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