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2003 Marauder

I have a 2003 Marauder and all of a sudden, the heat in the car will start blowing cold air. It does it more often then not as the RPM’s are at idle. Engine temp is fine which rules out thermostat. Any ideas? 80K miles and kept in great shape. Would having a pressure coolant change help?

My guess is a vacuum problem. When running slow (also should happen when you accurate fast) the vacuum can drop, especially if there are some leaks in the system. Some parts of your heating/cooling system are vacuum controlled. It is also possible that the water pump is nearly gone, but that would be unusual in a car that new IMO.

Nice car. These have electronic controls (EATC). The blend door is controlled by an electrical stepper motor. I suspect a problem with that control unit. I am not very familiar with them, but there is a way to do a self diagnostic on the EATC unit. Ask your question and any others you have about your Marauder at www.crownvic.net. Several posters there will know how to do the self test.

I think vacuum issue too. The car has three vacuum motors on the heater/ac system. A vacuum leak can be anywhere from the switch, the vac motors , or the hoses. You could have a vacuum leak in an area not related to the heater/ac ( out in the engine area hoses ) but you’re only noticing the heater mal-function because of the spring-back default of the vacuum motors.

I vote against it being a vacuum problem. Vacuum would only effect where the air blows out, not the temperature of the air. (The blend door is electric.) It is either an electronic control issue (as was stated) or a coolant flow issue. Check and make sure the coolant is at the correct level and there are no leaks. Make sure it builds pressure when the engine heats up.