IF you replaced the heater fan motor recently then that should not be the issue. However it could be the control or the motor speed resistor in the plenum. Look at the location for the fan, there should be a set of wires that connect to a plug that appears to have screws in the edges and it attaches into the plenum. This holds the resistor in place. They can and do go bad.
Check the simple things first! 1. ) Check your spark plug firing order. There is some bad info on the internet, make sure the firing order you're using is correct and DOUBLE check. 2.) Check your vacuum hoses. Make sure the hoses are good and none of the connections just came loose. 3.) Check your fuel filter. 4.) Check your engine timing. 5.) Check your Carburator adjustments.Most of the time these simple checks can take care of the problem.
depending on the car your serp belt may control all functions from alternator to power steering as well as the a/c make sure it was the a/c compressor that caused this it could have been coolant from the radiator or the high pressure release valve if its running rough it might be an issue with injectors is the check engine lite on. I would recommend a garage or dealer
If you have the Auto temp control; and you have checked the coolant level then you may need to replace the module that is under the dashboard on the right side footwell,. this is a common problem on these cars. When you remove the plastic under dash panel, look up and there is a module in the duct for the heat that determines the temp so the Climate control works correctly. I pulled mine out of a junkyard car and saved about 80 bucks.
If you click the link you can watch a video of the job on a 96 Impala which uses the same engine as your Buick that being the Chevy LT1 5.7L Corvette V8.
