Different carbon buildup on heads
 

Pulled my heads to replace turbo/manifold seal (long story).

Anyway, one head has a little carbon buildup (what I thought was normal). The OTHER head is clean. All around the valves, where the spark plug is threaded. Clean.

In one photo you can see the silver valves are blackened a little. In the other photo, they're all clean.

The car has only been run with these heads for less than 30 minutes in total.

What does it mean? Am I leaking coolant in one bank … or burning oil in the other?

When I replaced them the head gaskets were obviously replaced.

EDIT: and new head bolts, of course.

My fear is that my overheating incident last year caused the block to crack, somehow.

The heads themselves have been inspected by a machine shop, and machined when I did all the valve work.