Why did I suspect a head gasket? A couple of reasons, the truck was getting hot then just as quickly when I turned off the engine, it would cool down. I was concerned about a circulation problem so I took the truck to a radiator shop who told me that there was exhaust gases in the coolant "which means head gasket" according to the tech. I have not noticed a water lost problem at all. No smoke if it was burning it, no water in the oil either. No loss of power.

As a follow-up, I did remove the thermostat yesterday and drove the truck for two hours or more. The temperature remained at a level I thought was appropriate for an engine with no thermostat,and coolant seemed to be circulating normally. The radiator and all hoses got warm as expected as well.

So, coolant circulation seems to be there. Additionally, I hand tested the pressure needed to open the thermostat and it appears to function normally.

Thanks for the response.

Ken