I'm having trouble diagnosing a rough-running / no power condition on my '94 940T. Here are the symptoms: At cold start (actually luke-warm start, 60 degree F ambient) it will start after 10-15 sec cranking but will not idle without throttle manipulation. I can hold it at a steady high idle for several seconds before the rpms drop rapidly to 0. I can overcome the stumble with more throttle manipulation, but the same stumble starts several seconds later. After a couple minutes of this, the idle stability improves somewhat (seems to be a noticeable step in stability, not a gradual improvement) to where I can hold a consistent high idle, but it will still stall with no throttle. Even as the idle stability improves, I still have no power. Under load it will not accept any more than a touch of throttle without bucking and stumbling. The threshold for this may be the point at which the TPS signals opening throttle - that's how little throttle it will accept. If I keep it running in this condition and driving slowly around the block, it will eventually "snap out" of this state and run normally. The takes between 5 minutes and 25 minutes. I have been through this cycle about 5 times now and it seems to be taking progressively longer to get to the strong running point.
After reaching the running point, there are no problems at warm start. The condition returns after the car sits for 5 or 6 hours.
What I've tried: No OBD codes set. I replaced spark plugs and rotor. Plugs were light gray and dry at electrode with no deposits. I've checked the ground straps and all vacuum hoses. Several months ago I had similar symptoms that were apparently solved with a new fuel filter. All sensors were replaced a little over 2 years ago when I bought the car with a blown head gasket.
I guess I could try another fuel filter, but the relationship to the warm-up period with an abrupt change to normal running makes me wonder. Would a bad coolant temp sensor give this behavior? How about fuel pressure regulator? (I don't have tools on hand to check pressure but could work on that if it is likely.) Any other thoughts? -thanks