Coolant backing up out of reservoir, overheating

1998 SATURN SL2
194,000 MILES • 1.9L • 4 CYL • 2WD • AUTOMATIC
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JESSE987656789
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Gets up to the middle on gauge, coolant goes backwards out of pressurized reservoir. Changed thermostat, new reservoir and cap, new relay. The fans not coming on, but hot wired the fan and I know it works. Help!
Jun 20, 2018 at 5:50 AM
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SATURNTECH9
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Have you checked the coolant temperature sensor? That would explain the symptoms of the gauge reading normal and not coming on.
Jun 20, 2018 at 1:47 PM
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JESSE987656789
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Have not replaced it but I will. I ordered new connector and sensor, I do not think that is it though. The person I bought the car from unplugged the check engine light and or wires and or computer or something, it does not come on when you turn the key on. Scan tool will not work for OBD, before I put the car back to how it was supposed to be the car did not do this. The people I bought the car from rigged it up oh, I put everything back how it was supposed to be, coolant flows constantly with heater on high and blower on high, not overheating, yet.
Jun 20, 2018 at 3:50 PM
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STRAILER
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I would replace the engine coolant sensor and if that does not work go over the wiring using this guide following the fan wiring diagrams below. If the wiring is okay I would swap out the PCM for a used one to see what happens.

https://www.2carpros.com/articles/how-to-check-wiring

Check out the diagrams (below). Let us know what happens and please upload pictures or videos of the problem.

Cheers, Ken
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Jun 23, 2018 at 2:22 PM