Here's the problem. Recently I noticed a smell from the exhaust, kind off smells like when you start the car in the cold winter mornings and let it idle for 5 minutes. Rich gassy smell. I get this smell after some highway driving (30 miles) and a few short trips after. The entire exhaust system (header back) is very new, even the O2 is new. The tail looks clean although there is always a tiny amount of carbon present. No codes, and fuel economy is about 23 mg. Tune parts and plugs are have about 20K miles on them.

The only thing I've done is disconnected the hot air duct and thermostat. It's been about 40-30 F here. Could this be the cause?