2002 S40 1.9 Turbo 68K, recently started having rough idle, check engine light came on, code at parts store indicated cylinder 3 misfire.
Replaced spark plugs, one was noticeably more dirty than others(the third from the left looking into engine), idle remained rough no improvement. Examined ignition coils and all plug shaft boots, no hairline cracks of any kind.
Replaced air filter and cleaned MAF sensor, no improvement.
Replaced fuel filter, no improvement.
Put Seafoam in a vacuum hose, oil crankcase and fuel tank, idle improved no check engine light but acceleration sluggish. Changed oil after 50 miles with seafoam, idle became rough check eng light back on. Put more seafoam in oil crankcase, idle improved but slight hesitation still detectable and acceleration still sluggish. What is next area to examine/replace to resolve problem? Also, there was no smoke when Seafoam was used, used it the same way as shown at this site http://www.channels.com/episodes/show/9862...aming-The-Volvo
Thanks!

 

If you can, I would try and switch up wires/coil.

This I tried on other cars and it moved the misfire from one cylinder to the next.

From what I see on my 2001 s40, there are 2 coils. It may be that they fire two cylinders at a time (like my old Hyundai) one on the compression cycle (fires the gas) and one on the exhaust cycle (really does nothing).

I think, if there is too much resistance on one wire, all the energy can go to the exhaust plug.

I don't know if you can physically switch the cables on this car, but if you can, it is worth a try.

Just a guess.