928S4 Idle/Run Issue Driving Me Nuts!!

[Rod Schneider]

OK, bear with me, this may get a little long....
1988 Porsche 928S4 Automatic, 106,000 miles.

I've had this car a little over two years, and up until this August, it's been a delight to drive. Now, I'm getting a bit frustrated with it.
It's developed an unstable idle and will frequently die when coming to a stop.
It also seems to be running a bit rich from cold start to normal temps, judging by the smell at the exhaust. I sometimes have a bit of a miss under acceleration up to around 3,000 RPM or so.
I've been fighting this battle for a while now, without much success.
To date, this is what I've done, not necessarily in this order...
1, Replaced LH computer with a Roger rebuilt unit (Thanks Roger!)
2, Replaced the Crank Position Sensor (Twice)
3, Replaced the Temp II sensor
4, Replaced the Throttle Position Sensor and verified the Idle and Full Throttle switches.
5, Replaced the Idle Air Valve with a used unit.
6, Replaced the Mass Airflow Sensor with a used unit.
7, Replaced the Spark Plugs.
8, Verified no fuel present at the various pressure regulator vacuum lines.
9, Swapped the Ignition Modules, one at a time, with a spare Module.
10, Verified proper continuity and resistance of both Ignition Coils.
11, Inspected both caps and rotors for cracks or carbon tracking.
12, Verified wiring integrity from the LH and EZK computer plugs to each other, the MAF, TPS, TempII, Idle Stabilizer, Injectors, Ignition Modules.
13, Replaced the "53" relays.
14, Verified proper fuel pressure.
15, Replaced Fuel Filter.
16, Pulled the intake manifold and carefully inspected all the rubber hoses.
17, Measured the resistance and verified operation of the fuel injectors.

The only thing I can't really check is the Hall sensor on the R/H Cam Gear, and the EZK computer itself.

The spark plugs, wires, coils, caps, rotors, and ignition modules were all brand new about 18 months/10,000 miles ago.

I've done a LOT of searching on this forum and have found quite a number of threads concerning running issues, but it seems like no one posts a definitive fix.......

I'm about at the point of taking the car somewhere (Garage 928 most likely), but I'd really like to find the solution without shelling out a bunch more cash.....
__________________

[harborman]

Fuel injectors??

[stepson]

When you replaced the Throttle position sensor, did you adjust it such that as it comes off idle, you can hear it click. And conversely, when the engine comes back to idle, can you again hear it click? The click is what alerts the Idle Control valve to begin working to maintain a constant idle. Does the throttle cable allow the linkage to come back to click the Thottle Position Switch on it's own? It has to be able make the switch click to work properly.

Finally, when you replaced the Idle Control valve, did you plug it in under the intake mainfold?
Don't ask me how I know, but the car will not idle for $hit if the Idle Control Valve is not plugged in.

[George928]

#6, have the used MAFs tested. Send it to Injection Labs.

Injection Labs Inc.

[Rod Schneider]

The idle switch and full throttle switch are adjusted and the operation verified at the LH plug.
And, yes, I did remember to plug in the idle stabilizer!

I am leaning toward the MAF since the car seems to be a bit rich, hot or cold. The O2 sensor is about 6 months old and is a Bosch unit.
The MAF will be going to Kevin at Injection Labs this week. At least I'll know it is good when I get it back.

Thanks to all for the replies! Ahhh, the fun of a 28 year old car........

[Mrmerlin]

why did you install a used idle control valve?
thats not the smartest thing I would suggest.

[MikeB7]

Rod, Did you ship your MAF to Injection Labs? Did you speak with them before shipping it out? Turn around time/costs?

I have mine packed and ready to ship, but was waiting to get a return call/email from them before shipping it.