Hi Steve , I just fixed an idle surge problem on my 88. The cause of my problem was the throttle position sensor was on the edge of making and breaking which causes the idle speed solenoid to sometimes work and sometimes not. When the idle speed solenoid is not working the car goes very lean and starts surging. This can be verified at the EZ or LH computer with resistance checks to verify the switch is where it is supposed to be. I don't have my manuals here but if I remember correctly it was pin 5 and 18 on the LH plug. Throttle closed should be less than 10 ohms and anything over 1 degree of movement you should get infinite. If your throttle position sensor is not working (adjusted properly) your idle solenoid will not turn on. I had to take my intake manifold off to get at everything but it was worth it because someone had misadjusted the throttle plate and it wasn't closing all the way and was blocking some high speed bleeder hole. The car idles extremely smooth now and decelerates very smoothly which my car has not done since I have owned it. I hope some of this is a help because for now this is all of my experience in the idle circuit.

Mark Baistrocchi 88 auto


Hey Tim, sounds like cleaning the MAF helped, but just in case the problem returns, at least on an S4, a bad idle switch can cause serious idle surging. Mine worked fine when the engine was cool but when hot, it was very erratic. Since replacing it the idle works great. Checking it is easy with an ohmmeter.

88 S4


Hello Emanuel,
I also have an '87. Your problem sounds similar to what happened to me, but not nearly as drastic.
I'll suggest that you get your fuel pressure checked at the fuel rail port. My fuel pressure regulator was sticking closed, then open, causing overpressure, then falling off to nothing, before slamming the 100 psi gauge to the pin. My idle was acting as you describe; @ 800rpm, then falling off to nothing, then surging up, back and forth, until it'd stall.
Then, hard to start! IF it would even start!!!

There's a lot of info on this in the workshop manual!
'87 Cassisrot S4 auto