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Temperature Gauge

The temperature gauge in my '95 Cutlass Supreme (3.1) always reads high in spite of having the thermostat replaced 2-3 times. I replaced the motor 65K miles ago along with the radiator and all new hoses (didn’t replace the heater core). When I’m doing 75 down the interstate, it reads at the line just before the red area. When I slow down to 55, it comes down a tad. Heat & A/C work fine. What can be wrong?

Could be the gauge is inaccurate. Overheating problems are usually caused by one or more of the following

  • Thermostadt not working or not installed correctly
  • Radiator plugged
  • Water pump malfunctioning
  • Coolant low or incorrect mixture
  • Hoses plugged or collapsing
  • Air in cooling system

I assume you have fresh name brand coolant with a 50/50 mixture. Since you have a new radiator, it seems unlikely to be plugged. The heater wouldn’t affect summertime engine cooling, so that’s unlikely to be a cause. hmmm … I’m thinking either there’s air in the cooling system and needs to be bled out, or the new radiator isn’t up to OEM specs for you car, or the thermostadt isn’t working or wasn’t installed correctly. I’ve had brand new thermostadts be bad right out of the package. That’s probably where I’d start.

You unfortunately are in the position you need a qualified mechanic to check this out, there are so many what if possibilites…

Someone suggested that the fan may not be operating correctly. I’m assuming there’s a temperature sensor that causes it to kick in and off. Any thoughts?

If you are sure the temperature gauge is working correctly and is accurate then you might have a block check done to see if exhaust gases are getting into the coolant due to a bad head gasket.

yes the fan comes on at a pre set temp. or when the ac is turned on.

Let’s assume the gauge is accurate and you’re having a legit cooling issue.
It may not be the case if that thing reports the wrong temperature, though.

One would assume that when you’re driving 70 there’s enough air moving around the radiator to cool it off, no?
You could, by way of a test, replace the fan relay’s contact with a dead short, forcing them to stay on. Then drive it at 70 to see if it overheats.

Along with @cougar very valid concern, if the exhaust gases somehow make it into the coolant, you can often tell this condition because the coolant overflow bottle will bubble because that’s where it tends to get out. Is that happening?
You could do a coolant system pressure test with one of these gadgets. I bought the very same one. It works quite well: