03 Montero Limited, 190k, overheats only at idle after highway driving
 

Hello...I took this rig on a 4,000 mile road trip from CO to TX to FL and back and while in TX it was 105 F in the day time and I noticed the temperature gauge, which always stayed at halfway or under, started creeping up toward the "H" while in a fast food line. I then noticed after any highway driving it would do this if I was idling for more than 1-2 minutes. It comes back down to the usual halfway mark when I start driving it past 20 MPH or so for a couple of minutes. I also noticed that when I turn off the A/C and place the Heat on inside it does seem to help if I'm trapped in a idle spot (like freeway traffic jams). I had to do that once in TX...it was 108 F on the road and we got locked into some traffic for 10 minutes or so. Turning on the heat full blast seemed to keep the temp from reaching the upper 90% of the gauge.

It only seems to do this AFTER it's very nice and hot and I've been cruising on the highway for many miles.

I drove this Montero for 8-9 months around town and short (30-50 mile) highway trips in CO but this was the first long road trip to a hotter climate.

Does the electric fan on the front of these actual cool the radiator or is it just for the transmission cooler? It is mounted in front of the tranny cooler. If this fan is coming on at idle when it's hot, might it be a dirty radiator fin problem?

I'm going to check the hoses again when hot and if I'm not getting a swelling or collapsing condition as it overheats I was going to:

1. pressure wash the radiator and tranny cooler the best I could and see if that helped.

2. If that doesn't work I was going to yank the radiator, rinse and drain it out, and then fill it up with 100% white vinegar for a good long 3-day soak before rinsing, reinstalling, and refilling it with 50/50 coolant/water.

Does anyone have any additional suggestions? The thermostat is new and I drained/flushed the coolant when I got it. There aren't any leaks or coolant loss. Thanks!

the fan mounted on the front of transmission cooler will push air through the other radiator as well. Make sue that fan turns on. If not, you may have a bad temperature switch.
With the conditions you describe, I'd suspect that you have ether radiator not functioning at full capacity, or a water pump that is failing and not able to pump coolant fast enough through the system. Almost seems like radiator is #1 suspect. Your plan to clean/flush it is a good one. Before you do that, you can put a cooling system cleaner in and drive with it for some time to make sure deposits get dissolved all around an not only in the radiator. After that you can try to do an additional cleaning on the radiator and flush the system.

Problem was the AC condenserfan fuse (25amp) was blown. I'm hoping just because of the south TX heat (108 F) and the long road trip. I'll be watching it sense I'm back to CO now with cooler climate and less highway usage.

The electric fan is a little nosier on this one when it comes on (I cycled it several times) compared to my other 03. Which may be why it blew a fise...might be going bad (fan bearings?).

I thought it was for transmission but actually is for AC. It only comes on when the AC is turned on. On this one and my other 03. This is why I like to have 2 of same year, I can compare! Lol

Does the vehicle actually get hot when the temp gauge reads "H"? Reason I ask is these models are known for faulty temp gauge in the cluster. Mine goes all the way to "H" and back to normal all the time. There is a resistor on the circuit board that needs to be re-soldered, that I have not attempted yet.