C-15 losing coolant from overflow

The quickest way to see if it is a head gasket or pressurized cooling system is put a pressure tester on the radiator and if it builds up pressure by doing two things. One is run the engine to high idle and let it run at that rpm for about 30 seconds then let it fall to idle, the other thing is that cylinder compression is greatest on acceleration so if you repeatedly accelerate and the engine from idle to max RPM and back to idle and see what the pressure gage shows it will indicate the following. The pressure should never exceed 10 PSI which is what the original cap was you have a possible head gasket issue, cracked cylinder head, or liner protrusion was not correct when it was rebuilt and has blown the head gasket. There is a kit out there that will test for compression gases in the cooling system but the pressure tester is the easiest. You can't isolate the air compressor that easily, but if you have air controlled cab heat valve that may be a cause but it would pressurize the radiator even at idle.

 

ShadeTree answered it about the heater valve.

Someone also said something about the radiator neck being bad and thats a good point. I have no clue how you'd check it.

I got something like a 25lbs radiator cap on mine. If you spew water out with that on you got a problem for sure.

 

I'm gonna say yes if you had compression in the radiator.

 

Why do you have a 25lb radiator cap on that truck?

 

Cause I wanted it on there.....

Actually my old cap was junk and I got tired of mine always spitting out small amounts of coolant. For some reason mines always done it. Put the bigger cap on and it stopped.

Not 100% sure its a 25lb cap, it might be smaller.

 

You have a blowed head gasket, cracked liner, or cracked head. Most of those other things people talk of are just wishful thinking. Get ready to pull the head off. good luck

 

Sounds like head gasket.

Something else to keep i mind is if it has an aftercooler. My Accert does and is in getting replaced as we speak. As soon as the manifold pressure got higher than the coolant pressure it would start pressurizing the coolant system. I just don't know what all has an aftercooler.