ago)
* New Air filter (four months ago)Other people have suggested adding injector cleaner but I doubt the new injectors can become congested in such a short time.
I am ready to raise the RPM on my own. What do you think?
Thanks.
A. The recommend idle speed for that engine is 780 to 820 rpm so you are a little low. If you kept the 1990 system, then the idle speed is supposed to be between 670 to 770 rpm. If this is the case, the idle speed is just about right. Since the idle speed is computer controlled on both systems, there is no provision for adjusting the idle speed.
From what you describe, it sounds like you have a bad Auxiliary Air Valve (AAV) or Idle Speed Control Valve (ISCV). Either of which can cause the symptoms you describe.
The AAV bypasses the throttle plate to give you your cold high idle and as it heats up, a bi-metallic spring slowly closes off the bypass tube bringing the idle speed down.
The idle speed control valve supplies the engine with the proper amount of air, through a bypass around the throttle plate, to maintain the correct idle speed. It takes over the function of the AAV which is no longer needed when the engine is warm. The idle actuator is controlled by the ECU.
Data processed within the ECU to determine idle speed include, engine speed, coolant temp, idle speed signals, drive gear position, and whether the A/C compressor is activated. With the ignition switched off, the idle control valve maintains a set position (limp home mode). With the ignition switched on (and the engine off), the idle valve is activated by a predetermined voltage signal. Once the engine is started, the ECU will vary the voltage to maintain a constant idle speed.
If I had to guess which is the more likely possibility, I would say the AAV. They have been known to go bad on this car often.
Additional Information provided courtesy of ALLDATA
