For the entirety of my ownership of my '91 240, there has been a fairly inconsistent issue, but weird happening where the car gets up to temperature, then the temp gauge skyrockets up past the red, lingers there for a bit, then falls to the "normal" marker. It may do it a few times while driving. Now, it has become an every day, every drive occurance.

I have recently bequeathed the car down to my little brother who has grown nervous as the car approaches 350k of being able to detect when the car actually has issues or may really be overheating.

We've replaced all of the usual suspects, including the thermostat multiple times. Clearly it (probably is) an electrical issue (which I know can be like trying to find a needle in a haystack) but figured i'd reach out and see if anyone else had experienced the issue and had any luck with a fix.

Thanks in advance

I had the opposite problem - temperature gauge would intermittently drop to "ice-cold". Turns out that if the wiring is either cut (infinite resistance) or shorted (zero resistance), you get either cold (infinite) or overheated (shorted).

You can check for a short or run new wires - even running them temporarily will help you rule out a short.

Did the 91 have a faker in it? I know it was only certain years.

Good Luck!

Pretty sure it was 1986+ but certainly a 1991 North America model.

hi sages- i surmise i have a simple solution to this problem. get a basic small round temp age from an auto parts store. mount it under one of the square pop out cover panels next to the cluster. should fit right in the hole there. should be 2 terminals on back of the gage. run wires from there to the terminals on the screw in temp sending unit plug on the cylinder head. if only one terminal on the plug , fasten one of the wires from the gage to it and the other to a ignition switch controlled hot wire from the switch. when the engine is running and warming. the resistance wire in the sending unit will sense the temp and the gage now bypassing the faker board will give an accurate temp reading.did this a few times on dodges and fords(12volt neg ground cars). what think you? thanks tons oldduke

Sounds to me like a clear issue with the Temperature Compensation Board, aka Temp Faker. It's a small circuit board inside the instrument cluster. I'm sure you can find more detail by searching brickoard for it and by reading this page:

http://cleanflametrap.com/tempFaker.html

It takes the actual temp measurements provided by the sensor and converts them to a dumbed down, cold, medium, or hot reading on the gauge. I recommend pulling the instrument cluster and seeing if the board has become slightly dislodged. You may be able to just push back in to connection and have a proper reading. If it is seated correctly, it may have just gone bad. You can replace it, but most brickboarders like to bypass it so they are actually getting a more accurate reading of the temp (see link).