Idle problems 90Q (NG engine)

[Huw Powell]

> I'm having serious idle trouble with my 90Q with NG engine.

I've been lazy in responding, but since it isn't fixed, I'll chime in...

> Main problem: It will either idle high, hunting up and down between 1000 and 
> 1500 rpm, or it will idle so low that it almost stalls, sometimes it does 
> stall.
> 
> * I've tried cleaning the ISV, did not help. Cleaned the throttle body too, 
> no difference.
> * I measured the idle switch. When closed, it will stay at 18-20 ohms. Time 
> to replace it?

Well, closed should be zero ohms - so, yes, time to replace, or try to 
clean.

> * If I jump the connector while the idle is hunting, the idle will go down 
> to 5-600 rpm or lower, and the car will stall.

To me that says that 1. the switch is bad and 2a. the car has perhaps 
been mistuned to compensate somewhere along the way.  Or; 2b there is 
another problem exagerating the effect of the ECU not knowing it is 
supposed to be idling.

The "hunting" is probably the result of the OXS loop running without ISV 
compensation (ISV only used when ECU knows its supposed to idle)


> * If I unplug the ISV when it's hunting, it will continue, but a bit slower. 
> If I unplug it while it's low, it makes almost no difference.
> * I've checked all the vacuum hoses and they all seem OK. No cracks and no 
> open ends.
> * I've checked the throttle basic setting.

How?

> * Injectors and injector inserts (the black plastic things) is less than two 
> weeks old. The problem persisted after replacing them.
> * Disconnecting the DPR when idling makes no difference at all.

You said that has changed, didn't you?

> * Engine feels a bit weak and mileage is not very good, about 1litre/10 km 
> with mostly highway driving. It's more than before.
> * The ECU fuses are OK.
> * The charcoal canister valve will sometimes operate, sometimes not operate. 
> IIRC, it operated all the time at idle with warm engine before this problem 
> appeared.
> 
> It looks like the difference between idle hunting and the too low idle 
> depends on the idle switch resistance, but it should not act like this 
> anyway. I'll replace the idle switch, but that alone will only make the car 
> idle at 500 rpm or stall all the time.

Do that, then check the settings at idle.  I bet the DPR current is 
wayyy off.

-- 
Huw Powell

http://www.humanspeakers.com/audi

http://www.humanthoughts.org/