Bought a 2001 with 176,000 miles on it. Inspected timing belt and it looked near original... I've never seen a belt so evenly cracked! So drove it just a little up and down my road for perhaps 20-30 minutes, let it idle for 30 minutes at a time, etc, to see if I'd notice any other problems before doing the timing. It seemed to work fine, other than it didn't heat up well and I assumed the thermostat was either locked open, or the heater core was clogged.

So after setting 8 hours aside to do timing belt/idlers/auto tensioner, crank pulley bolt, thermostat, and water pump...16 hours later (yes straight thru; I temporarily lost my mind, lol) I got that done Sunday. What a B and a Half!!! I will pay the $800 next time and gladly, lol.

Anyway, while installing new thermostat, due to noticing the HVAC heat took forever to warm up, I found no thermostat in it! Just the rubber seal, luckily, so I could fit the new one.
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​​​​​​Now I think I know why there wasn't a thermostat. The previous owner removed it trying to correct the dash temperature gauge reading, I bet. What's weird is that it never overheated without the thermostat in it (it actually read cooler than Montero's do normally, like never passing the bottom 1/4 of the dash gauge), but now after the new thermostat it's reading Hot, goes down a little to about the 2/3 point, goes back up. Sometimes it's in the middle, but it mostly reads too hot, like all the way on the "H", and stays there. I haven't driven it more than 10 minutes after the timing job out of fear of ruining the block or heads.

I had to pull the radiator to do timing (to get impact wrench on crank bolt) and of course 2/3 of the fluid went too, so I could have an air pocket, I guess. So I'm giving that a few days to work itself out to exclude it. Will park it uphill with the cap off, use the bleed screws, etc, to see if that helps.

So has anyone figured out a way to really test if the block is getting too hot? I guess I could maybe install an aftermarket temp gauge and sensing/sending unit, but not sure if that's available to buy for a gen 3 Montero.

What I'd like to do is test the block temp when it reads Hot and see if it's just s false reading, sensor, or dash gauge.

If it's actually not hot, and I can't install an aftermarket temp sensor, I may just leave it be. I'm not into ripping the entire dash apart on a 16 year old SUV....no thanks.

Any troubleshooting advice or repair actions that you've had regarding engine temperature would be appreciated!

 

Bought a $25 laser temp gun. Ran Montero for 45 minutes on ranch, various times of revving to 3,200rpm for 20 seconds at a time.

Coolant manifold (area around thermostat housing) never went past 195 F (mostly stayed around 185). Upper hose maxed at 170. Lower hose at 140. Ran it with heater on and off. Would ya say it's OK? Faulty coolant sensor or gauge?

 

I just took readings on my identical year/model Montero that I know is good. Its coolant manifold area was 172 F; upper hose was 145; and lower hose 120. That's about 20-25 degrees lower difference between the two exact same models (and similar miles 175k) vehicle readings. Do ya think this is acceptable? Or is something up with my newer one that is reading 20-25 degrees higher?

What can 50/50 antifreeze/coolant get to before it's problematic for the vehicle?

 

I think you are dealing with the electrical issue in the instrument cluster. There is a common issue with Monteros where the temperature reading on the instrument cluster shows "High", while the engine actually runs at completely normal temp. There are actually two separate temp sensors - one for the PCM and one for the instrument cluster. The sensors are working correctly, but the gauge on your dashboard is not showing correctly.
Your laser thermometer reading kind of confirms it. You can have the cluster professionally repaired ($$$$) or DIY. Here is one thread dealing with self repairing it:
https://mitsubishiforum.com/forum/mi...p-gauge-47609/

 

BTW Ive had the temp gun, from HF, vary from small to wildly in readings ( same spot test seconds apart). Cant always rely on their accuracy being consistent.