Pressure in antifreeze overflow tank

Does your air compressor have coolant circulate thru the compressor head?
This could be the source of pressure in the cooling system.

 

Maybe you just need a new rad cap? How much pressure? Need to do a combustion test.

 

Dropped cylinder liner. Did they measure liner protrusion when head was off? Common. Most don't invest in a good sled gauge to measure.

 

cap off coolant circuit through the air compressor when fault is active. if bubbles stop problem is in compressor if not then its in the engine.

typically if the cooling system takes a while to pressurize and does it more so under load it will be the engine causing it.

dropped liner is common on those engines. it allows combustion gas to bypass the fire ring and leak into the cooling system through the head gasket. if they did not cut counter bores at over haul time then this is likely the issue. you will need to cut counterbores, install shims and reset liner protrusion

 

My truck did that recently, it even bulged the plastic reservoir.
Then I found a gallon of diesel sitting on top of the antifreeze and found the problem. O-rings shot around injector cups.

 

I had an n14 that did that one time.After sitting overnight it would sometimes hydraulic lock and would not turn over.Thoufgt it was a cylinder liner,come to find out no liner problems,put engine back together with new head gaskets and did a pressure test on water tank with oil pan off.the leak was coming fromair compressor.It had a crack in coolant head that would allow air pressure into coolant system while running (causing expansion tank overflow,and high pressure) also when shut down overnight after air pressure bled down excessive coolant pressure would reverse from crack and push coolant up air compressor intake into manifold causing water in engine .This caused hyd.lock

 

This is true...mine ended up being a bad headbgasket