It started on mine of dripping fluid, I fixed all the drips.. (New hoses, replaced the big metal one that was always 'wet' under the vehicle as well.. Then as I drove it, got more drips on the top where the new hoses sealed to the front of the engine.. Then got that resolved.. and then I was driving and the radiator boiled over (pressure cap let go, boiled out most of the fluid...). Couldn't find anything wrong.. (exhaust was NOT white in any way for any of that!). Then refilled the tank, it happened on more time (just sitting in the driveway while I was diagnosing things... and suddenly I started getting white in the exhaust (and the radiator cap kept releasing). So I knew I had a head gasket or head problem (or block cracked).
Brought it into a shop, since winter was just starting I had to rely on someone else. Anyway, they got it apart confirmed that there was a tear in head gasket on one side.. BUT they also found a crack in one of the valve seats on both heads, but the block was good.. cylinder walls looked good, no sign of damage. (The cylinder heads themselves had some marks on them, looks like parts of a glow plug went though it while it was running, but no damage to rings or cylinder walls, that we could tell.). So 2 new aftermarket heads and head gasket.. and she is sealed up. (But the shop I took it to, I'm a "backup job". When they have down time, they're working on the HMMWV. It's still not done, almost all assembled, once they finish they're going to drive it for a few days to verify it's all holding together properly.
So it's POSSIBLE the crack in the exhaust valve (on both heads) was part of the problem, but there was definitely a bad spot in one of the gaskets. The belief is that is was 'self-sealing' when cold, and more or less acting as a one-way valve for a long time letting small amounts of exhast into the coolant, but eventually failed.
