Gas smell coming from vents "Please help!"

SO I just purchased a 1996 Plymouth Voyager from my dealer two days ago. It purrs like a kitten and runs smoothly. HOWEVER, when I am ideling with the vehicle on an incline I have this real strong raw gas smell coming from the vents. This ONLY happens when the vehicle is on an incline. I let it idle for a half hour yesterday while on the flat and nothing, nothing at all. No smell, not even a normal musty smell that you get from older vehicles. I took it back to my dealer yesterday prior to knowing that it happens only on an incline and they could not find a single leak anywhere at all. So I got it back and that was that. Well when I got home again and the vehicle was on an incline that is when I realised that was the key to getting the smell back. When I called my dealer again and explained what I found out, he was baffeled, explained that this doesn't make sense whatsoever and told me that he needed to investigate this prior to bringing my vehicle back into the shop. I am hoping that someone will be able to get back to me and help me out with this. If you want just email me...or reply here in the forum either or, but I would relly like to find out where the raw gas smell is coming from.

check your evap canister ?