245k miles. Pulled off head last night to replace head gasket, temperature sensor, and heater hoses.

Head bolts were tight as the dickens. I broke a 3/8" drive 6-point Snap-On socket on the first one and had to go buy a metric 1/2" drive Craftsman last night to remove the head bolts.

Pistons all have some carbon on them, mostly on exhaust side, around 1/16" thick at the thickest point. Valves are all clean on backside. Gray and powdery looking on head, EXCEPT for the #4 intake valve. It has thick black blistery looking carbon on the head. #4 spark plug looks light gray, just like all the rest. There was perhaps just a bit more carbon on the head of the #4 piston. This car does not smoke at all.

There was some crusty crud blocking some of the water ports, but mostly around Cyl #2 and #3, not around the #4 intake valve, so I don't think it is failing to cool.

I'm thinking about sucking some Sea Foam through it when I start it up to test for leaks before changing the oil today, to help get some of the rest of the carbon off.

Any thoughts what might be going on in that #4 Cylinder?