Nissan truck overheats sometimes

If the engine is at normal operating temperature, then you can't take the cap off, the system is designed to run at a pressure above atmospheric which raises the boiling point of the coolant above 100 degrees C.

If the system is pressurized and you remove the cap, then some of the coolant will explosively turn to vapor as it comes to equilibrium. This will normally blow a huge amount of wet vapor out of the system as the cap is removed - easily scalding anyone close.

Is the coolant mixture correct? Have you tried flushing the system? Or back-flushing the radiator? Have you tested the thermostat?