Mrmerlin

you could also pull both of the coil wires off the coils and caps and see if they are corroded, also inspect the hot post cover and the 14 pin connector above it for corroded pins.
Check the thrust bearing is a must

Paul Bakker

Thanks guys for all the advice.
Today I also got a tip from a Porsche 928 clubmember it could have something to do with the rotary idle actuator.
Seems to be a common problem with cars that didn't run for a while. If lucky some WD40 could be the solution. Prior to my buy of the car it had been standing for quiet a while (1 year 30km).

As a non mechanic I should be able to spray some WD40 into the thing but where do I find this actuator ????

Thanks.
Paul

Landseer

Search the forum for ISV. Idle stabilization valve. Read a few posts and it will outline it for you.

There is also a TPS or throttle postion sensor that has some effect on idle.



Paul, have you checked you flexplate? Grounds? 14 pin connector? Some of us (not sure why) deal with these cars 7 days a week and from your symptoms we are wondering if you've ever addressed these frequent maintenance items. If you were idling high or surging we would have asked about your ISV.

Paul Bakker

I'll start here first.
I was given the article below. It may be of some interest to you.

---
The "idle stabilizer" is a variable valve that controls the air flow into the engine at idle.
Most (maybe all, I don't know) are made by Bosch and are "torque motors" - what looks like an electric motor, but with the armature constrained by a clock spring so it can only rotate about 1/4 rev. It is attached to a rotary valve that controls the air flow. More current means more rotation, hence more air flow and the idle speeds up. Problem is the clearance in the valve is very small and small dirt particles can cause friction. The computer says "speed up" and applies more current, but the motor sticks. Engine slows down more, more current is applied and suddenly it breaks loose, opens too far, computer reduces current, valve sticks open, well you get the picture. Result: idle surge, sometimes enough to stall, always annoying. Others like Hitachi have tried to make this type of thing work, never with any success. Almost all new cars have stepper motors working a valve on a lead screw. Reliable, but more complicated.
---

Sounds exactly like my problem. Have this checked first by sprayin WD40 in it.
Keep you informed!

Thanks.
Paul

Paul Bakker

Hi.
I'm only 2 months in posession of the car so no I didn't check these (yet).
First I'll try the idle stabilizer because the sypthoms I get could have something to do with a stucking valve.

Thanks!

Bill Ball

Sure, could be a stuck stabilizer, although this one sounds very intermittent, and it doesn't quite fit that the car runs fine for a few minutes, then suddenly develops a problem. Could be thrust bearing failure, but I would expect the car to be hard to start (slow crankover) as well when hot. Need to rule out the ignition monitoring system if this car has cats, and that's so simple to do by pulling up the passenger floor carpet piece that is over the CE panel and seeing if one of the LEDs lights up when the problem happens.

taffelman

If the car got problems with the flex plate, wouldn't you notice it while cranking? It seems logical to me that if there is any friction caused by the flex plate it would crank slower than normal? EDIT: Bill just answered that

Paul, in the last test did you notice the engine temps? is the problem related to engine temperature in any way? Take a look at my post regarding fluctuating idle during warm-up phase: https://rennlist.com/forums/928-foru...mup-phase.html could it be a similar problem? or is it also dying after the engine has reached full operating temperature?

Also don't ignore that the problems started after a wash, that might indicate that you have bad connections at the ground spots in the engine compartment. Spray some electronic contact cleaner on the ground spots for the coils, the ignition modules (two boxes on passenger side on the front valance) and on all the contacts and ground spots you can see. Let it dry a few minutes with the hood open and re-do the last test while noticing engine temps. If you get a slight improvement you know you have bad ground spots and should clean them up first.

Paul Bakker

Guys (and girls),
Just removed the airfilter and airflow sensor.
Took a can of WD40 and extended the "pipe" so it could reacht the bottom of the black intake.
With throttle fully open I sprayed a lot of WD40 into the device.
No improvement.
Discovered that after starting the car and doing nothing the car ALWAYS sudenly stops after exact 75 seconds. As if something electronic takes over after this time.
What happens after 75 seconds ????

So when I do nothing the car always goes off 75 seconds after starting. When driving the problem also arises but after these 75 seconds the engine keeps running at idle only unregulary. When put into Neutral the "sawing" noise is bigger then when I let it stay in Drive. The revs then go slightly up/down. Though I get impression it fluctuates less then yesterday. Maybe WD40 did something.

After a 15km drive I once noticed at a stop the engine was doing fine at idle but when I got to the next stop the problem again arise.
In a minute I'll drive the car again, maybe the WD40 takes time to do it's work fully.

Thanks.
Paul

OJ GTS

Have to say if I was getting spurious electrical warnings (particularly from engine sensors) I'd be checking the engine earthing strap and the quality of the battery negative connection to the chassis.

Paul Bakker

Nothing changed although I noticed when approaching a trafficl-light (after 75 seconds of driving) and I give an extra throttle befor stopping (kickdown) the engine seems to be a lot more stable at idle.
Because of the 75 seconds which is always the startingpoint of the problems I don't think it has anything to do with a faulty earth or loose contact.

Maybe the WD40 will do its work in the next days (fingers crossed), we'll see.

Thanks,
Paul

OJ GTS

Another thought, 75 seconds could be the duration of the start up fuelling routine, at which point it switches to normal.

Perhaps it's 'ignoring' a sensor or three while it's warming up, and then as soon as it enters normal running, a dodgy signal is upsetting it.

Paul Bakker

Hi.
I've done this:

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Locate the throttle linkage on the drivers side. Directly below the linkage, you will see a black hose that runs fore to aft along the manifold lower edge. Locate the 'y' fitting in the hose. Disconnect where the 'y' connects to the hose that goes under the manifold (aft end of the manifold). Squirt WD-40 into the hose that leads under the manifold. Re-assemble the fittings and let sit for a few hours.
---

I used compressed air to force the WD40 through the tube. At one moment I clearly heart the engine going from raw idle to perfect idle when I was pumping air into the hose! I thought I had solved the problem but after a few minutes the engine stalled again without warning....

Maybe the WD40 needs to settle first...

SQLGuy

I think you're on the right track here. The car is probably running open-loop during initial cranking, ASE, and a bit of WUE. If it's been warm-started, it's pretty quickly going to go closed-loop. If one of the sensors needed for closed-loop operation, like the oxygen sensor (or the MAP?) is bad, it may be getting over or under fueled at that point.

Paul, it might be interesting to see if, after a bit of low RPM running, a sample spark plug is either bone white, or soot black (normally, they should be a light tobacco tan).

Paul Bakker

OK, i'll keep you informed about status of the spark plugs.

Bill Ball

I'm going to assume now you don't have cats since you still haven't mentioned the ignition monitoring relay LED status when your problem appears at 75 seconds after starting. If no cats, then no relay is present and I can stop mentioning it. Sorry to be a pest.

The consistent delay after start-up could be that or switching over to O2 sensor control, if your car has one.