Spark plug fouling. Bad coil or cylinder problem?

[Clemens]

A really bad valve stem seal could leak oil and cause the fouling while still delivering good compression, I imagine? You would see the usual puffs of smome after idling or startup, though.

[FireFox31]

I found a local Volvo specialist to look at this issue. They think the piston rings may be stuck, allowing oil to slip by. They used a borescope to look at the cylinder head and saw that it was wet, presumably with oil. They added a top engine cleaner to try and free the stuck rings. Once that's run its course, they will drive and recheck the piston head for oil.

I thought the cylinder rings were attached to the piston, slotting into grooves. When I asked the mechanic, he replied that they "float", allowing for some movement. Is he suggesting that the piston rings are not affixed to either the piston or the cylinder walls? Thus, having them stuck (to the walls, I assume) would allow some oil to sneak by.

[erikv11]

This (stuck or damaged rings) is what misha and trying be were pointing to, a stuck ring on cylinder 5.

The piston rings sit in the groove around the top of the piston, there is no bonding of any kind. Imagine stretching the piston ring open where it is cut, and snapping it into place on the piston. That leaves plenty of float possibility. The combination of the piston's groove and the surrounding cylinder walls keeps the ring in place during operation, yet allow for float. The ring's job is to fill the tiny gap between the piston and the cylinder, and keep the piston centered as it travels up and down. You don't want the meaty piston head contacting the cylinder wall, imagine all of the friction! So a small amount of oil (from the bottom, by design, after all you want to lubricate the piston ring as it travels up and down) and combustion gases (from the top) will always be getting by the ring in use, but it should be a very small amount.

One thing about all of this that doesn't fit well with the stuck ring theory is that the wet cylinder has great compression numbers. Usually they would be down. I would be inclined to keep in mind Clemens' post about valve stem seals - oil can also get in from the top, and it is a known weak spot on these engines when the mileage gets high whereas piston rings are not a weak spot.

If the top end cleaner doesn't help, a better bet to free up stuck rings is something like: pull the spark plug on the offending cylinder and pour something in there, like SeaFoam or better yet 1:1 MMO:ATF or 1:1:1 MMO:ATF:acetone (many different favorite cocktails here), pour enough to car the while piton with an inch or two, let it sit until that drains through, crank by hand to ensure all drained and turning smoothly, then change the oil and drive it. A bit more aggressive than just a top end cleaner.

[FireFox31]

Thanks for the additional input. The shop called today and said a stuck fuel injector is to blame. Simply replacing that cylinder's injector will resolve the problem.

I'm not sure how they determined this. They said that immediately after driving, removing the spark plug revealed that the top of the piston was wet and it shouldn't be. If it's not wet with oil, perhaps it's wet with gas. And perhaps that gas sat on the ring after every drive, gumming it up. After all, the spark plug turns black, but who's to say if it's oil or gas.

A bad injector seems to agree with the slight misfires I felt at idle, under light acceleration, and under hard acceleration. When the new injector installed, I will report back on the misfires, oil consumption, and plug fowling in the long term. Thanks for all of your input!

[erikv11]

Thanks for posting the solution!

[volare]

The Falcon sprung a misfire after running fine all year and then not driven for about a month, now when started it's running "something is very wrong" rough, to the point it's not drivable. Codes only indicate cylinder 5 misfire. Haven't poked at it yet; thanks for this thread, do please let us know how the repair went!

[wizechatmgr]

The Falcon sprung a misfire after running fine all year and then not driven for about a month, now when started it's running "something is very wrong" rough, to the point it's not drivable. Codes only indicate cylinder 5 misfire. Haven't poked at it yet; thanks for this thread, do please let us know how the repair went!

Swap coil for #5 with coil of #4 and see if it follows. If it does, then it is likely the coil. If it doesn't, then it is likely something else.

[RickHaleParker]

After all, the spark plug turns black, but who's to say if it's oil or gas

Black and wet = Oil
Black and dry = Carbon from incomplete combustion. There is a limited amount oxygen in the cylinder, too much fuel would result in incomplete combustion.

[erikv11]

The Falcon sprung a misfire after running fine all year and then not driven for about a month, now when started it's running "something is very wrong" rough, to the point it's not drivable. Codes only indicate cylinder 5 misfire. Haven't poked at it yet; thanks for this thread, do please let us know how the repair went!

Swap coil for #5 with coil of #4 and see if it follows. If it does, then it is likely the coil. If it doesn't, then it is likely something else.

Next up, in whatever order is easiest for you, would be
- check compression (this is definitive if it gives you a very low value)
- similarly, try swapping injector #5 with injector #5
- check the coil pack lead on #5 (I would probably do this last)