I have a 1998 Buick Park Avenue when I turn the heat up to 90 it will blow hot air out on both sides of the car when I turn it down to 80 it blows hot air out of the passanger side and cold air out of the drivers side

SOURCE: 99 park avenue blows cold

As snowbird said, it's most likely your passenger climate control system. Could be a short in a wire or a burnt acuator that moves the passenger door open and close. If you don't have the money or time to troubleshoot the problem you can pull down the glovebox and you'll see a metal rod about 8 inches long connected to the acuator. If you pop it off by the white connector, you can move it to full hot or cold. If it was the heater core it wouldn't matter where the air is comming out, it'd be the same temp.

It'll take you 5 minutes tops to simply check it by moving that door.

Good Luck

SOURCE: 1998 buick park avenue drivers side blows cold, passenger side hot

sounds like the drivers temp door actuator is not working should be located on the drivers side of the air box down by the gas pedal or center dash area.

SOURCE: Replace Thermostat in 1998 Buick Park Avenue

if you follow the top radiator hose to the engine you will see where it connects to the engine...you will want to drain some of the coolant from the radiattor but once you loosen the clamp holding the hose on you will need to pull the hose off and push it to the side out of the way. there are two bolts that hold the housing on that you need to remove where the hose was hooked up. once you take those bolts out you should be able to pull the housing out and the thermostat is right there.
i do wish to offer you an opinion on the thermostat i dont think its the problem..i think maybe your heater core is partially clogged and not letting the coolant get into the interior of the car as you are driving.if its blowing warm when you start it up and let it run and it gets warm and then get cooler as you drive thats going to be in the heater core since the thermostat is opening to let the heat in when you first start the car up to warm it up.

Marvin

• 85242 Answers
• Posted on Apr 13, 2009

SOURCE: my 1997 buick park avenue

U need to replace the air temperature control blend door actuator motor, this requires removing the dash to access the motor for the blend door.

Donnie Matney

• 426 Answers
• Posted on Jun 19, 2009

SOURCE: 1998 Buick Park Avenue Ultra, AC was low on

still low on charge usually problem.