1988 K1500 Losing Coolant Passes Pressure Test

[Rogue]

I rented a cooling system pressure tester for the week to check my '97 K1500 and it is good, but on my father's '88 K1500 with the 5.0 TBI it's losing coolant. I pressure tested it cold, and it held 15 PSI for more than five minutes no problem...think I left it on there for maybe 10 minutes and it was still holding. Odd thing is that it's still losing coolant over time. Not a ton, but noticeable and the coolant level in the radiator is about four to five inches low and reservoir is about three inches low when cold.

When he had it at the dealership a year or so ago, they said it was leaking coolant at the intake, so does that mean the manifold, or at the water pump intake from the radiator? I've been thinking of hooking up the pressure tester and turning the vehicle on and seeing what pressure it has when warm.

Any thoughts on this? Could it be head gaskets since it's losing coolant somewhere when running but holding pressure when cold? The the thermostat got stuck closed about three years ago and the engine did get a bit warm, not sure how hot though.

As always, always appreciate everyone's thoughts on this. I'd hate to lose the engine on his truck with less than 140,000 miles on the engine (It was replaced in early 90s/late 80s for free by GM due to a defect on the original engine after 10,000 or 20,000 miles.)

 

[Ironhead]

Intake manifold gasket leaks are said to be a problem with these engines, if the cooling system was not maintained and kept filled to the proper level.

I'd also check the crankcase for coolant contamination in the oil, and have a combustion gas test of the rad to see if there's a head gasket issue. And, finally, a good look at the water pump weep hole would be a good idea. If the truck leaks coolant when it's hot, the coolant may evaporate off the hot block, and not leave a puddle on the ground.

 

[Rogue]

Intake manifold gasket leaks are said to be a problem with these engines, if the cooling system was not maintained and kept filled to the proper level.

I'd also check the crankcase for coolant contamination in the oil, and have a combustion gas test of the rad to see if there's a head gasket issue. And, finally, a good look at the water pump weep hole would be a good idea. If the truck leaks coolant when it's hot, the coolant may evaporate off the hot block, and not leave a puddle on the ground.

Perhaps I'm mistaken, but I thought that the intake manifolds were a known/common issue on the Vortecs, not the TBIs?

I'll have to check that other stuff you mention, I have a tester somewhere for head gaskets. I hadn't thought of the water pump, is the weephole on top of the pump or on the bottom of it?