Hi. First some specs on SUV:
1994, 3.0 liter, 165,000 miles, automatic trans, 4x4 w/overdrive, first winter in CO, engine and tranny runs very good, all dash indicators have always worked well, coolant is mixed 50/50 and full, heat controls and vent controls seems to be working fine.

About 2 weeks we had a deep freeze cold snap in CO where it got down to -15 one evening and stayed under 0 for about 3 days. The day after the temp dipped I got in the Montero, started it right up and started driving it after about a 2 minute warm up. It was about -5 degrees out that morning. The Montero would not warm up. Then suddenly the temp gauge started climbing and kept climbing until it got all the way into the red. My lights started dimming and I stopped the SUV and had it towed back to my house. The next 5 days I started it up and idled it for 30 minutes or more. The temp climbed to the normal range, but it would not get warm inside the vehicle with the defrost/heat on high. So I drove it a few times and it would get luke-warm inside, but would not ever get hot.

I thought that maybe the thermostat had failed in the closed position, keeping the inside from getting warm, while causing the motor to overheat, so I changed the thermostat. After changing the thermostat the temp gauge does not rise at all now, and the inside heat temp is again cold. I took the radiator cap off and let it breath to let any trapped air out, as I ran it for about 45 minutes. Then I disconnected the upper radiator hose and poured antifreeze down the hose towards the thermostat, and it went right through it, indicating the thermostat was open, even thought the heat is cold, and the temp gauge shows the engine is cold also.

What is going on? If the heater core was clogged, the engine temp would at least get warm, but it does not. When I changed the thermostat, the cold problem got worse.

I thought to check the heater Control valve (some vehicles have these) but the parts stores tell me my Montero does not have one. Is this true? What keeps the hot water from flowing through the heater core in the Summer time then (when the A/C is on), making the cab hot???

Then today as I drove it about 20 miles round trip to the store, I noticed the tranny is not going into 4th/overdrive gear. This is new. The switch seems to be working fine, but it never goes into 4th gear. Certainly this can't be related to the engine and cab temperature problems, can it?

thanks!
chris

Transmission not going in to OD yes is due to the motor not getting warm there is a sensor that will stop it for going in to OD tell the truck is warm.

MMM maybe the thermostat you put in it is crap.??? I have in the past bought cheap ones or..... maybe its not seated that well and its leaking around it. I there way I would pull it out and re-install another new one if I was you. You might even want to replace the radiator cap as well. Just for good measure.

Check your engine fan. Make sure it's turning freely. Sounds to me the fan is pulling to much air though the radiator. You can slip some cardboard or some metal sheet material (air dam) between the radiator and air-condition condenser like the trucker do. Start big and than trim smaller to find the best results. There is a pice of plastic trim between the two, just remove it and slip in your air dam.

My 3.0L runs on the cool side also, but thankfully I live where we don't get the cold you get. Still it's not the best, but it will have to do.

Your symptoms are unfortunately consistent with a blown head gasket. The water level is below the heater and the coolant temp sensor, so neither one works. You should be seeing steam at the tailpipe when the water is topped up, and bubbles in the radiator.

Park the truck on an incline, where the front of the truck is higher than the back and check for this. Bubbles = no bueno...

Otherwise, bad thermostat, not closing.

John B.