Fumes in cabin while sitting in traffic
 

Just noticed that the cabin starts smelling exhaust fumy when sitting in traffic; checked/replaced cabin filters with no noticeable difference. 4.4 exhaust is stock and hasn't been monkeyed with recently, didn't see any signs of exhaust leaks at any of the joints. Any ideas?

A couple of things:

I am not familiar with 8 cyl, but could you have a valve cover leak that is dripping oil on exhaust ( a common 6 cyl thing)?

you might pull the large assembly that holds the fresh air intake tube above the rear of the engine and in front of /under wipers. I bet the foam seal has deteriorated..you coudl replace that foam for a better seal.

Check any other rubber grommets/seals in the firewall.

You could have developed an exhaust manifold leak - but you might hear smell that with car running.

What you're smelling is most likely oil leaks from the valve covers. They leak onto the exhaust and the smell comes in through the vents.

Its not the valve cover gaskets as they were just replaced and it doesn't smell like oil as I had that issue on my 540, more like exhaust. Didn't realize that entire intake assembly above the motor is removeable but I'm betting that after 18 years those gaskets are shot.