Virtually all 4.0L exhaust manifolds develop a crack where the downtubes meet the collector. Welding up the crack usually just makes a new crack appear farther up the tube.
Step 3: Smoking Pipes
We did a lot of research when choosing a header and after-cat exhaust system for our Cherokee. We needed 50-state legality, we wanted complete stainless steel construction and we insisted that it not be loud. In our opinion, there's nothing worse than a loud exhaust on anything except a V-8. We found exactly what we were looking for in Gibson's stainless 4.0L header (PN GP400S) and after-cat exhaust (PN 617200). Since we're lazy, we took our Cherokee to Gibson in Corona, California, where technicians Guy Parsons and Shawn Seidelman installed the header and exhaust in about three hours. Not surprising, since Parsons is the company's header designer and Seidelman designs the after-cat systems.
The header hooks to the downpipe with no problems, but on later H.O. engines with the rounded intake manifold, you'll need to make sure the intake and header aren't contacting each other as the bolts are torqued down. Gibson's install instructions should cover everything the do-it-yourselfer will need to know.
The stainless steel after-cat exhaust features a mandrel-bent tailpipe, a polished stainless exhaust tip and a stainless steel chambered muffler. Allow us to gush here for a moment. This exhaust system just floors us with its power and sound levels. From inside the cabin it's every bit as quiet as the stock exhaust, with only a slight burble noticeable from outside the vehicle when idling. The fitment is great, with no rattling or contact anywhere.
Seat of Pants: Dramatic difference in power. Off-idle response is very snappy and almost violent, with much improved acceleration across the board. Feels like there's much more low-end torque as well as high-rpm horsepower.