John S. Markowitz

I did get my mechanic to run through your tests and everything seemed to check out OK.

I'm about to get a new and experienced person to take a look at everything- sitting around with this unresolved is getting very old.

worf928

I do not routinely replace the ISV during an intake refresh. Why? Because they are stupid expensive and I have yet to see a dead ISV on an 87+. Sticky, dirty? Sure. Dead? No.

So, if the ISV nicely "clacks" as a result of the multimeter test and the idle switch closed is detected on both the EZK and the LH then...

... what is the history of your mass-air sensor? If rebuilt, by whom?

Stalling while coming to a stop is usually an out-of-spec MAS and/or the idle switch not closing. Was the idle switch test done with a heat-soaked engine?

Hilton

Disconnected O2 also causes the hunting (to stall) idle.. once the car switches to open-loop. Can run ok for a min until that happens.

worf928

Well, a disconnected 02 sensor is not a 'normal failure mode.' I didn't think we were going to list the ways that we could !@#$-up a 928 so that it stalls...

Even so, I'm not convinced that *only* disconnecting the 02 sensor on an otherwise near-perfect 928 will cause it to stall when coming to a stop. But, very few 928s are near-perfect.

In combination with something else then, sure.

On an automatic S4 stalling when coming to a stop is more-often than not, in my experience, an out-of-spec MAS as the primary failure mode. If the MAS has been rebuilt by someone reputable, and a known-good O2 sensor is connected, then...
... look for overly-rich near idle: TEMP-II? Busted vacuum diaphragm in a fuel pressure dampener?

... would be the next thing on my list.

worf928

This is also, exactly, the behavior when the idle switch doesn't close.

DJF1

Have exactly the same problem on my 91. We did the WD40 process and its not working either. When cold and warming up no issues with the idle, when warm and at proper operating temp when coming to a stop idle is hunting really low and dies. cant even put it on reverse when backing out as it dies... Otherwise she runs like a dream.

Christopher Zach

I have an ISV that was bad, have opened them up, and have seen exactly how they work. They can test ok with a multimeter and still fail.

A good test is to pull it and use a 9v battery to try and move it with your finger in the end hole, pressing lightly on the valve to provide a bit of load. If it moves smoothly against the pressure it's good in that direction, now try it in the other direction by hooking up to the other coils. If it feels like it's stuttering in one direction, the brushes and/or armature is bad, replace.

Sure it costs money, but it fixed my problem in one shot. I hate wasting time. Also as I said, don't try the $24 Ebay ones, they are cheap, and in my case the plug would not even fit because the metal skirt was that bad.

worf928

John S. Markowitz

Hi Dave
First a disclaimer I should probably have in my signature line- I am highly limited in my knowledge of the mechanics of my car even though I've owned one 928 or another for probably 25 years.. (yes, I've spent a lot at shops!). Anyway- some things I will DIY but highly limited. When I solicit info drom some of you guys I am generally doing what I can handle with it or passing this on to a stumped mechanic. Now back to the car....

* First, on the stalling- Let me say that although the car has stalled I would say that the main issue is generally that it is "threatening" to stall during traffic stops and idling can drop very low but doesn't quite cut out, then bumps back up and seems fine. The other characteristic isinconsistency as it will go along for a period while driving where everything seems fine and normal- at traffic stops etc...then it starts again, goes away, comes back etc.

* I would love the issue not to be something that's going to require the intake to be removed....again this issue did not exist prior to the intake refresh. From reading here I had gotten the impression that the ISV was something I should have definitely replaced during the intake refresh and have been feeling remorseful about not replacing it- my mechanic said he did clean it though so... BTW- I did find them since that seem to be of good quality and not so expensive.
https://www.ebay.com/itm/IDLE-AIR-CO...-/152135500864
To some of your questions...
*On the MAF- I forget who the rebuilder was but it came through Roger over at 928sRUS
*The testing was done with a hot engine -not sure about the term "heat soaked: sorry.
* On the O2 sensor - I'm not sure if this has been re-checked yet

Next week I'm hoping to get the car over to a seasoned Porsche mechanic in a nearby town who comes highly recommended. I am hopeful that a fresh look by an experienced person can put this issue to bed and I can enjoy the car again.

Your input has been greatly appeciated

John

dr bob

Think about the things you worked on, disassembled/reassembled, that would cause the symptom. Most Probable:

-- Idle position switch. You can VERIFY that the switch is working correctly with your DMM by reading the switch contacts at the contacts at the LH controller plug. Since you have a digital dash ('89+) car, you can use the instant MPG display to do this test with no fiddling. Drive the car with instant MPG displayed. When engine RPM's are above about 1500 and throttle is closed, the display should go to 90 MPG. As RPM's drop below 1500 or so, the MPG numbers will fall, and get to "--" when you are stopped. If they don't display 90 MPG at the right time, the switch isn't reading the closed throttle.

-- ISV plug not seated. This is a too-common fault. When setting the manifold, this plug is about the last thing that gets seated, and it must be correctly and completely settled on the ISV. You can use an inspection camera to peek in there and look, plus you can verify with DMM at the LH connector that it's connected.

-- Vacuum Leaks. Test with the fixture described in Dwayne's excellent intake refresh instruction, and a little compressed air. Or buy a smoke machine. Either way, you'll locate any leaks. The Edsel Murphy Repair Guide tells us that the leak will be someplace you can't reach without lifting the manifold again.

-- Look at the Temp-II connections and test the sensors from the LH and EZK controller plugs, using your DMM. This way you get to test the sensor and the connecting wiring, in case it was damaged during the work.