1995 850 NA....188,000 miles:
Changed the plugs, cap, rotor, filter, wires about 300 miles ago. All Volvo parts. Now, I get a slight miss upon acceleration...a hiccup, if you will. Very slow acceleration from idle gets this miss, burp. It's freezing here in N.Y.. My fuel milage has dropped from 21 to 15. The crankcase venting system was changed this past summer. I do get oil going into the flame trap, as I can put my hand under the unit and it is wet...so I know that oil is going into the throttle (if that helps in any diagnosis).
Could I have used the wrong plug? Sorry that I don't know the exact model #, but it's the multi prong ones you get in the Volvo pack from Groton.
This car has never, ever, run uneven. Always smooth and responsive, so this is why it concerns me.

 

What is the recommended fuel octane in the owners manual? What are you running in it? Have you used fuel additives like Lucas? Could be a number of things.

My guess in order would be:
1. Low fuel pressure. Failing fuel pump.
2. Vaccum line loose somewhere.
3. Bad MAF.
4. Worn motor, have a compression test done or do one yourself, pretty easy.
http://www.atthetipwebs.com/technolo...ips_repair.htm

 

Can you correlate this change in the way it runs to when you changed all those parts? If so, possible you did something incorrectly; is that possible? Rotor or cap loose maybe?

If this started some time after your tune up, then it could be something else. Any CEL codes set? If your mileage dropped that much i would expect a CEL.

 

I had this problem with a 94 850 - the vacuum elbow to the fuel regulator was cracked - it is under the regulator whick is covered by the rail guard - I did not find it until I saw injectors leaking and had to replace the injector o rings

 

How abrupt is the miss? Could it be a misfire with the plugs? What gap did you set them to?