So after my montero (2002) limited full size is at normal operating temp, it will start to overheat or so it says on the temp gauge in the gauge cluster. However, only if im sitting stopped/idle or in bumper to bumper traffic. It will peg to the top of the gauge, but never see any smoke etc. The fans do turn on automatically at this point so I know they are working.

My remedy which I've had to do regularly because I dont want to test the situation is this:
- if I'm in traffic or stopped and its doing the overheating temp gauge, I simply put it in neutral and hold the revs at 3K and the temperature will go back to normal
- once I start driving, things normalize again.

But only for it to happen again if I stay stopped or idle.

No leaks or anything. Any idea what this could be? If I'm driving it doesnt bother me because I can always counter against this situation which has been happening for 2 years, BUT sometimes I lend the vehicle to my wife and I feel she may forget to do the trick mentioned above. I want to fix it once and for all.

 

You need to figure out if this "overheating" is a true temperature event, or just an instrument cluster glitch. There is a known issue with some contacts on the instrument pannel going bad and the temp gauge showing High temp while engine is actually running at normal temperature. There are several independent coolant temp sensors on these vehicles. One is sending data to the instruments, the other to PCM. You can get (borrow) a scanner capable of monitoring live data stream. With that plugged into OBD II port, select to display engine temp. Drive around and try to reproduce the condition you observe. If the gauge on the instruments climbs to high, while data stream show normal temp - you know you got instrument cluster issue. If data on both shows high - you have other problems. In the second scenario, most likely culprits are (in no particular order): 1. Water pump going bad, 2. Clogged up radiator core, 3. Air pocket in the engine, 4. Restricted coolant passages in the engine (corrosion, deposits, etc.)

 

thank you! i will try to find someone with a reader who can stream real time temp from the sensor. that seems the easiest to confirm

 

In have a third gen like yours and experienced the exact same issue, after a lot of work trying to figure the cause it turned out the overflow tube (aka "puke tube") from the radiator to the overflow bottle was clogged! After removing the clog and cleaning the tube, the gauge has always read correctly.
The reason is such: at idle your radiator wants to expell the excess coolant because of the heat expansion from sitting still, if the overflow tube is clogged and the coolant is trapped, excess heat will be stored in the top end of the engine (where the guage sensor is located) and until you rev and circulate the coolant the guage will read hot....even though the vehicle is no actually overheating. if your upper rad hose looks collapsed when the engine has completely cooled down then that is another telltale sign your overflòw tube is cogged.
Good luck

 

Thank you! I still have the problem to this day and your post makes sense. I will check on it being clogged

 

The same condition - collapsed radiator hose when engine cools off - can occur when radiator cap goes bad. If that happens, get OEM cap. Stuff from the parts stores does not fit, even though they supposedly "match" it based on VIN.
I do not know how much coolant moves from radiator to overflow tank when you idle. I seriously doubt it is much (if any). Most of that expansion happens when engine initially goes from cold to warm. Them when you shut it off, and engine cools off, coolant flows the other way (into radiator).
Most likely cause of temp increase at idle is a weakening water pump. When was it replaced last time? Is replacement a Mitsibisi part, or an aftermarket?

 

Have you checked your fan clutch?

 

Fan Clutch. I just went through the same thing, replaced thermostat, radiator, flushed the system 10 times, finally read about the fan clutch. Who knew? Too easy. Wish I'd red that article earlier.

 

Check this out guys.

https://www.pajero.guru/how-to-fix-t...-nm-2001-1003/

 

ok... since I had what sounds like the exact same issue, I'll throw in what fixed mine.

My first indication was the drive-thru. Like you, I found that raising the RPM to 1500 and turning the HEAT on HIGH (Windows wide open.) would cool it enough to finish the line if it wasn't too long.
All the other situations you mentioned were present.
I acquired my 2002 XLS from my brother who bought it new. He swore to me his mechanic had changed all the basics.

My short story: I decided to start over as if I didn't know anything. (Pretty accurate).
1) I took my scanner like HunterD suggested to see if the gauge and the live data from the PCM read the same. YEP... they read the same.
2) Dug in the manual and found there are two sensors side by side. One for the cluster gauge and the other for the PCM.
3) Stopped by the Advanced Auto and replaced both of the sensors along with a new thermostat. (I know they read the same but just because.)
*The only thing I noticed was when I removed the old thermostat the JIGGLER was @ 3 o'clock.
The diagram shows it @ 12 o'clock. I installed it @ 12. (I know it sounds silly... I was just trying to get a little traction)

4) Voila temps back to normal on both the gauge and the live data from the PCM.

Which actually fixed the problem?
Since I had to drain coolant to replace the thermostat, I just decided to change out both temp sensors. (Cheap too.)
Since the temps both read the same, before the fix, my "guess" would be the thermostat/jiggler position.
It's only guess though Hope this helps a tiny bit.
jc