I have a 2001 Expl. 6cyl. 4.0. I looked under it to see the exhaust system because I suspect the catalytic conv. is clogged and causing pressure build up in engine. What I found is what looks like a small cat. conv. coming down off the headers, and then down from there on a bolted flange to a large cat. conv., and then a little further down another cat. conv., and then down from there to another bolted flange to the pipe leading to the the muffler. Seems like a lot for an exhaust system. Is this correct. A small one, then a larger one, and then behind that one another large one, and then the muffler. Single tailpipe, mind you. Just feels like the vehicle needs to fart. Pardon the language. Vaccuum ok. New EGR valve. New PCV. But seems like the gases don't have a clear path of exit.