2008 g35x bought at beginning of summer with around 50k miles on it.

Drove fine all summer AC worked great, never smelled any gas smell inside. Winter got here, it got cold, now when it turns pretty cold and I run the heater I notice a strong gas smell inside the car. It is strongest when car is stationary like at a stop light and when air flow is coming from outside. I can minimize the smell if i turn the air on "re-circulation".

I took it into dealer, they jacked it up and checked for leeks, but said they couldn't see any, there is no overheating and fuel mileage seems normal 17-20 gpm.

Also took it to a private mechanic who ran a camera down by the air intake to check if there was any leakage around there and said he found some valves or something that weren't completely tight and they tightened them....seemed to help for a few days but also the weather wasn't to cold at that time and when it dipped down into the low single digits i have noticed the smell even worse now.

Anyone have any ideas?