what mods have you done to your truck?

 

I'm guessing that you have a blown head gasket. You can have a chemical block test done to find out if you have carbon monoxide in your cooling system, that will tell you for sure. Another way you could sort of check is this: Start your engine cold. If the upper radiator hose is hard (pressure in the cooling system) in the first few minutes of just idling, you likely have a blown head gasket(or two).

Then, if you want to take it one step further, remove the inner fender liners, remove the glow plugs, and put shop air into each cylinder(one at a time). Watch for bubbles in your coolant overflow jug. That will help you determine which bank.

I recommend that if you do one, you do them both. The last one I had apart only tested bad for cylinder #5(pressurizing the cylinder-bubbles....), but when torn down, there were 3 cylinders that had damaged fire rings on the head gaskets, and both banks were affected.

 

^^^ my truck is currently torn apart and im waiting on studs and a gasket to show up

i found out my HG was blown because i was pissing coolant out my overflow like it was going out of style.

 

We just ended up staking and JBweld'ing in the freeze plugs.. No HG leaks, just lots of coolant flow/pressure which an under-drive or electric pump would have also solved...