Temp gauge going funky, Updated! Changed Thermostat but still does the same prob Options

[reko]

As i was driving on the highway on friday night i noticed that my temp gauge was almost at cold which isnt normal for a sunny summer day.

After rolling for 10mins, it went to normal temp. but started to go slowly from normal to cold back and forth 4 to 5 times after what it stays at the first 1/4.

It did it only once, but then on the next day, it did it 2 times and went back to normal.

Nothing happened yesterday, but still, i dont really like this.

I'm thinking thermostat, but who knows?

Is it bad? I mean I went to school today (which is about 20kms from my house)... should I change that tonight?

**UPDATE**
25/08/08: Changed changed the thermostat... The shaft bent so it was stuck open so I just put one for Camry since the one for Celica 2.2L didnt fit (dont know why but anyway....) Still does the same problem as before... temp gauge isnt stable...

So what's next?
Water pump maybe

This post has been edited by reko: Aug 25, 2008 - 5:47 PM

[GriffGirl]

I have to disagree about going with the dealership on this one. I really think a NAPA or whatever thermostat is just fine, as long as it's the correct part. I have one from like Autozone or Schuck's that works just fine.

IIRC the thermostat can really only go in one way, it won't fit if you put it in "backwards". Yes, make sure you replace the gasket. (sounds like you did) Check your coolant level - if you're low, you could overheat. Run the car for a few minutes with the radiator cap off to get excess air out of the radiator. Did you replace your radiator cap? If your cap is old and doesn't hold pressure, you'll overheat there too. Replace your cap.

If after doing this it's still acting up, try replacing the coolant temperature sensor (located on the engine block, should be just below the thermostat if it's the same on a 5s as a 7a). This should run you about $21 at NAPA or Autozone or whatever you guys have up there. One of these things should fix it - if not, it's time to explore other options like water pump or heater core. But rule out the easy stuff first.