I installed a new radiator but when I climb a hill or mountain in 90 degree + heat the cooling system heats up. If I turn off the a/c and turn on the heater full it usually cools back to normal. on any other terrain other than a steep hill, it stays in the normal range with the a/c on. It doesn’t leak or use any water (no leaks). Only in hot weather and on an incline.
RESPONSE
No more hills for you, Just kidding. Sounds like the thermostat may be sticking open a bit. When this happens the coolant doesn’t stay in the radiator long enough to cool under stressful conditions such as heavy acceleration or climbing hills.