After contributing answers to a few questions I now feel qualified to submit a question! Has anybody had experience of, and more importantly, found a solution for the following problem I have with my 1996 Audi A4 2.6 SE auto (ABC engine).
The problem is as follows. The idle speed is not being controlled properly, but it is an intermittent fault. Sometimes (about 25% of the time) when driving and returning to idle the engine speed dips right down and then bounces back to normal again. Occasionally the car will stall completely, usually in the most dangerous and embarassing situation. With the car in neutral, if the accelerator is blipped slightly, the rev counter will rise, fall back and almost stall, bounce back by itself, and then fall back again and cut out completely. On separate occasions, say once every few weeks, the idle speed will lock too high, bouncing around between approx 1200 and 1500 rpm. The only solution is to turn the engine off for a short while and restart.
This problem has existed since I got the car 2 years ago. It had done 53k miles, it has now done 109k miles. My previous identical A4 2.6 SE auto had exactly the same problem most of the time- from around 80k miles when I got it up to 270k miles when I sold it.
I have tried the following to fix it over the years:
Put on computer at local specialist- no fault codes.
Replaced the idle control valve stepper motor thingy on the inlet manifold
Removed & cleaned the throttle body assembly many times with carb cleaner spray. This seems to give a temporary cure for a few 1000 miles.
Many full services- inc. air filter, plugs etc.
Fuel system / injector cleaner additive in the fuel tank a couple of times.
So- is this an inherent design issue, maybe explaining why the 2.6 engine was dropped in 1998, or is there something I have missed? Thank you in anticipation, Chris A.
|
Chris
Might possibly be the ECU's coolant temp sensor. Checked it's reading in live-data?
|
Could be a few things, but the fact that it 'resets' when you turn the ignition on and off suggests that it not a bad connection.
I would put a 'scope on the idle air control valve drive signal. Look at what the ECU is trying to do. Is it trying to open up the valve when the engine stalls (indicating a fault with valve and TB) or is it actually trying to close off the valve when the engine stalls (indicating a problem with the data its receiving). This would be a good way of 'decomposing' the fault.
|
Great answers so far- thanks guys. Re. the temp sensor, I will probably replace that speculatively then- the cost will presumably be less than the £25 or so to run it up on my specialist's computer. Re. the signal to the idle air control valve (TB= terminal block?) I can see the logic and that will be the next move- the problem being of course the law which states that when it's all connected up to the diagnosics at the garage it will thus perform faultlessly!
Thanks- any more ideas or experience gratefully received.
Chris A.
|
Does the car have dashboard diagnostics?
Idle Air Control Valve should be the first check. Can it be cleaned? Cleaning can clear the fault for a while but never seems to clear the fault completely in my limited experience. Carb cleaning solvent can be a bit aggressive on the seals to the IACV IMHO, I've used petrol (perhaps that's worse!). Check wiring to IACV.
Also check for air leaks.
TB = throttle body?
|
|
Yes, TB is throttle body.
|
Thanks Micky. Don't know if it has dashboard diagnostics but it has been on the diagnositic computer at my local independant but no faults shown. I have cleaned the IACV many times (and the TB)- and as you say it seems to clear the problem for a while and then it returns. So there must be an underlying problem. The whole assembly is coated in oil inside when I pull it apart but the car does not burn any significant amounts of oil between services- i.e. it never needs topping up. The ventilation / breather pipes from the cam covers to the air intake are clear but have a coating of 'mayonaise' inside them. So I think I will try cleaning the TB & IACV again but this time leave the breather pipes disconnected- with the holes at the air intake blocked off (but not the pipes) and see how it goes.
Chris a
|
|
|
