928autobahndreamer

My 87 928 Auto with 101K miles has been running great for the past year and a half. It has never left me stranded and has started everytime I have asked it to (except once, quick fix of cleaning the ground strap). Over the past couple of weeks it has developed a new problem.

When I start the car, it will no longer hold an idle. It drops to 500 and then dies. I can keep the car going with extra throttle, and once underway it drives fine. Once the car is warmed up it will hold idle fine, but until then I have to keep a little throttle on at stop lights to keep the RPMs around 900.

I am thinking that this may be MAF related. When the engine is cold it is in an open loop and doesn't have the feedback from the O2 sensor to compensate for the gradual drop in voltage from an aging MAF.

I have gotten the testing info from JDSPorsche to try to troubleshoot but will not have a chance to work on the car until this weekend. Anyone have any additional bits of advice for what I should be looking into? Or just let me know if my reasoning is faulty. I plan on testing the MAF and also the Temp II sensor.

Other info, I do not hear any clicking of the injectors with the key in the ignition but the engine off. The fuel filter has been recently changed.

If this is a MAF failure, how much does it generally cost to rehab? Ballpark is fine. I was just looking to replace my aging ignition system and I am due for new rear tires and am just trying to look at my available 928 funds.

Thanks in advance for everyone on this forum. I could not do it without you guys/gals.

JP Rodkey

First thing I'd check is your engine temp sensor. If that checks out, I'd focus on the TPS and ISV.

928autobahndreamer

I did the WD40 spray into the tube leading to the ISV. It does seem to be sticking some as if I rev the engine once warm in park, the revs drop briefly before recovering its usual baseline idle.

no doubt

Idle Air Control valve


(Auxillary Air Regulator)

928autobahndreamer

I guess that is good news. But, it does mean pulling the intake. That's OK. Something I have been meaning to do anyway as I have a vac line that feeds the air pump that has been disconnected that I need to take care of.

Still a little curious why the situation improves when the car warms up. I guess it is possible there is more than one problem.

Any other thoughts on what I should be looking at?

no doubt

It's my understanding that the Idle Air Control valve opens only when the engine is cold. It isn't needed when the motor is warm.

If it fails to open, then you need to press the gas pedal a bit to crack open the throttle a little to allow that extra air to come in when cold.

928autobahndreamer

I see. Thanks a lot for the info. I am going to try some more WD40 as a temp fix and see if I can resolve the symptoms.

Is there a way to rehab the ISV? Or is it time to replace?

no doubt

It's part #2 in this diagram...but also look at the hose #15 that goes to/from that idle air regulator.

A clogged hose...perhaps a $3 part, would cause the same problem as a failed idle speed adjustor (AKA auxillary air regular/idle air control valve).

dprantl

No this is not true. The idle control valve works at idle whether the car is hot or cold. When you have your foot off the gas and the throttle plate is completely shut, the only air that should be getting into the engine is the air provided by the ISV. Of course some air will leak past the throttle plate, but very little. It is the TempII sensor that modifies the mixture differently depending on engine temperature. When the car is cold, the tempII sensor tells the brain to add more fuel and make it run more rich. The first thing I would check is the TempII sensor.

Dan
'91 928GT S/C 475hp/460lb.ft

no doubt

I'm not so sure, Dan.

The "solution" to his problem is cracking open the throttle...but that doesn't make the car run richer per se...what it *does* do is open the throttle body butterfly so that more air comes in.

And that's what I understand the idle air regulator to do, as well.

In contrast, the TempII sensor can't inject more air...it can't crack open the throttle to allow more air in...it can just signal the brain to inject more or less fuel.