Temp gauge fluctuating even after replacing thermostat and CTS

[Christgen]

A few months ago my temp gauge would fluctuate between operating temperature and below, so naturally I checked the thermostat. As I had expected it had broken and was stuck open which explained the drop in temperature. I replaced it with an oem thermostat and decided to replace the coolant temp sensor as well since it was the original. Now about 6 months later the coolant temp gauge is fluctuating again in the same way. I checked the replacement thermostat and it isn't broken. I hooked up a scan tool and the coolant temp does drop when I'm driving from 45 mph and beyond and comes back up to operating temp when stopped, just as it did the first time I had the problem. Any ideas as to what the cause of this could be?

[misha]

Thermostat.
If scan tool confirms drop in temperature on open road just like gauge....it's thermostat.
You can't know if it's bad just by visual inspection.
It can also partially fail....mid way open....not just stuck opened.
Oe & oem thermostats(read quality thermostats) always stuck open avoiding overheating of the engine.

[RB3]

I too am experiencing this as well after replacing the thermostat sensor not long ago. Though I'm not sure it's only at high speeds.

[erikv11]

To eliminate or pinpoint the thermostat as the issue, remove the stat and drive the car. If it still fluctuates, it wasn't the stat. If it stops, it was.

[scot850]

I have found that the P80's are prone to air-locks when playing with the cooling system. These will cause random temp readings.

Try raising and supporting the front of the car, run it up to temperature, and then let the engine cool over night. Although the heating system does have a by-pass it is worth leaving it on hot to help max flow and maybe remove any air. Then see if the problem persists.

The other guys have also mentioned the ways to ID if it is a thermostat issue.

Carefully check the connection to the temp sensor. They are prone to getting fragile with age and can cause intermittent readings.

I think you said you used an OE Volvo temp sensor. If not chances are you will have issues.

Neil.

[erikv11]

Typo alert, Neil probably already knows this fact: Actually the heating system does NOT have a bypass. Coolant always runs through the heater core, regardless of engine temp and regardless of where you move the control setting inside the cabin.

[Eddystone]

The air pocket theory sounds most likely to me, but the gauge doesn't really read temperature, but electrical resistance. It is suppose to react to changes in the sensor designed to change resistance as a result of changes in temperature. However, if you have a solder joint on the instrument cluster that has become high resistance or intermittent due to years of heating up and cooling down (on the circuit board) that change at the back of the meter will affect the position of the needle.

I think the troubleshooting advice given above is sound and should be followed. Just pointing out that something a simple as a bad solder joint will cause your meter to fluctuate, as well.

[abscate]

To eliminate or pinpoint the thermostat as the issue, remove the stat and drive the car. If it still fluctuates, it wasn't the stat. If it stops, it was.

That’s a powerful method, controlling all variable but one while manipulating it and monitoring outcome. You should patent that and name it something.

[RB3]

Gonna try this today. Stat and sensor replaced last fall. Not sure if OEM. Also had water pump and timing belt replaced 2 mos. ago so naturally coolant was involved. I haven't noticed any fluctuations since and only topped it once. But after you mentioned the air, I did just run the heat the past couple mornings/evenings.

Been having random electrical issues as well. Replaced battery 2 days ago.

Thanks for tip on Stat.

[RB3]

Removed the thermstat. It ended up fluctuating tho it never seemed to go as low as before and doesnt seem to be as often. I didnt drive too far. Store, gas, etc. So do I assume stat is ok? What's next? I'm having what might be a fuel/fuel pump issue as well. hesitatiion when accelerating. Not sure if the 2 are related in any way? Oh and license plate light is on when hatch is open but goes out when hatch is closed halfway That one is annoying. I dont think I can deal with that. Dont even want to think about it. One day (problem) at a time.