coolant loss

91 Montero, 3.0, Automatic, 118000 miles.

Experiencing loss of coolant through resevoir overflow tube, only during highway driving (is fine at idle).

Things I have done:

Idle test for 1 hour with A/C on, no overheating or coolant loss.

Changed thermostat, radiator cap.

Backflushed cooling system.

No visible leaks on water pump, hoses or radiator.

Tested radiator by warming-up vehicle and feeling around the radiator to make sure that it was evenly hot. It was not until we revved-up the engine to 3000 rpms.

Has anyone else had this problem or know what could be causing it?

Thanks!

Anybody out there??

I have a 95 montero LS and have just recently noticed that I'm losing coolant. I don't know how or why, just that it's disappearing. My montero has always had a problem w/ burning the oil and using the oil but that's about it. Now, I noticed this problem w/ the coolant. I am also smelling some kind of exhaust or something - don't know if it's related or not.

So I am no help to you b/c I don't know what's going on - but just wanted to let you know that I'm having a similar problem.

Have you had any insight as to what's going on since writing?

It has something to do with the pressure being excessive in the coolant system, or it could be a bad radiator, thermostat stuck closed. Something is creating the pressure. One mechanic seems to think that I have a bad head gasket and exhaust gases are going in the coolant system. I find that very hard to believe.

good luck figuring out the prob - hope you get it fixed - if you do, maybe you'd post a quick reply as to what the prob was. I'm looking to sell mine now so I'll just let the buyer know what's going on -