Smelling Exhaust/oil in cabin

[GNICETRY]

What do I have to do to find out the problem? When I decel it is very noticeable. Like don't want to drive it noticeable. It used to be when I had the sunroof only open it would smell, now even with the roof open and both windows on decel it is pretty bad.
I do have an electric cutout (looks like it is closed), oil leaks, missing driver's side weatherstrip and both valve covers vented to a catch can with a breather.
I am leaning towards exhaust since it is mostly on decel after boosting.

 

[TurboBuRick]

You running a pcv?

 

[GNICETRY]

GM PCV and check valve (don't know which one) with catch can. I do want to get the US Plastics check valve though. Lots of vacuum on decel after accel.

 

[groumoutis]

Do you have the cowl to hood seal? My car didn't when I bought it and had the same issue.

 

[GNICETRY]

Passenger side seal is on. Driver's side is not (I cut it a while ago). I don't get the smell from the vent with all the windows closed. I am going to buy the US Plastics check valve, new GM PCV and then put the complete seal on the cowl and will see what happens.