Blown head gasket??

What truck? What engine?

 

Could be the air compressor if there's no combustion gases

 

Look for black specks floating around in coolant. If you have them it’s a sure sign of combustion gas. The combustion gas kit has been known to lie on all makes all models

 

Those combustion leak testers are trash IMO. Never had one work yet. Even a little Perkins with a 1" long crack between the valves didn't make it change colour.

I'd rule out the air compressor first.

 

My 12.7 did the same thing. No bubbles or excessive pressure at idle. Increased with RPMs. Coolant would blow out overflow while running to a point, then quit. Meanwhile it was also leaking into oil pan. It will separate and settle on the bottom. Might want to let it sit a little while. drain a bit, see if there’s any coolant in oil.

 

Are you losing coolant?