My 928 ran fine.
I washed the car, and parked it.
I started it once every few days and allowed it to get warm.
One day I started it and it had a hard time starting, but started.
Ran rough, so I gave it some gas, which made it run better.
Allowed the car to warm up.
Reversed up my driveway, she suddenly started running rough again.
Allowed her to warm up, giving a bit of gas to smooth running.
CAT FIRE!! Caught it in time, smelling heat I had the hose ready.
Next day I replaced the cat and started the car. Ran rough again.
So I shelved it for a week, trying to start every day, same result.
Rough running.
Left it a couple of months.
Tried new MAF; new ignition output stages; checked wires and fuses and relays (in th interim the starter relay had failed).
Upon replacing the MAF I found the reason for the problem (not MAF-related). Turns out the plugs were all horribly fouled and wet.

Anyone want to guess what the problem was? It's fixed now, all I did was to replace the plugs. All the info you need, is in this post. I can tell you that I have never seen this before.

 

So where was the water ?

 

Good work, guess I was pretty obvious ... I found water in the throttle body. And on the plugs. I remembered I had washed the engine

 

Well, I'm not a wrench. In fact, mechanically I'm more of a wretch! But I can't resist a brain-teaser. So I'm gonna look stupid here, but I don't mind looking stupid if I learn something.

Sounds like the cat fire was due to unburned fuel getting into the exhaust system and coming into contact with a hot surface.

If the plugs were all fouled and wet (with water?! Or more likely fuel.) then it's not surprising that the fuel wasn't burning properly. Sounds like a way too rich mixture, or a fuel leak...

Did you have one or more cracked plugs?

 

Wait a minute! You didn't tell us you'd washed the engine!

You washed the engine???

 

Inside and out!

 

Heinrich...I too had a simular problem as yours. I'd wash the car and it would run rough for a bit. If I was to park it over a grassy area for more than two days the beast again would run rough. Jim Bailey fixed my problem for me. He sold me a new set of plug wires, now no problem how often I wash the 928 or where it is parked. Sure doesn't take much to foul up one's day, in your case one's plugs...
Good-luck

 

 

Bottom line is you don't drive Sharky enough! Letting the car idle to warm up is not enough.
When I came back from 3 weeks of vacation and the ATF leaked fluid all over the place, I took Sharky around the block to get the fluid level down in the reservoir.
It didn't work!!!
Took it next day on the highway and drove it hard....since then everyhting is fine!
These cars want to be driven....

 

Bigs that would have been waay over-obvious :-) ... but yes, I di and the throttlebody was a swamp!