I have a 1990 D350 1 ton dually, and the ole 360 LA engine is building a mad amount of pressure in the radiator. The engine itself runs super smooth for its age. Temp gauge rises above normal after about 20 mins at idle. She's a throttle body intake engine. I have an aftermarket 3 core radiator, with a high rpm two stage fan, & 30 amp relayed fusible power supply. I bought the truck used, and replaced the thermostat, upper and lower hoses, and a new high flow long nose water pump. When I done the trans, I thoroughly inspected the freeze plugs and they were all ok. I would like some advice on trouble shooting the overheating issue. If I need heads or head gaskets etc, I can manage that. I'd like advice for a starting point. No coolant is mixed in the oil neither, but there are little bubbles inside the radiator at idle.
This is my project truck. So Over time, I will hopefully have it all nailed down.

 

Perform a Combustion check in the radiator first....

See if you have a problem there first.... If thats good we can move on...

 

With the water pump belt off, start the engine and look into the radiator. If you see bubbling or water moving about you have compression air seeping in.