I have a 2007 Saturn Relay minivan. The front heater blows hot but starts blowing cold when you stop even for a red light but will blow hot again when you speed up and the rear heater never does blow hot, what could cause this? This vehicle has about 93,000 miles on it.
Have someone check for air trapped in the cooling system. If there’s air trapped in the cooling system, and the engine goes to idle speed and the water pump slows down, this air can accumulate in the heater core because it’s the highest point in the cooling system and you get no heat. Once you take off from the stop, and the water pump starts to spin faster, this air is forced out of the heater core and the heat returns.
Tester
Tester summed it up pretty good. But first make sure the coolant is at the proper level, it might be really low.
The coolant level is full. How do I check for trapped air?
Your vehicle doesn’t have a pressure cap on the radiator. So this makes expelling any air out of the cooling system difficult. But here’s a quick way to do it.
Get the engine up to operating temperature. With engine idling, loosen the upper radiator hose clamp. Take a small flat-bladed screwdriver and insert it between the upper radiator hose and the hose neck on the radiator. Allow the engine to idle until all the air is purged and nothing but coolant comes out. Remove the screwdriver and retighten the hose clamp. After the engine has cooled down, check the coolant level in the reservior.
Tester
some vehicles have a bleeder valve to let the trapped air out. Check to see if yours is equipped.