+1 on false gauge readings - and possible faulty thermostat. If the water pump is working correctly and your thermostat is installed correctly as well - your top radiator hose would be hot to touch and your heater blowing nice and toasty in the cab. Your engine is more than likely now at its normal operating temp due to the big clean out. Heat is the enemy of these engines (along with being abused by monkeys - but that's a different story all together...) Test your gauge by grounding the sender wire from the temp sender unit and see what impact it has on the gauge. If it peaks then the gauge is working and the issue is the sender. If you get little response then you have a wiring fault somewhere between the engine bay and the instrument cluster.