I have a 2003 Montero Sport Limited with the 3.5. If the outside heat gets above 80 degrees or so, it starts running hot unless I run the heater. For best results without baking my family too badly, I run the ac fan with the temp set at hot and the interior fan off. As long as I do this, the engine runs cool. As soon as I turn on the ac, the temp needle begins to climb. As soon as I turn on the heat, it falls back down. I have put in (and bled) a new radiator, thermostat, fan clutch, and overflow tank cap. Tomorrow, we have to cross the Mohave and cross northern Arizona to get home and, if I can't fix this problem, it will likely be a miserable trip. Your help will be greatly appreciated.

 

Oh, yeah. No oil in coolant and no coolant in oil.

 

got the right amount/dilution of antifreeze?
Id run the heater on full blow without the ac. Wind front windows down and blast heat up the windscreen.
Take it easy, carry water and top up if needed.
Check engine oil level and viscosity, too.

 

I have 50/50 coolant and water in it. Also, it does better at higher elevations. Weird, but true. We have been up and down mountains from New Mexico to Washington state and it always runs hotter at the lower elevations.

 

thats an air fuel mix thing.
Running lean (hot) at low alt because of denser air.

 

Well, many months later, I replaced the timing belt, tensioner, and water pump and that took care of the problem.