The TJ 4.0L used a few different manifolds through its run; some years used a tubular steel design similar to a header and others used a cast-iron design. Earlier versions used a straight 3-into-1 to 2-into-1 collector design; later versions used a 3-into-1 with dual output design. The latter used small catalytic converters right after the manifold followed by a 2-into-1 collector.
Performance exhaust systems are not cheap but often can last for the lifetime of the Jeep because most are stainless steel. If your Jeep has high mileage and is in need of replacements to the exhaust, consider a high-performance system. MagnaFlow, Borla, Flowmaster, and aFe make replacement and performance exhaust components for the TJ.
aFe Performance Exhaust Installation
If you need to replace a rear main seal or oil pan gasket, now is the time to plan for that. Removing the oil pan usually means dropping the exhaust, so you might as well do both.
This TJ has more than 193,000 miles and the check engine light is illuminated, indicating a problem with the catalyst system. Onboard diagnostic (OBD) codes can be viewed on a TJ by turning the ignition key on three times in 5 seconds. The cycle is on, off, on, off, on. The speedometer will display any code generated by the Jeep. In this case it was P0432 indicating that the catalyst system was below efficiency. The issue is likely an oxygen sensor, but the old cats are way past their useful life. The muffler is not leaking yet, but the outer shell is falling off from rust. Replacing the whole system is money well spent. A performance system from aFe was chosen; it included new catalytic converters, mandrel-bent pipes, and a free-flowing muffler all constructed of 409 stainless steel. The factory manifold is in good condition and will be retained. All four O2 sensors will be replaced as well with the system.
O2 sensors are a necessary evil in modern engines. These sensors measure the oxygen level in the exhaust gases, which allows the computer to adjust the fuel mixture to achieve optimum burn efficiency. Later-model TJs used four sensors that read O2 before and after the catalytic converters. When replacing sensors, check with a reseller to make sure the proper sensor is used because Jeep used a variety of sensors.
The TJ exhaust always crossed under the engine in some location (depending on the year) to allow the pipe to run down and eventually exit on the passenger’s side of the Jeep. The crossover was most often below the oil pan. Dropping the oil pan requires the removal of the exhaust system because of this crossover.
Before starting the project, disconnect the battery and properly support the Jeep to allow easier access from underneath. The center skid plate will need to be removed to allow access to the catalytic converter and other sections. Support the rear of the transmission/transfer case with a stand or jack. Remove the transmission mount nuts and then the six skid plate bolts. If the Jeep has an automatic transmission, you may need to deal with the transmission pan skid plate. Lower the plate and move it out of the way. It’s easiest to start removal of the old system from the rear and move forward.
1 Remove the center skid plate by supporting the rear of the transmission/transfer case, remove the mount nuts, and remove the six frame bolts. The muffler and catalytic converter are easily accessed with the center skid plate removed. This difficult-to-reach area can trap mud and trail debris that increase rust potential and wear, so use this opportunity to clean here.
2 A reciprocating saw makes removal of the old system quicker because unbolting old clamps may be almost impossible thanks to rust. Cut the pipes between the muffler and tailpipe, muffler and catalytic converter, and the catalytic converter and crossover pipe.
3 The aFe tailpipe is compared to the stock pipe. The stainless-steel construction will allow this pipe to serve the Jeep for a lifetime and the smooth mandrel bends along with the larger diameter pipe allow the exhaust to flow more freely. The hangers attach at the exact stock locations, which eases installation.
4 The aFe performance muffler, compared to a stock muffler that is rusting away. The smaller size and freer-flowing internal design adds horsepower to the Jeep along with a pleasing exhaust tone. The stainless-steel construction will likely allow this muffler to serve the Jeep for its lifetime.
The later-model TJ dual mini catalytic converters are located right after the manifold. They are equipped with a downstream O2 sensor immediately after the mini cat. The crossover pipe goes under the engine oil pan, taking the exhaust to the passenger’s side.
With the rear sections removed, the crossover and lead pipes are next to come out. These are difficult to reach and will require patience. Unplug the lower O2 sensors before removing the bolts. With the lead pipes unbolted, remove the lead pipe and crossover.
Because of this Jeep’s excess mileage, all of the O2 sensors (four in this case) will be replaced, so their removal from the old lead pipes isn’t necessary unless the sensors are to be retained. At this point, the upper O2 sensors are unplugged and removed, followed by installation of the new sensors. Access to the sensors with the lead pipes removed is much easier.
Attach the sensors back to the wiring harness. Then install the mandrel-bent section of the lead back to the main catalytic converter. Install the supplied band clamp loosely.
5 Installing the new O2 sensors is easy with the lead pipes out of the way. Each sensor should come with some installation lubricant that aids installation and prevents the sensor from seizing over time. Tighten to manufacturer’s specifications; be careful of the wiring hanging from the sensor.
6 Install the new O2 sensors in the lead pipes’ section and tighten to the manufacturer’s specifications. Raise the section in place and loosely bolt the section to the manifold with the four bolts. Observe the front driveshaft clearance and adjust to prevent rubbing.
7 Position the aFe converter in place to line up with the factory bushing hanger. The aFe directions indicate to cut the lead pipe 3.25 inches from the end when using the aFe converter. Tighten the supplied band clamp loosely.
8 Complete the installation by installing the muffler to the converter at the flange and attaching the tailpipe to the muffler using the supplied band clamp. The factory hanger locations should match up.
After everything is installed loosely, begin tightening from front to back until everything is properly tight and fitted in the hangers. Be certain to attach all of the new oxygen sensors to the factory wiring. Start the Jeep and listen underneath for leaks. After 50 to 100 miles of driving,