Heater not strong, 89 Buick LeSabre

Can't get strong hot air flow on cold days. Either in auto mode with temp setpoint at 85 degrees, or in manual mode with heat full on...

I have noticed sometimes you are cruising on the highway for more than a half hour and the car is fully warmed up. Then you pull off and go to idle in park. All of the sudden plenty of hot air is gushing out.
Both modes work fine then, and hot air is strong.

89 Buck Lesabre, 3.8L V6, 4spd automatic.

I have noticed with the vehicles I have owned that on the days that the outside temp is really cold ( under +10 degrees) no matter how warmed up the car is, the heater/ defroster still doesn't put out enough warm air. I've attributed it to a good cooling system, as you are driving, the radiator is doing it's job at dissapating heat,and when you are idle the heat is fine. Have you owned the vehicle long.( i.e. over a year?) If so, has it happened in the past winters? Sometimes a stuck thermostat or air in the cooling system can cause problems like this. From what you have posted that the problem seems more prevalent on colder days, I would chalk it up to a normal situation, as my vehicles do that also,( and I'm talking about a Lesabre, C1500 pickup, and a Chevy Venture.) And I remember my Monte Carlos all did that , my Chrysler LHS, my wife's Beretta, and the company vehicles I used to drive.

I have a 1987 buick p/A with the same problem. I have changed the thermostat, checked both heater hoses, replaced the climate control unit but nothing seems to help. I did have success when I disconnected the vacuum manifold block below the glove box and only had use of the defroster. If you know where I can find a HVAC diagram showing where all the air flow doors go that would be a big help. From what I have figured out the vent door is not closing all the way when the heater is on during a cold day and the amount of cold air coming in is to much for the heater core to heat up. It almost appears the cold air is bypassing the heater core entirely but without a HVAC duct/ door schematic I am as lost as you are.

Try putting a small piece of cardboard in front of the radiator. Not the whole radiator just a section of it. In the winter months the radiator has a tendency to get frozen up by the wind when driving on the highway. You might need to just but a small barrier on the radiator. Make sure you take the cardboard off when the temperature outside starts to rise.

Try putting a small piece of cardboard in front of the radiator. Not the whole radiator just a section of it. In the winter months the radiator has a tendency to get frozen up by the wind when driving on the highway. You might need to just but a small barrier on the radiator. Make sure you take the cardboard off when the temperature outside starts to rise.
Yes, that should work when all else fails. Thank you for mentioning it as I forgot to in my previous post on the subject. Up here in the Northeast it is a sight to see all the vehicles sporting cardboard in front of their radiators in the winter. Must be a widespread problem.

Actually I'm in Oregon so it wasn't that cold, probably around 40 degrees. I'm suspicious of the door vent he mentioned because of when I pulled off the freeway and all of th sudden I've got really hot air pouring out of the heater. It would explain if the door suddenly opened up...This problem has happened in the summer also. Can't get heat during the night if I want it...