capnbob928s4

Hi all and thanks for looking. I have a 88 928, 5 speed with 100k miles. I have replaced the LH control unit, mass airflow sensor, both coils and coil wires, idle stabilizer valve, two knock sensors, reference sensor and throttle position switch. I am still getting idle fluctuations of about 200 RPM. Any suggestions? Also, I am going to be storing the car for a while and bought the Porsche battery maintainer. When I plug it into the cigarette lighter socket and then into the wall, I can hear a pulsing noise and what sounds like electrical motor sounds coming from under the hood. What is going on? Thanks for any help.

worf928

Idle: Fluctuations of 200 rpm about what base idle rpm?

Underhood noise: get a helper. Have them plug in the maintainer while you are standing in front of the open hood. (IOW - localize the noise a bit...)

Steve J.

Since you have eliminated many ignition or electrical possibilities with the new parts, I suspect false air leaking around the throttle body somewhere. Verify proper operation of the new idle stabilizer and the pulse width modulated signal that actuates it. Inspect vacuum lines for cracks or splits. Sorry, no guesses on the electric motor sounds under the hood. Good luck.

Mark

Not sure about the S4 - but on the earlier cars, the Cig lighter circuit is OFF with the key...you may be backfeeding something.

Any of the inexpensive batery mainainers work well...I'd connect directly to the batt or the charging posts up front.

Allanr

I have exactly the same problem on an 87 928 std transmission, and have replaced the idle control valve, throttle position switch(which is properly adjusted with an ohm meter), LH, all vacuum lines, MAS air sensor, O2 sensor.
Engine will occassional kill when rpm is coming down from 3-4,000 rpm. The MAS is a Bosch rebuilt unit. Since the manifold has to be removed to replace the idle control valve and throttle position switch all intake manifold gaskets are new. A preplexing problem, any suggestions?
A. Allbritton

John Speake

If the loop is hgunting around the required rpm, then this indicates the main loop components are OK.

You could try an idel adaptation with a diagnostic tester.....if this doesn't work, then consider the possibility that interaction between the idle system and the O2 loop is part of the problem.

It would be a good idea to do a diagnostic fault code readout.

killav

A couple of months ago, I had a no idle condition after a manifold R&R. I could start the car as long as I pumped the accelerator, but it would not start and idle on it's own. When I unplugged the MAF, it would start and idle perfectly. It turned out that the black rubber hose underneath the manifold that runs from the MAF elbow to the Idle Control Valve was disconnected completely, therefore the computer was not reading any air that should have been moving through the MAF during idle. After finding this, I re-connected this black rubber hose to the "Y" in the MAF elbow and this fixed my problem. (There are two Y's in the MAF elbow, if you are facing the engine, the right Y fitting is the one I am talking about). However, the idle would fluctuate up and down every once in a while if I abrubtly shut the throttle blades, It would hunt up and down about 200 rpm. It turned out that I did not get the clamp tight enough on the hose. It was pretty loose as a matter of fact. I tightened this down and the problem went away.