There are lots of tips about replacing the blend door actuator, but rarely do you find the solution to the problem if the blend door works fine. So, here is the solution. It is the heater core! After time, if not properly cleaned and flushed (like most people do), the antifreeze gets nasty! Especially if you use the GM Dex Cool, and add tap water instead of distilled water. The heater core has very small passages, so it is much easier for it to get plugged up. One side of it can get plugged, and cause the exact same symptoms of a bad blend door actuator. Fortunately, most of the time its only a matter of flushing the system, and rarely replacing the heater core! It is also easy to do on a 3.8L Buick Lucerne! All you need is a garden hose, a nozzle, and pliers. images 2 and 3 show the nozzle type and a shutoff valve.
First remove the clamps and hoses shown in the first picture. Turn on the nozzle to its highest flow point, and insert the nozzle into the small hose (1). Aim the large hose (2) down, or into an extension so the waste water does not get everywhere, especially not on the serpentine belts! Turn the shutoff valve on full for 2 to 3 minutes so it flushes the heater core. Water should start running clear after a few seconds, and at a good flow rate. Then reverse the flow by moving the nozzle to the large hose, following the same procedures. You can put it back together at this point and top off the antifreeze, however, I would highly recommend flushing the rest of the system first. Remember, the old antifreeze that blocked it in the first place is still in the rest of the cooling system! Easiest way to do this is put the large hose (2) back on the motor, reinsert the nozzle into the small hose (1) and SLOWLY turn on the shutoff valve. The old antifreeze will start coming out from the port where the small hose goes. When the flow comes out clear, remove the radiator cap and block off the port where the small hose goes (with your thumb, whatever, but remember, the port is plastic so don't get aggressive) and the water will start coming out the radiator fill. When it comes out clear, shut off the water, and put the hose and both clamps back on. Fill the radiator with (concentrated) long life antifreeze for all makes and models and dilute to proper gravity with DISTILLED water! Follow the maintenance instructions for this procedure. Don't forget to fill the overflow tank to its max level, and you WILL need to bleed the air from the system, or you will have no heat. Turn on the heater to maximum heat on high. So I don't reinvent the wheel, go to this link, and it will tell you how to bleed the air. You will need to let the car cool down after, and then refill the radiator again. Drive around for a while, let cool down again, and top off radiator and level overflow tank again. After that, your heat should be working great again, on BOTH sides. It sounds like a lot, but is actually very simple. Other than the filling and cooling down part, the whole procedure only takes around half an hour. Hope this helps.