Truck ran supper till I had to change the water pump at 150.000 mi. After that , it seems to build pressure in cooling sys. and blows antifreeze out the over flow. Sometimes it would take 500 mi. under a load to do it, and other times it would be less. I had it to a GM dealer and they pressure tested the engine, all OK, no leak down. Spent $500.00 there. Still have the problem. Took it to another diesel shop, and they changed the thermastat, and something to do with the turbo valve, another $500.00, and now it's worse. Now it will spue out coolent at an idle. It now seems that when I haul a load, it will blow out some coolent till it gets down to a point,then the low coolent light comes on , and will run ok till I put more coolent in. Then it will blow it all out again. Both shops said that they believe the heads, and gaskits are ok. I now has 187,000 on it. And have used a lot of coolent to say the least. Now what do I do???