This problem is not with one of my Fords, it's a 6.5 GMC diesel, and is a general diesel question. Combustion chamber corrosion has caused the inlet valve on one cylinder to blow back into the inlet manifold. It has no head cracks or blown head gasket, and there is some pitting on the piston. Has anyone seen this happen? What! I can't add a photo!

 

Could be a bad head gasket leaked coolant into that cylinder, causing the corrosion. Or the engine ingested water at some point.

How do you know the head gaskets are good? Have you done a leakdown test?

 

The head is off, and the gasket shows no signs of being blown. There were no bubbles in the coolant tank when the engine was running. The coolant had to be constantly entering the cylinder for some time to cause such major pitting. I am going to try to see if the inlet port is cracked.

 

There is a crack in the inlet port. The high pressure/temperature erosion from the small amount of water is extreme, pitting is 1 mm (40 thou) deep. Shame that I am prevented from posting a photo. Needs a new head. Thanks for your reply, flainn.