I have a 1986 Volvo 240DL and the temperature gauge keeps going up to the top even though the car is NOT overheating. It can happen at any time and then it just slowly creeps back down to the middle area.

I've already swapped out the thermostat, thermostat housing, the temperature sensor and the actual gauge itself inside the cluster.

I've also checked the engine area when the gauge is at the top and the car is definitely NOT overheating.

Any ideas what could be causing the gauge to give the impression that the car is overheating?

 

Sounds like a ground wire problem. Did you check resistance and voltage to/from the ECT sensor? If not there, then it could be in the gauge.

 

I swapped out the gauge.

 

Disconnect the wire at the water temp. sender in the block. Tape it off so it doesn't touch anything. Start the car and see if the gauge does anything. If you short that wire to ground the gauge will go to the hot mark. There may be an intermittent short in the wire or harness.

 

I can't be much help with troubleshooting, but my 89 245 did this a few years back. The issue was a failed temperature compensation board. From my understanding the board will read a temp, and send the needle to a predetermined spot for a particular temperature range. When this fails it will send the needle all over the place.

Check this out. It's an easy process but I lack the skills to really properly describe it:

https://www.volvo-forums.com/t14694-...ard-bypass.htm