mazdatweaker
+1y
With the truck idling and the radiator cap off, bubbles at the filler neck is usally a symptom.
Rusty looking coolant is an indicator.
Having to add coolant is an indicator, but that could be a small external leak.
Seeing a grey plug insulator when the others look brown is an indicator that coolant is being burned in a cylinder.
White foggy smoke on startup which doesn't really go away once the engine warms up.
Smell of antifreeze at the tailpipe.
If you put a pressure checker on the cooling system, and start the engine, if the gauge reading pulses, that indicates a cylinder pumping issue. Disconnect and reconnect one plug at a time to test and to isolate the at-fault cylinder.
If the gauge reading jumps immediately to a high pressure, that is an indication of gas pressure pumping into the water jacket.
Easy test. Pull the radiator cap and look at the sealing rubber. If it has gotten hot enough to expand beyond the edge of the metal part it sits against, the truck has run hot.