[joe92k1500]

I would agree with KimMG. The heater core needs to be backflushed on its own. When the system is refilled, make sure the coolant tank stays full. Sometimes not enough coolant is put back in and the system will suck in air when the engine begins to cool down. That will cause the temp to flucuate. When the car is hot, the coolant level should be at the hot level on the tank. If not Fill it up to that point. Hope that helps.