Overheating at startup, fine afterwards

[TintedRam50]

Problem is when first started up temp gauge goes about 3/4 of guage (just before red area) before thermostat seems to open then after that late opening it maintains good temp (below 1/2 guage). Seems as if a air pocket blocks the thermostat from reading at first until finally gets hot enough to open up late.

Thinking about drilling a small hole in thermostat so the hot coolant can surround it better but don't want to be the unknown problem. Anyone got experience of this before on these trucks?

No smoke, no mixed oil with coolant, everything seems fine otherwise.

Any help on this one? Thanks

[Law Dog]

You might have an air pocket that’s not filling with coolant I had to Jack up an engine at an angle to fix that problem before.

[SubGothius]

Or maybe just drive the front end up on ramps and let it idle there a bit while fully warmed up?

That said, I haven't heard of this (or other workarounds like drilling a bypass hole in the T-stat) being required for our trucks, since the highest point in the cooling system should be the top of the radiator and the hose to the T-stat. Just keeping the rad topped off should sufficient, and my '87 2.6L has run a quart or so low (due to a mystery slow-leak somewhere) without any odd warmup behavior or other adverse issues.

Maybe the T-stat is sticking when cold and needs to warm up a bit to loosen up and open? Or maybe someone replaced it with one spec'd for an opening temp too high?

[TintedRam50]

Pulled the thermostat and it's a 180. Went ahead and a extra one I had had also 180, took off that little jiggle bypass thing and drilled out the hole. But, I drilled it to big cause now it stays cold around 150 maybe 150 according to temp gun (not that accurate I believe). Overall slow cold warmup.
Something odd going on. I've had this problem since parked years ago. During my ownership the water pump, head gasket, radiator all have been replaced but I'm concerned about compression getting into cooling system. But no signs off it. I have a leak down tester for compressor but doubt that would work good on cold engine without turning over motor. Maybe on of the chemical testers at atozone that test for gases? Maybe water pump impeller ate up?