The head gasket is probably blown out. It would need a set of them, so pulling off the heads would be the best place to start. You can inspect the psitons and rings as well as the heads for signs of cracks and check for warped heads with a straigt edge.
You can still end up with cracks or head warpage you cannot see that will cause problems, so you must take it to a machine shop to shceck these items.

Answered by DrCranknWrench (expert)
3,292 answers provided