Bad smell comming out of vents Acadia 11

Make sure you have the switches in the correct position to switch from gas to propane.

what motor ?

only found this gmc acadia specific forum
http://www.acadiaforum.net/index.php

sounds like the blend doors are stuck shut. when you switch it to defrost does the air come out the windshield vents?

There is an antibacterial aerosol spray available to spray into the inlet of the vents to kill the bacterial growth in the vent pipes.

Throttle position sensor or mass air sensor, more than likely TPS sensor going bad and hasn't thrown code yet.

I think you are getting a rotten egg smell,not natural gas. I would check for exhaust leaks from the catalytic converter for starters. Have you checked the grill at the base of the windshield and on top of the firewall? folks have been known to drop eggs down there as a prank.

gas is a very high evaporative liquid..fumes extremely strong..if overfull it will smell..and if emissions equipment has issues it will smell also.. when the engine gets hot, any gas present will vent off until the engine cools..thats the fuels evaporative nature..still keep an eye out for issues..

Virtually any vehicle can be converted to LPG. The key is understanding what is available in your area and what is more cost effective. This will involve a conversion kit. It is not as expensive in fuel injected and Direct injected motors as it is in carburetor motors but there is still considerable cost. You are talking about adding a Pressure tested tank and the cost for that will depend on whether you go natural Gas or Propane. Both are doable. Both require a regulator be installed. Natural Gas typically requires 1300 PSI in the tank where Propane is at a much lower pressure. Both are cleaner burning and theoretically, you should notice I used the word "Theoretically", as both natural gas and Propane are said to have an equivalent Octane level of 104 versus the high octane rating for unleaded or 93 to 97. Because of the higher Octane you should get better mileage but in conversions it doesn't always work out that way. It has been my experience that in factory built LPG vehicle to experience a higher mileage rating but that is not always so in Conversions. You also have to consider that unlike Gas powered vehicles in most states you are required to get a registration sticker that is sold to at a cost of your estimated usage because you are taxed once for the gas instead of by the gallon. You have to estimate how many gallons of LP gas you are going to use and pay the taxes up front instead of as you use it. If you choose propane and have Propane used in your house you can have what is called a wet line installed in that tank and be able to fill up at home, this is not an option for the Natural Gas conversion. At first glance that 73 cent a gallon for running propane is very tempting but when you add to it the upfront cost of taxes every year that actually ends up running some $4 a gallon making it more expensive that regular gas powered vehicles. I looked into all of this when I considered opening a conversion business a few years ago and got the shock of my life. Then working for a company that used conversion vehicles that ran on LP gas found out that on LP, in the conversions anyway, they were prone to vapor locks and slow acceleration or hesitation at acceleration. It wasn't so bad in the vehicles that started out from the factory on LP but those were still problems. Add to that that there was not a marked improvement in mileage, in fact there was a loss in most cases, it is not cost effective to make this conversion.

Dead field mouse or cat? Usually caused by build of smoking and other odors that stick to the condensation created by the a/c and eventually becomes kind of a permanent smell in the vents.

burnt or unburnt gas? if burnt gas smell look for bad manifold or manifold to pipe (donut) gasket.
if unburnt smell look for gas line leaks