Verified answer

if the exhaust system is tight, then look at the pcv system, it captures fumes from the engine and burns them out the exhaust..

One of the most overlooked culprits is a leaky trunk seal. Even when the exhaust system is tight, it can be drawn into the car from the rear when in motion. If you've ever driven a station wagon or something with the rear gate/hatch window open, you've probably experienced this. New trunk seals are readily available. Some folks have found a new seal still won't do it. In extreme cases one person gets locked in the trunk while another goes around the edges with a bright light to spot gaps from the inside. Suprisingly often the trunk lid is warped and/or out of adjustment. A new seal can only do so much. Any other openings/holes in the trunk area can also draw exhaust in.

Other things can be responsible but for some reason air leaks at the rear of the car can draw exhaust stink in like a vacuum cleaner.

I agree with the other gentleman. Have exhaust checked at once. You do not need to get ill. Never mind the new interior and your clothes. You may check yourself by looking for black around connections under car. Another way is to have someone drip trans oil slowly into carburetor. While you check exhaust for the, now tell tale, smoke.

Having your exhaust welded at each connection assures seal. Check for a cracked exhaust manifold. Also the connection there.

One more thing. Rapid entry of fumes indicates you have not properly sealed the interior. Remove carpet and check your firewall and floor for holes and unsealed metal seams. Include your trunk, package shelf board and the thick paper seal behind rear seat for this. IE you should not be able to see your rear seats inner springs from the trunk? This is also a great chance to check and see if you've proper sound deadener and heat insulation. With all properly sealed up you shouldn't have really had enough fumes coming in to stink it all up. Except at a dead stand still and dead air.

probably coming through the firewall. pretty common if you dont seal every little hole up good. steering column etc. sort of a sick motor exhaust smell. the oily fumes off the motor get in you clothes and everything else. you better get it fixed bro or you might end up in the slammer for D U I.

did you check all of your exhaust connections ? do you hear a odd noise when it is running? check for exhaust leaks .do it soon before you get sick from the fumes or worst. take it to a mechanic to find the problem if you can,t.