Why is my car making a pop sound?

Question

I have a 1995 Mercury Grand Marquis. My car recently started making a popping noise while my heat is on. Usually shutting my heat off makes it stop but today after I shut it and my car off there was a pop louder than what it usually is. I don't know if it matters but my heat also sometimes will work but after a little bit will blow cold air and switch back to hot after a while.

Answer 1

It's likely that your blend door actuator is making the popping sound.

The blend door is in the heat/ventilation/air conditioning duct system and it is responsible for mixing hot and cool air to achieve the temperature you want. To move the blend door, there is an actuator which is like a black box with an arm that pushes or pulls the blend door to adjust its position.

There are several possibilities. The blend door or its linkage could be broken, causing the actuator to make popping sounds as it tries to move the door. The actuator could be broken or defective, making the same sound. Or, if you're lucky, the blend door could have lost its calibration, so its actual position is different from the position it thinks it is in, and it keeps pushing or pulling when the blend door is already at the limit of movement.

If the actuator has lost its calibration, this can be repaired at a dealer or a well equipped private shop using a factory-level scan tool to recalibrate the actuator. This is like a computer programming repair and nothing has to be taken apart.

If there is a physical problem with the blend door or actuator, the repair will involve removing part of the dashboard including vents, ducts, glovebox and the passenger airbag. Then the defective parts can be replaced.

Working with the airbag requires specific precautions to avoid being killed by it, if you will excuse the hyperbole. So I suggest that if a physical repair is needed, only an experienced mechanic should do the work.

The first step is to get an in-person diagnosis to confirm that the blend door actuator is at fault, and to test calibration by using a scan tool. The thing not to do is to go directly to blend door actuator replacement without further diagnosis, because it may be unnecessary if recalibration will fix it.