Where do I look next ... rough idle ...
 

Hi, I have a project 2000 986 2.7.
It has a rough idle and very occasionally throws a misfire on cylinder 6.
I have, I think, discounted all the obvious and easy(ish) to discount problems.
- Vacuum Leaks
It had some. AOS replaced, this was leaking. Oil filler pipes (two pieces) cracked/split. Replaced. Smoke tested now and no leaks that I can see, and pressure is held for a short time when the air line is turned off.
- Throttle body cleaned. Was quite clean, but a bit oily due to the AOS failing. Cleaned and reset.
- MAF cleaned (and tried a replacement). I don't think this is an issue and the idle changes when disconnected.
- Sparks and coil packs are new and only just changed by the previous owner.
- Spark tubes changed as I did see some oil leaking.

I can't get the Oxygen Sensor off to inspect or replace them.

I am getting white smoke after a couple of days of not starting the car, this clears within 10-15 seconds, but I would consider it excessive compared with my other 2003 986.

Fuel quality is likely to be non-premium, but I do not think stale.
There is a change in idle if I remove the fuel filler cap.

Unsure if I have an injector issue, but no errors suggesting so.
Will be getting it on the road in a few days and getting some premium fuel and injector cleaner in the tank.

I'm not getting any really worrying noises.
I have not dropped the oil/filter to check for deposits.

I'd appreciate any suggestions on where to look next.
I am considering finding an Indy, or going to Porsche, to see if they can get any more useful diagnostics from the car.

By “non-premium” fuel do you mean lower octane? I’m curious if that fixes your issue.