My 850 will not "accept" much throttle. It starts fine and idles smooth, and will rev ok if the pedal is pushed very gradually. But, moving the pedal quickly or very much will result in an engine mis-fire with severe stumble. In other words, if I "baby" it from idle, it runs ok. But, if the pedal is mashed, it shudders, shakes and won't rev. No CEL is present now, but it did light for a brief period several days ago. Plugs and O2 sensor have less than 20K miles, rotor and crank-position sensor about 3k miles. Original plug wires (103K miles). Not sure about fuel filter. Any suggestions?

I replaced my plug wires on my 98 R at around 155K miles, just because I happened to have a set on my work bench - the car I was supposed to put them on was totaled. The old plug wires were still "good", just getting old with all of the engine heat from the turbo.

The fuel filter makes sense! Read the other post close to this one where the owner of a new V70 has no fuel filter???

Klaus
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Common sense trumps a PhD

Problem seemed fixed, but I started the car last night (after driving then sitting for 2 hours), and it had a miss @ idle and wouldn't accept much throttle w/o stumbling. After an "Italian tune-up" it seemed much better.
Maybe a fuel injector is leaking?

Feul injector problems are rare, but it is a possibility.

If a high rev tune up helps, it means that the plugs were dirty. If the plugs are fouled with fuel then you could have a vacuum leak or a MAF problem.

Go ahead and inspect the plugs if it starts to stumble at idle again. I hope the plugs say Volvo on them, otherwise it is time for new plugs.

The high rev tune up also lets the valves rotate, which they need to do so they don't get fouled up and sticky. The engines love to run above 2500 rpm, unfortunately most driving in automatic mode is at 1500rpm.

Klaus
--
Common sense trumps a PhD

Drove the can again last night. It dropped a cylinder sporadically all the way to redline. Didn't run bad under light load. Problem seems worse when engine is hot. Will check plugs tomorrow, but I installed them 20k miles ago....

Forgot to mention that the check engine light is on for about a minute when the car is started, but then goes out...

Things happen in cycles around here due to the weather. It could be a failing MAF, your plugs are getting flooded.
Or a very dirty throttle body.

Try to force a code if you have a scanner handy.

Klaus
--
Common sense trumps a PhD

Klaus-

Thanks for you message! I replaced the fuel filter last night, and the problem seems to be fixed. When I removed the outflow hose, only a few drops of gas came out. How long do the plug wires normally last?

Bill