check engine light
If an incorrect coolant reading is coming back, that will throw off the car. If the connector is good, I'm wondering if the sensor is cracked or something like that and letting water in.
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I recall seeing similar threads where these weird happenings were due to water getting in to the PCM connector. The catch is that once you open that connector, you are getting in to a whole new bag of worms. GM issued a TSB that says that anytime you unplug the PCM, the connector seal needs to be replaced. (So, if you go that route, you'll want to have another seal handy for when you put it back together.)
Any chance that a previous owner may have unplugged the PCM?
Before you start guessing with the PCM connector or a new sensor, maybe you could try some tests:
- Measure the coolant temp sensor (as demonstrated in the Wells article) in a good condition and then again when you are having the problem. If it measures good in both cases, that would rule out the sensor. You'd just want to make sure the car is up to temperature for both tests.
- Spray the sensor down with some water and see what happens. (Probably best to avoid drenching the rest of the engine.)
- Spray the PCM connector down with some water and see what happens.
Maybe you can test the resistance on the coolant sensor If you can't see any cut
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