On hot days here in FL. the temp gauge goes up to the red zone. Changed thermostat and temp sending unit-no help. Took it to Honda and they say the car is not over heating. On cooler days the temp gauge stays about the middle and fluctuates a little as the cooling system cycles. Both fans work. Any Ideas?

 

Clogged radiator would be my first guess. Probably enough circulating through to keep it cool under normal conditions but can't flow enough on the hot days. I'd try flushing it a couple times with radiator flush and then put new coolant in it and see if it gets any better. Pulling it out to clean it would not be a bad idea either as if it's really clogged the gunk won't make it out the bleeder bolt, but can make it out the coolant passages if you rotate it around and flush it.

 

Get it laser-tested. Find out the exact temperature.

 

Good logic. Good thinking. Thanks!