Ihave an 89 3.9L and I can barely drive 20km without the truck almost overheating. I've changed the thermostat and water pump and tried running without the thermostat and nothing seems to help. Theres no oil in the coolant or leaks. I hope someone can help me!

 

try flushing the cooling system, if that doesn't work, check your fan clutch relay.

 

replace the radiator cap, if your system doesnt hold pressure it will overheat.

 

Ihad the same problem and did just what you did, no luck.My radiator from the engine side looked fine butI have A/C so could not see the front side behind the A/C condenser coil. I took the radiator out and the front side was all but gone, the fins all eaten away. Installed a new one problem fixed.