I have a 2005 Honda Civic with 150000 miles. I was out of town and on my way home about 200 miles in 100 degree heat with my AC blasting. My temp gauge is always at the 1/4 mark. Well the last 20 miles of my trip the gage went to 3/4 so I shut off the AC and turned on the heater and the gage went to 1/2. But every time I stopped at a light it would shoot up almost to the red line. What could be wrong? Or just a fluke cause I was running it so hard?

 

When the car has sufficiently cooled, check the coolant level in the radiator and top off the radiator and reservoir if necessary.

Also, there are two cooling fans under the hood (a radiator fan and an A/C condenser fan); both should be running at the same time when the A/C is on or if the coolant temperature at the radiator fan switch is above 199 degrees F. With the A/C on, check that both fans are running.

 

To add to tech8's post, I would also be doing a compression test with the gauge getting that high. Just to be on the safe side.