* HVAC System Blower Motor: This is the most common cause. The blower motor is responsible for moving air through the ventilation system. If it's running when you don't want it to, it could be a malfunctioning blower motor resistor (controls fan speed), a faulty blower motor control module, or even a problem with the blower motor itself. It might be running constantly, at a specific speed, or intermittently.
* Vacuum Leaks: Your Alero likely uses vacuum lines to control various functions of the HVAC system, such as air direction (dash vents, floor vents, defroster). A leak in these lines can cause unintended air flow.
* Blend Door Actuator: This is a small motor that controls the mix of hot and cold air. A malfunctioning blend door actuator could cause air to blow unexpectedly, even if the temperature control is set differently.
* Mode Door Actuator: This actuator controls where the air is directed (dash, floor, defrost). A malfunctioning mode door actuator could be causing air to blow in unexpected locations.
* Leaking Heater Core: While less likely to cause *blowing* air, a severely leaking heater core could force air out of unintended areas due to pressure buildup within the system. This would typically be accompanied by other issues like coolant leaks.
To figure out the problem:
1. Describe the air: Is it hot, cold, or ambient temperature? Is it blowing forcefully or weakly? Is it constantly blowing or only under certain conditions (e.g., when the engine is on, after turning the car off)?
2. Describe the location: Where is the air coming from? Is it only from the vents, or is it coming from under the dash, or other unusual places?
3. Check the controls: Are the fan and temperature controls set to "off"? If so, the problem is likely electrical or mechanical within the HVAC system.
Once you provide more details, I can give you a more specific diagnosis and troubleshooting steps. You might also want to consult a repair manual specific to your 1999 Oldsmobile Alero.
