My Dad has a 2003 LeSebre and just had a new fuel pump installed. Now occasionally when he starts the vehicle thick white smoke comes thru the ac vents. The mechanic said to turn on the key to accessory, wait 5 seconds for the pressure to build up and then start the vehicle.

I am suspecting an electrical issue, possibly insulation burning, in the firewall area.

Any suggestions?

 

I believe you are on the right track. Usually you can smell the odor of burning insulation. Just how it is getting into the ductwork is at this time a mystery.
It is no doubt something that is activated by turning on the ignition. One way to possibly find it is to remove the fuses that are live when the key is turned on. Then start replacing them one at a time to see which one produces the smoke. That will let you know which system is involved. If you can do this and pin it down to which fuse , I should be able to assist you possibly locating the problem. What do you think? ,,,hanky

 

I'm thinking it's the heater core module. I told him to turn off the AC before starting the car and he doesn't seem to have the issue. Now I need to figure out why its doing it.

 

If you keep a close check on the coolant, it could be anti-freeze from a leaking heater core. Does it have a burning "fragrance" or stink like anti freeze? If you can check at the bottom of the heater assy and if it is the heater core you will feel the slimy feel of anti freeze. I can't think of a better way to describe it, also the carpet under the heater might be wet with anti-freeze. If you turn the defrosters on when you first start the car and it fogs the windshield-- it is the heater core. Not a hard job to replace. ,,,,,,,,,hanky