Question on overheating/heat control valve/heat control slider….I’m sure it is one or a combination of these things.

Recently acquired a new 1991 240 wagon (auto, 150K). While driving the car home it occurred to me that the PO had the recirc button depressed. When I turned it off I found myself enjoying plenty of warm air even when it was cool out, temp slider was all the way to cool, and blower fan was off.

I also noticed that the temp gauge had crept up. If the beginning of the orange zone is 100%, it was around 90% of the way there. It remained at that level even after leaving the highway.

I drove the car to work today (no highways) and the gauge stayed about in the middle...no problems. At lunch I decided to run a few errands and once I headed out to the highway (pretty good incline over a bridge) the temp went back up. I turned off the recirc button, moved the heat slider all the way to hot and turned on the blower….temp gauge promptly went down.

At that point the problem seemed to be mitigated. The drive back to the office looked good as well (with heat on though) until I entered our 9 story parking garage….as I went up and up so did the temp gauge. Even with the heat and blower on the needle rose...never into the orange zone, but definitely in that 90%-95% range. After parking, the temp didn’t seem to go down either.

Can anyone shed some light and/or help me focus on what I should look at first? I guess I don't completely understand why turning the temperature slider to hot helped the way it did.

Many thanks!
Chris