Temperature gauge

Tiny
MICHELLE STRAW
  • MEMBER
  • 1997 PONTIAC SUNFIRE
  • 2.4L
  • 4 CYL
  • AUTOMATIC
  • 155,000 MILES
I replaced the thermostat and water pump and upon driving it 3 miles, the temperature gauge was in the red. When I pulled over to check things, the overflow tank had a little fluid coming out from the cap. The cap is also new. Could the problem be the coolant sensor, or is it the radiator?
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Saturday, August 8th, 2020 AT 11:27 PM

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Tiny
CARADIODOC
  • EXPERT
Neither. The coolant temperature sensor just reports the temperature. It's only involvement in controlling the temperature is the Engine Computer uses it to know when to turn on the electric radiator fan.

In this time period, most engines used two coolant temperature sensors. One with a single wire connected to it is for the dash gauge. We know that is working properly because the gauge told you about the overheating. The other coolant temperature sensor has two wires. It is for the Engine Computer and it is much more accurate. The computer uses that data to determine when to cycle the radiator fan on. During the overheating, if you observed the fan running, that circuit is also working correctly. A quick way to check it is to unplug that one when the ignition switch is on. The computer will detect the break in the circuit, set a diagnostic fault code, turn the Check Engine light on, and since it won't know coolant temperature, by default it will turn the radiator fan on in case the engine is running hot, or to prevent that from happening. When you reconnect that two-wire sensor, the fan will be turned off within a few seconds.

The radiator was apparently working properly before the repairs. It didn't suddenly become plugged from replacing the other parts. Besides having plugged tubes, engines can also overheat when you have an extensive butterfly collection plugging the cooling fins, or when those cooling fins are corroded and crumble like a rotten chocolate chip cookie when you touch them. Those corroded fins can't give up the heat to the air flowing past them.

Those radiator problems typically show up after 20 - 30 minutes of driving. The corroded fins cause more trouble at highway speed, and more running too warm when outside temperatures are high.

By far the best suspect is you didn't bleed the air out of the system. This is less common on V-6 and V-8 engines. When you refill the cooling system, there is no way for the air to escape, and this is worse on GM cars where they didn't put a cap on the radiator. Thermostats have to be hit with hot liquid to open. Hot air won't do it. With air pooled under the thermostat, it won't open, and the coolant won't circulate, so no hot liquid will reach the thermostat. With it staying closed, the engine will overheat very quickly. The biggest clue is when you mentioned the engine overheats in three minutes. Engines don't get hot that quickly when there's no air in the system.

Check out this article, especially step 16:

https://www.2carpros.com/articles/replace-thermostat

If there's a bleeder screw, it will be on the thermostat housing. If your engine doesn't have one, you can usually find one of those coolant temperature sensors or a threaded plug to remove. The engine doesn't have to be running. Just add coolant until it reaches that bleed hole. What little air that might still be in there will get pushed into the reservoir once the thermostat opens for the first time.
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Sunday, August 9th, 2020 AT 12:01 AM

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