I have a 1990 Honda Accord. It usually runs with the temperature
guage in the middle range. Frequently the guage will quicky go up
to close to, but not at the high end red mark. This happens even
when its 50 degrees out and I am going 45 mph. If I turn on the
heater the temperature usually goes back to middle. It goes down
very quicky, within 10 seconds of turning on the heater. I dont
believe the heater could cool things off that quickly.

I have a new radiator and antifreeze. I am not loosing coolant.

Could it be a hose or chamber that is blocked/pinched, and turning
on the heater is allowing the coolant to byupass the blockage ?

Any helpful comments appreciated.

G