Hello all,

First a confession, this is a picture of a ford ohv v6 head, not my Buick. However, there are so many smart people on this board I thought I would post it here. I have started getting misfires on the #3 cylinder so I pulled the head this weekend and saw that the number 3 exhaust port was black not grey.

What would cause that and how can I fix it?

Thanks!

the color doesnt matter much but did you take compression readings before you took it off? try putting the head level and filling the reliefs with water and see if any of the valves leak

That black cylinder has an excess of carbon build-up. There are a number of things that could cause it including a bad plug, bad plug wire, bad valve or worn rings on that cylinder. Would have been best to run compression and leak down tests before you tore it down, but Justin'* suggestion of checking with water may work.

Thank you for all of the suggestions. The plug wires and plug are new. I am trying the water test now. I just finished pulling the other head. All 5 of the piston heads are black with carbon. The #3 is almost perfectly clean. Does that suggest anything? Thanks again.

Hi Bretton.

Sounds like it has been running rich for a while from the looks of it.

Could be unburnt fuel making it'* way into the exhaust manifold and causing a backfire.
Could even be something as simple as a fuel regulator.

I will throw a new one in as I put it back together.

I'd check the valve guides for excessive wear, then re-seat the valves, and replace the guide seals if the guides are still within spec'*. Kinda late now, but check and see if there are any signs that the head gasket may have been leaking in the area around the problem cylinder, or in the area of the one clean burning cylinder. Kinda odd that only one cylinder was burning as it should. Maybe that one good cylinder had a minor vacuum, or coolant leak?

Also, make sure that the deck of the head, and the block are straight/flat, before putting the head back on.
Just a little friendly advice for future use. If possible, it'* always best to perform a compression test, before removing a cylinder head. The results of a compression test can say a lot about the condition of an engine.

Best of luck.