Sometimes pieces of the original plastic impeller get stuck in the cooling system causing it to run hot and even ruin the new water pump. If the small hole in the thermostat is plugged the thermostat will not work correctly and can air-lock the cooling system.
The fan has a low speed through the resistor and a high speed bypassing it. If the fan is not working correctly you will run hot or overheat at idle, but not while cruising.
Do a compression test and check that the cooling system holds pressure. Backflush the system thoroughly.
