scottK:

I had a similar problem a couple of years ago, on my ex-volvo, a 740. It was giving off black smoke on acceleration and generally choking itself. Turned out to be the fuel pressure regulator. It is at the end of the high pressure fuel rail that distributes fuel to the injectors. But again, it could be the air flow box or even a temp sensor in its last hours causing the problem. The latter often causes the engine rpm to go up and down while idling. If you have time, read the troubleshooting page for the 7xx engines which applies to the 850 as well:

http://brickboard.com/FAQ/700-900/EnginePerformanceSymptoms.htm

In any case, get it checked soon before the rich mixture messes up the catalytic converter.

yt855
1994 v855 5spd, na, 110k