Temp gauge related problem

The car is a '99 chevy cavalier with the 2.2 4 cyl. Going down the highway running 50-55 mph the temp gauge is reading normal . When you get around 60
mph the temp guage will drop to the cold mark and wander back and forth between cold and middle of gauge and you can feel the torque converter unlock . 3 speed automatic. Sometimes it felt like it was dropping back to 2nd gear. What are some possible things to look at. No check engine light coming on.
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Kevin wrote: "The car is a '99 chevy cavalier with the 2.2 4 cyl. Going down the highway running 50-55 mph the temp gauge is reading normal . When you get around 60 mph the temp guage will drop to the cold mark and wander back and forth between cold and middle of gauge and you can feel the torque converter unlock . 3 speed automatic. Sometimes it felt like it was dropping back to 2nd gear. What are some possible things to look at. No check engine light coming on." *************************************** I'll take a stab at this although not really experienced with that particular car. It's no surprise that the converter unlocks if the ecm percieves the engine temp to be cold. And the indicator on the dash reads cold as well. This _suggests_ that the ECM uses the same sensor for engine control as the dashboard indicator uses -- but that isn't always the case. But I would say the easiest thing to try first is replacing that sensor, since it's probably easy and inexpensive. I presume the coolant level is full.
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If the check engine light is not coming on that simply means you car is not polluting. It has nothing to do with fluids. Rather than just changing parts, based on a description of the problem to those in a NG, take it to a competent technician and have analyzed and corrected WBMA

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It suggests that the engine is actually running cold when the dash guage fluctuates. The ECM uses a " coolant temp sensor " to monitor engine temps. The dash guage uses a " coolant temp sending unit " to work the dash guage. The two circuits are completely seperate from each other.

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Actually, 99 Cavalier uses ECT sensor for temp guage operation. The PCM supplies the info to the IPC via Class 2 serial data. James was correct about the lock up not operating when cold. The symptoms suggest low coolant level to me, however.

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Sounds like low coolant level or the thermostat is sticking open. You may just want to replace the thermostat righ away before it sticks closed and overheats.
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The coolatnt is at the proper level. I was leaning towards a bad temp sensor, but looking for other opinions.

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Try wiggling wires while someone watches your temp gauge. Also found a bulletin (#99-06-02-010)about trapped air at the temp sensor. Loosen the sensor until coolant starts to weep out around the threads, this will allow any trapped air out, then try it again. Free fixes are the best. :)

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