I used to be able to smell it inside the car - it wasn't good

If I'm not mistaken the car requires a certain amount of 'back pressure' to operate properly, so a badly cracked manifold or perhaps a completely worn out gasket between the block and the manifold could cause larger problems that would stem from improper EGR sensor readings caused by a lack of pressure. I'm not 100% sure though.

I think thats the source of my current poor idle, my exhaust is in awful shape. I'm going to take it in soon and I'll update this thread if I notice an improvement in gas mileage or idle performance.

Mine used to do the occasional backfire inside the exhaust manifold ..... Yeah definitely put a new manifold on...it might cost a bit but you'll make gains in fuel efficiency in the long run. It's also healthier for you & your car