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Pulled a bypass hose and it was full of air and the truck began to run normal. Hopefully is a real fix and it's not a liner or wrinkled head gasket. Washed out radiator from bottom to top

Yes, that was the same thought my mechanic had. The dirty coolant that smells funky is what is strange. I Bypassed the air compressor to check if the gasket was leaking on it and it did not even have the slightest bit of air coming out of the water jacket

Yes, there is one on this engine. This is a 2002 CL 700 Oilfield Vac Truck

Pushing coolant out of the cap and the coolant was very dirty. Would overheat at 60 MPH and then cool down at 30 MPH. Used the NAPA combustion gas kit and it tested positive for exaust gas. So the head gasket was the first thought. I've just discovered the suction from the radiator is cold when at operating temperature, yet the rest of the system is hot The coolant smells strange as well, the short hose from the thermostat housing was also sucking flat at 1200 RPM and going back to round at idle. I changed it to a reinforced hose and that helped the overheating, but not the coolant purge out of the cap

After head replacement truck runs fine for 50 miles then gets hot at 60 MPH and will cool down at 30 MPH immediately. Coolant system pressures up and then will push coolant out the cap at 1200 RPM, but no pressure at idle. My mechanic is stumped on this problem. Any help is appreciated.