My 2000 S80 has 168K miles and the engine is a little noisier at low speeds noisier than I'd like. I removed the coil packs and noticed that the coil pack/spark plug well closest to the timing belt is pretty dirty while all the others are shiny clean. The plugs all look like they are all in the same condition. Could this dirty spark plug well be an indication that there is something wrong with that spark plug, coil pack or cylinder? Wouldn't this show up in the onboard diagnostics?

 

To me I think that looks like blow by.
That plug wasn't torqued to spec or the washer on the plug failed in some way. The explosions were able to blow by the plug.

Just get a new one, or if it's time or close to time for a total plug change do them all. Make sure they're torqued to spec. over tightening isn't good either. The car will be fine with a new plug and the noise should disappear.

 

Thanks Don. Makes perfect sense. After a successful day yesterday replacing the radiator I figured I change all the plugs. There was a little dirt around and underneath the spark plug in the well that was dirty so I gently loosened it up with a screwdriver and vacuumed it out. When I started the car with the new plugs there was a pretty substantial knocking noise and when I shut the car off there was air rushing out of a device to the right of the cylinder head. (although other than that the engine seemed to run smoothly). I thought that perhaps one of the new spark plugs was bad or I had missed covering the spark plug with the coil boot, so I put the old plugs in again and the same noise happened. Any idea what could be causing this knocking noise? I can't pinpoint a particular cylinder that is causing it. I worried I'll ruin the engine if I drive it.

 

I take what people say in relation to noise with a grain of salt.
See what I or you think is a knock, tap, click, tick etc.
Lifter tick, rod knock etc and top or bottom of engine? Left or right side?

You've never noticed the noise before ever? My 2001 cavalier as well as my 1999 Volvo S80 T6 both have lifter tick when they're cold. Until oil circulates.
Lifter tick is top side and it won't be noticeable really of which part of the top since you have them across the engine.

It won't hurt it to run a little unless your throwing check engine lights and it's running like hell.
If it's got no codes and it's running fine just warm it up see what happens.
Maybe the lifter tick developed while she was a little noisier and now that the plug is sealing you hear the little noise.
Lifter tick can be ignored i think?
I haven't bothered to care about it on the cavalier which did it as new when cold. Dealer said not to worry and 141,000km later no issues from it.

 

My mechanic pulled the plugs, looked into the holes with his scope and didn't see anything unusual, so he turned the engine over a couple of times with the plugs out and shot some compressed air into the holes. He reinstalled the spark plugs, started the engine and it quieted right down. He thinks a piece of plastic blew out of one of the spark plug holes. This figures because the black plastic corrugated hoses that surround the coil wires had disintegrated into tiny bits over the years and I had recently replaced the hoses. I noticed some small pieces of the plastic were hiding beneath the coil packs when I changed the plugs.
Whew! Thanks for your helpful insights on this forum.

 

Yeah something was causing that compression washer on the spark plug not to seal.
It got quieter so it sealed a little more the second time but not completely.
I guess I should have thought and mentioned to have you clean it out in there.

Glad it's good though.