I don't post much in here cuz I drive a Jetta but my brother has a Civic 92 VX (d15z1) and he has a very weird problem... and I'm trying to help him

The temperature gauge shows that the car is getting really hot after just a little bit of driving, the needle goes all the way up almost reaching the "H" (nothing strange yet) but if he puts the car in 4th or 5th the needle starts going down and if he drives for about 10 minutes in 4th or 5th the needle goes all the way down to the "C" (as if the car were turned off)
What gives? What can be causing this... he changed the thermostat & coolant about 1 month ago, and the radiator doesn't have any cracks or leaks...

I hope I made myself clear... [8D]

Please help...

 

Might be a heat sensor...

 

What else can be causing this besides a heat sensor... the mechanic checked that and he said the sensor was OK...

 

As someone once told me in this very forum, it might be air in the cooling system. I don't really remember exactly what he said, but it was something about draining the system and adding coolant little by little as the engine was turned on. To be sure, try searching for one of my posts about dacing temp guage...

 

If there is air in the cooling system, you can bleed the coolant (similar to bleeding the brakes). There is a bleeder screw on top of the thermostat housing.

 

^^^^It was this dude...

 

Bleed the coolant, make sure there's no air in it. Otherwise, he may have popped the headgasket.

 

the problem is solved... kinda I guess... the gauge cluster is f'ed up (electronically speaking) causing the evil "dancing temp gauge"