i think it's the gasket in the up pipe. I noticed to day when I was trying to pull off the heat shield on the up pipe that it smelled like fuel and felt some air. I couldn't tell too much because I couldn't get the heat shield off because the bolts were too high and i did not have an extension. when I get a chance i will pull everything off to make sure. can you tell if there is a crack in up pipe or header? thanks for responding.