perfectphoto

My 87 s4 has just started this idling problem. I will start fine and start to run fine. When I put the A/C on and put it in gear (Automatic) it starts to hunt for idle, it surges and most likely will stall. It runs great going down the road, however when I come to a stop sign it sometimes will just die. Any thought as where I should start? It feels like fuel to me.

The Forgotten On

When the AC is activated in a 32V 928 it sends a signal to the IAC valve under the intake to raise the idle. It sounds like there is something wrong in that circuit.

perfectphoto

Is there a way to test it without removing the intake manifold?

GregBBRD

In the '87 to '95 injection system, a surge at idle is almost always an overly rich mixture. The fuel mixture is controlled by the idle micro switch, the temperature sender, and the MAF when the engine is cold. Once the engine runs for 60-90 seconds, the O2 sensor takes control and begins correcting for the relatively "crude" control that the MAF has.

We always start with the O2 sensor and measure the voltage it creates. With the O2 sensor connected, measure the voltage on the "black" wire. It should start out at about .5 volts cold. With-in 90 seconds, it should be fluctuating from .2 to .8 volts, relatively quickly. If it is stuck and doesn' t fluctuate, replace it first. If it still doesn't fluctuate wirh a new sensor, you will need to check the idle microswitch and the temperature sender, before you replace the MAF.

It is common for people to replace a MAF and still have the same problem.....or worse. This has to do with the quality of the rebuilt MAF.

​​I've had extremely poor results with MAFs rebuilt in the states.....the calibration on these MAFs is generally very poor. (It's very rare to find one that is correctly calibrated and I am contantly removing these units and having them re-rebuilt.) If you find out your MAF is the source of the problem, I'd suggest you find a MAF that John Speake (In England) has rebuilt. Louis Ott is John's US distributor and I believe can test them, also.

Hope this helps!

perfectphoto

Thanks for the help, That was very detailed and that is what I need. I have noticed that there is a rumble coming from the underside of the car when it starts up. Like a cat going bad. I'm willing to bet that maybe it has taken out the O2 sensor. So I will start there and see what the O2 sensor tells me and let you know.

Mrmerlin

I dont know the history of this car but,
FWIW the cats dont usually fail on these cars.
Rumbling under the car tells of other issues.

Poor servicing does reduce the chances of the systems working as they should, think deferred maintenance.

That means you might need an intake refresh when your engine is asking for it.
Everything must operate perfectly for the machine to run well.

Rumbles can be bad TT bearings, loose heat shields, or ???.

Since your guessing on whats causing the running issues, and noises.
You would be wise to take the car to a savvy 928 mechanic for a more in depth diagnosis before other troubles crop up.

Gdjsk

Hi, sorry I had to ask as i am going through same issue , did you solve the issue ?
thankyou