Rodrv6

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.....

voskian

Rod, looks like you have tackled most of the usual suspects in trying to find the problem. I had a hot idle/stall issue I went thru similar testing and mine cleared up finally with a rebuilt MAF from Rich Andrade. He tested my old one when it was returned as a core and for sure was out of spec.

Can't be much more help.

GlenL

I've got three suggestions:

1) Replace all fuel injectors or have them cleaned. (Suggest: http://www.witchhunter.com/)

2) Look again at the Idle Stabilizer Valve. "good used" isn't certain with an electromechanical part that gets filthy.

3) Something that you're sure is right is wrong.

FredR

I would say your data suggests the ISV. The words "good" and "used" when it comes to the ISV is a bit of an oxymoron unless you have trustworthy knowledge of where it came from and how much service it has seen and even then it is a crap shoot.

As Glen says if you have not had your injectors tested/cleaned and do not know if or when they were done, it is another maintenance item that should be considered mandatory- doubt it will help your situation but if they were in a really chronic condition..? For sure a few years into my original ownership I felt the car was down on power- turned out on averge the injectors were under flowing on average about 5% or 15 bhp on top end. Cleaning did not do much for the idle but it sure livened up the plot some.

Also agree with Bob that you should know for sure that your MAF is on song. Have it tested or try a known good one to eliminate possibilities.

Rgds

Fred

jcorenman

Rod,

I don't see the O2 sensor on your list, that would on the top of mine. They don't last forever (pretty much done by 60K). What you want is the Bosch 13048 Oxygen Sensor, comes with the proper connector and available from your friendly 928 vendors as well as Amazon and maybe the local FLAPS.

This is the reference that the LH uses for proper fueling, and as the O2-sensor ages then the mixture gets messed up. One symptom is that the car runs better, for a few minutes, after the battery is disconnected for a minute (which resets the LH fuel adaptation).

My #2 on my list would be the MAF, those also age-- the "hot wire" erodes over time which throws the calibration off. Replacing it with another used one is not a good test-- it was probably sitting on someone's bench for a reason. Unless you have some history on either unit I would go ahead and replace it with a rebuilt unit. Louie Ott handles JDS rebuilt MAFs, and I think Roger also has some available as well as Rich Andrade.

Kevin in Atlanta

I have a rebuilt MAF on the shelf if you want to swap it for testing. I would also suggest shooting some WD 40 into the hose leaving the driver side 'Y' that goes under the manifold to the ISV to see if that helps with the ide.

davek9

What all others have said, only check the Temp II wires for corrosion as you have replaced the sensor.
Clean the EZK and LH connectors too. As Jim pointed out a new O2 is a must, they do go bad w/ time.
Get your MAF rebuilt and have the fuel injectors cleaned and tested or just replace them w/ new ones.

Maybe it's time to get an AFR gauge on the exhaust and see what it's at when cold, then Hot Idle and see what you actually have?
Cold ~ 13.6
Hot ~ 14.7

Dave

Kevin in Atlanta

I also have a AFR with LEDs that I used on my ex-86.5 back in the misty dim.

Rodrv6

Thanks for the ideas! I did replace the O2 sensor about 6 months ago with a Bosch unit. I've made sure the throttle valve switches are working properly.
I checked all the plugs on the LH harness for corrosion and continuity.
The Idle Stabilizer I put in checked great on the bench-no sticking and it looks pretty clean. The one I took out was dirty, but also seemed to operate OK with a bench check.
I'm leaning more and more to the MAF since the car seems to be a bit rich, cold or warm. I'll send it to Kevin at Injection Labs this week and let him test it and fix it if needed. At least I'll have a known good one as a baseline.
I have checked the injectors for resistance and function using an injector test box, and verified they are all firing when the engine is running.
Thanks again for the responses!

GT6ixer

Just because the injectors are firing doesn't mean they aren't also clogged. I suggest you look at Witchhunter.com and read up on injectors if you haven't yet. Very informative info over there. And if you decide to go with them, they are quick, thorough and affordable. And if it turns out the injectors weren't the problem you end up with better parts than before unlike replacing parts that weren't broken in the first place with new parts that perform exactly the same.