cooling system problem

My best guess is head gasket. I've seen that cause a geyser with the radiator cap off

 

Could the cap be faulty?

 

No a bad cap wouldn't allow the pressure to build

 

I've had a cap on a c10 cause the overflow to drip

 

Please suspect your head gasket last. The rad cap itself is may be faulty and simply cannot hold any pressure, that is the cheapest fix. Second, I would suspect the air valve for the heater, then the air compressor and finally a head gasket. You can pinch off the air supply to the heater valve, or leave your air tanks open and run the engine to see if the problem continues. If you have combustion getting into the coolant, pressure usually does not build up until the engine is hot. To test the air compressor, just disconnect the compressor outlet line and run the engine.

 

My brain is failing me right now. It has been many years since I changed one, but they are air operated and when they leak air the compressed air leaks into the coolant to cause issues like this.

 

The valve is under the bunk on the passenger side. It's right by the heater core. Just flip up you mattress and you'll find it, it's the only airline going to the HVAC box.

 

Get a water bottle that your overflow tube will fit in,you need a couple inches of water in bottle. Start truck,put tube in the bottle so the end is underwater. If you see bubbles,take the line off top of compressor,see if they go away. That will narrow it down some.