Bonneville blower has low output

I have a 1990 Bonneville. When my parents first bought the car the AC/heater blower would move large amounts of air. A few years ago the volume of air dropped and you can have the blower set on high and very little air will come through the defrost, vents or the floor. I inherited the car and have replaced the blower motor and fan and still the air volume is very low. When set to high the blower sounds like an airplane ready to take off. Is there a filter (like a furnace filter) that could be plugged? Or a baffel that is stuck? Any ideas/guesses would be appreciated.

Welcome to AF!!

There is no cabin air filter but you need to check the vacuum pot that controls the air direction doors. This is usually easy to find. If the door is blocked or not operating properly it can cause the issue you indicate. Also it is possible that the vacuum line is blocked or damaged that is causing your problem. Good Luck!

More than likely your A/C Evaporator has leaked refrigerant /oil and has became plugged with debris that has stuck to the oil. I have an 87 Lesabre (basically the same car) which had the same problem until I replaced the evaporator. A cheaper fix if your A/C is already non functional would be to just remove the evaporator and plug the holes in the air box.

On this car you can clean the evaporator core easily without removing it. In your engine compartment on the firewall you will find your evaporator case and mounted on top of it you will find the blower motor resistor mounted. You may need to move a relay panel to get to it. Remove the resistor and shine a light down through the hole and you will be able to see the plugged evaporator core. Find someone with an air compressor and a high flow air blow gun. A longer blow gun works best. Blast the debris off at a straight down angle with a sweeping motion. It helps to have the blower on high to flush the crud out of your vents, then vacuum up the interior and enjoy much more air out of your heater / air conditioner.