So I got my 2006 s80 AWD in January and a couple months in I had a misfire, so i changed the plugs from OE volvo to bosch platinum but noticed two of the old plugs had oil on them, from cylinder 3 and 4. After about 5000 miles i checked on the plugs and just the plug in cylinder 4 had oil on it. So i changed the again, this time using denso's but now every time i start the car it does a sudden rumble and then goes away and drives perfectly fine. Anyone ever experience this or know how to fix it? Any help is appreciated.

 

when you say oil its probably unburnt fuel residues unless you are losing oil through the head gasket. That said, you should probably consider replacing the coil packs as it sounds like the problem is weak spark (ie low voltage) not bad plugs.

 

Well the plugs and the packs were replaced at the dealership before i got the car. Could it be the valve cover sealant?