Coolant temp gauge is acting erratic. Climbs to 220-240+ then drops back down?

2000 CHEVROLET IMPALA

154,000 MILES • 6 CYL • 2WD • AUTOMATIC

TIMLICKY

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Coolant temp gauge is acting erratic. Climbs to 220-240+ then drops back down to 170, climbs back up, drops down within a minute or two. Ambient temp has been in the teens to upper 20's. No or very little, short duration interior heating, which brought my attention to the matter. Drove 30 minutes to and from work while watching temp gage fluctuate, but never got to red zone and no overheat light. Checked coolant level. Seemed OK in overflow reservoir. Low in radiator. Added approx. 1/3 gallon of coolant/water mix per the procedure in the owners manual opening 2 small vent valves and filling, then closing valves. Started engine, now engine check light is on (wasn't previously) and seems to be a slight vibration coming from engine. No visible leaks of coolant that I can tell. Only ran engine for a minute or two with engine light on - didn't get a chance to monitor coolant temp since engine ran only short duration.

Feb 2, 2009 at 8:23 PM

SQM

CAR REPAIR CONTRIBUTOR
6,383 POSTS

Hello,

It sounds like you might have an issue with the coolant temperature sensor.
As you mentioned the temperature gauges fluctuates. Typically when an engine overheats it will go up on the gauge and wont go back down very fast even if the fan started to work. It might not go down at all.
Many of these vehicles when the coolant temperature sensor faults it will make the engine temperature display incorrect info. Sometimes they will rear too high or too low. It can even go back and forth in a sort period of time.

You might find these articles helpful:

https://www.2carpros.com/articles/how-a-coolant-temperature-sensor-works
https://www.2carpros.com/articles/symptoms-of-a-bad-coolant-temperature-sensor

You mentioned your check engine light is illuminated. It will be a good idea to get that scanned and see what the code(s) might be.
It can surely point you to the right direction.

Here is a helpful video:

https://youtu.be/vMwTzURWXmk

https://www.2carpros.com/articles/checking-a-service-engine-soon-or-check-engine-light-on-or-flashing

I have attached diagrams for your reference.

Please let me know of any questions.

Thank you.

Images (Click to enlarge)

Aug 15, 2021 at 10:36 AM

Repair Safety Notice: This information is for general instructional purposes only. Vehicle repair can be dangerous. Verify all information, follow manufacturer service procedures, use proper tools and safety equipment, and consult a qualified repair shop when needed.