That's what they make O2 sims for...and why no header system for the 6 is CARB certified.

 

I had this problem when I first got my '04. I would smell it whenever I got out of the car after driving for a bit. I took it to the dealer but of course they couldn't recreate it but one of the mechanics told me to get gas from stations that had only 10% ethanol and that has worked ever since.