Chevy diesel coolant problem

I’ve got a 2001 Chevy 2500 HD with the Duramax Diesel. The coolant resevoir is pressurizing to the point that about an hour after shutdown, it burps out about a cup of coolant unless I manually loosen the cap and let the excess pressure go slowly. It seems to work fine while running hot, I drove 900 miles over 13 hours and it lost no fluid. Then, about an hour after shutdown it burped out the fluid. The local shop has flushed the system, repaired a leaky line, and replaced the cap twice. Same problem…any thoughts?

Bad thermostat first thought. Curious though you are letting air out when you loosen the cap? Head gasket? if you run it from cold with the cap off do you see bubbles in the filler neck once it is warm?

Me and diesels and head gaskets were an item. Hope its not but sure sounds like it. What about the temp guage? Is it steady or spiking from time to time?

Temp guage is steady, I fear it’s a head gasket but that was replaced in October according to records I got from chevy (I bought the truck used). I’m trying to work with Chevy to see if it’s covered by a warranty but the work was done in another state and in the past I’ve not had luck with dealerships honoring warranties from other dealerships.

Chevy said they’d cover the head gasket if it was bad but they said it was good, no leaks. they also found no hydrocarbons in the coolant. They replaced both thermostats ($560) and no change. It still runs fine when hot (holds about 7 psi pressure), then when you shut down and it begins to cool, the pressure rises until it purges fluid through the resevoir overflow. The dealer and another shop are totally puzzled.

7 psi sounds low. I would think 20 psi the norm but willing to be corrected.
Coolant burp?