RED928

88 S4 Auto started surging at idle. RPM runs up to 1400 then back to proper idle about every 2 sec. Besides the MAF what else might cause this?

Thanks,
Steve

FredR

Good chance you have a problem with the ISV [idle stabiliser valve]. They tend to get a bit sticky with age and then when it does not respond the controller demands a bit more and then finally it lets go with a bit of a surge.

Have you had the inlet manifold off at all? If you can remove the isv and take a look inside the shuttle area [the bit that opens and closes with the stepper motor] it may be a bit clogged with whatever gunge forms in there- cleaning can sometimes free things up and then it will run better. Alternatively you can fit a new item- I believe there is an aftermarket piece of kit that is reasonably priced if it is still available.

I very much doubt the MAF will be causing your issue but it always pays to keep an open mind until the problem is solved.

Rgds

Fred

soontobered84

+1 above

Or the other thing that could be happening is the Throttle position switch which controls the ISV could be malfunctioning.

With the engine off, and your head under the hood, move the throttle lever at the quadrant and listen for a tiny click. Then move it back to idle position and listen for the click again. That click is the throttle position sensor microswitch which activates the ISV as the throttle returns to idle. If it doesn't click, the switch could be non-functioning and that would be your problem. That being said, the switch could click and still be non-functional. You would have to use a meter and know for sure.

John Speake

The fact that it is surging shows that the ISV isn't stuck and also that the LH is doing it's best to find a stable idle speed.

Also, the same reasoning means that the throttle switch is OK as the loop is being activated.

I would suggest that the MAF may be the problem, and the O2 sensor is a possible culprit.
First I would try unplugging the LH ECU for a couple of seconds and then reconnect. See if the car runs better (you will need to drive it for a few miles to fully adapt.)

If that doesn't work then unplug the LH ECU then unplug the O2 sensor and leave it unplugged, then reconnect the LH. See how the car runs.

If neither of those work then the MAF could well be the problem, assuming the manifold vacuum is OK.

How many miles on the MAF ?

RED928

Thanks for all the feedback...

The MAF is original to the car, which has about 118K miles on it.

Steve

John Speake

In that case the MAF is a strong suspect !

FredR

John,

The key word here is "sticky". Some 17 years ago my late 90 S4 was showing similar charcteristics to those described. The local agents diagnosed a faulty ISV. At that point I bit the bullet and removed the inlet manifold for the first time and pulled the ISV.

Inspection of the shuttle valve area revealed a grey/black dry powder in there and the shuttle could be made to move but it was most definitely "sticky". Thus when a correction was demanded it would not respond so the controller would increase the correction and then it would jump- or that was my perception. Cleaned out the gunge best I could with cloth and cotton buds, squirted in some carb cleaner for good measure and after a bit of TLC the shuttle moved quite freely. Put the thing back in and today it is still in there but now I have a slight surge at idle that I put down to the thing being tired now that it has about 100k miles under its belt.

That the original MAF is tired and under reads is probably a racing certainty by now - whether it can cause the symptoms described I would defer to your fine judgement. For sure if the OP can find a known good MAF to test/eliminate possibilities all well and good. I also wondered if the wiring at the connector might cause issues given they are typically falling to pieces by now.

To the OP: FYI the ISV shuttle sits at a detente position that gives a relatively stable idle with no a/c when warm and thus if the ISV fails completely you can sort of get by until you fix it but cold starts require throtttle manipulation until things warm up fully. Disconnecting the ISV might give further clues as to what is wrong.

Before changing anything you need to be confident you have found the root cause of the problem.

As a matter of interest, have you had the inlet manifold off previously and inspected the kit under there? Have you had a diagnostic tester on the car at all? Do you know if your knock sensors and Hall trigger are working correctly?

Rgds

Fred

John Speake

Good points, Fred. If the MAF is too badly aged then the ISV loop can struggle.

Sometimes the O2 adaptation gets in a knot and that's why I suggested an LH disconnect.