We have tried many things to get the overheating to stop, but nothing has worked. We have had the water pump changed, flushed the radiator/engine, replaced the thermostat, twice, replaced the heat sensor, checked fans, hoses, and made sure no air was trapped by filling with bleeder bolts out. Overheats going up a slight grade after 5 to 7 miles & have to turn on the heater to keep from getting to hot until I can go in a different direction that is more level. Took bottom hose off & radiator emptied quickly so I don't feel it is the radiator. What am I missing.

 

This may seem like a stupid question, but is your Montero actually overheating or are you just getting a "hot" reading on your temp guage in the instrument cluster? The temp guage has a known fault in the circuit board behind it that starts to play up on almost every model year of this Montero. Truck reads "hot" after a few minutes, without actually overheating. Mine started doing it this spring. I plan on re-soldering the circuit when I get time.

 

Second Haynes on this one. I had the same issue a while back - the temp gauge shoots all the way to high while doing barely any driving. However, the engine was actually not overheating. It is not a pleasant feeling watching that gauge for a minute or two until it returns back to normal range. Haven't done it lately, i guess things rattled around enough and re-connected back together

On the off chance that your Monty is actually overheating - the book suggest checking thermostat (sounds like you've done that), drive belt slippage (I think this would be timing belt that drives water pump and serpentine belt that drives cooling fan), testing temperature sensors. If all those are OK, you can remove the radiator and flush it out with some cleaning/descaling material. Also, is your radiator fins clear or all jammed up with debris and all bent up? That may result in heat not transferring efficiently.

 

A new radiator cap is not a bad idea (cheap insurance).
In addition, the fans on the Gen III are clutch driven.
The viscosity of the fluid in the clutch can degrade over time.
You can recharge the fluid yourself if it has lost viscosity.
Just a couple more things to try if needed.
HTH

 

I'll third the faulty gauge but it still needs to be addressed. I have a bad gauge on my 2001 but got lazy and didn't listen to the wife when she said it seemed like it really WAS getting hot. As part of another job on it (lifters/valve seals) I was replacing the timing belt and water pump too and found that the old belt had loosened and was not fully spinning the pump. Lukily I think I caught it in time but if I had not been doing the other work overheating was probably in the future.

 

I had the same problem. I replaced temperature sensors, radiator and cap, fans, water pump and I still had the problem. I've been trying to fix for a year now and finally found a fix. I replace the cluster module. part # MR532787

 

Nice troubleshooting, Great post. Well done.