Q: My heater seems to blow hotter when I'm driving versus when it's sitting at idle or at a stop light. Also the heater takes too long to heat up the cabin department. Why?

asked by on November 15, 2016

When heating my car up in the mornings the heater doesn't get to it's full warmth until I start driving. Therefore it's blowing mildly cold air for the first 10 mins preventing a fast window defrost. I've made sure the coolant (50/50 mix) is filled to the line. First time experiencing this.

My car has 90000 miles.
My car has an automatic transmission.

Hi there. When your vehicle is in motion and driving down the road, then engine coolant temperature tends to get hotter on the engine before it gets to the radiator, so the heater will naturally heat up more when driving around. As far as the heater not heating up fast in the car, either the engine coolant is not hot enough yet to heat up the car or there is an issue with the heating system or cooling system in the vehicle. First check the engine coolant to make sure that its full. Top off the coolant if its low. Then check the heater control valve and see if its opening up when the temperature is at operating temperature. If the valve is not opening up, check the vacuum hose to it for any leaks. If nothing is found on the valve, then the valve needs replaced. If the valve is working correctly, then the thermostat may be sticking open when the engine coolant is cold.

The thermostat’s job is to bring the engine to operating temperature in a timely manner. If the thermostat is sticking open, then the engine not only will have a longer time to warm up the coolant, but the computer system will think that the engine is cold all the time and make the fuel system go full rich all the time. This will give the vehicle a poor fuel mileage and a waiting time for the heater. In this case, the thermostat would need replaced. If you need further assistance with your vehicle, then seek out a professional, such as one from YourMechanic, to help you with your heater not heating up in a timely manner.

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