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I took my car on a 30mile trip this morning. About 3 minutes the temp guage was reading at the 210 mark I thought thats odd I just started this car. Anyway 10 minutes later it was at 230 . I couldn't believe this so I turned off the headlights and the guage went to 120 or so. What could cause this problem?
Check the obvious stuff first: Loose sensor wire etc.. Do you have electric fans? if so is your alternator putting out enough voltage?
If you have stock fan, check the clutch, Fluid Level in the radiator etc..
I would have to guess your gauge cluster has a grounding issue.
Do the "normal test" for the temp gauge.Ground and unground the sensor wire at the engine.Then do the same with the lights on and off and see what diff it makes.(with ign key on of course)