[Stocker]

Bump for a 2-month old thread. Have owned my K20 for 41 years, never had a hole in the thermostat, never had a problem. Now there's a problem and I'm gonna drill a hole just in case it helps. Sounds like at the very least, it can't hurt.

Coolant was changed last year for the umpteenth time. Truck is rarely driven anymore but on a trip to town this spring, the heater/defroster stopped blowing hot air, just blew ambient. Temp gauge was normal, and the heater hose going to the core was hot to the touch, but the return hose was cool. First time that's ever happened. Truck sat for maybe an hour, then drove it and all was normal. I was told it might have been an air bubble problem.

The other day I was cutting firewood and the same thing happened. I have a sealed system with a coolant recovery tank and everything appears to be working as it should. I just backflushed the heater core and the water flows through nicely and comes out clean, so no apparent blockage in the heater core.

If the problem was air in the system, I'm hoping the thermostat hole will eliminate it. I'll update this thread in a few days or so with the results.