The
Ocala Jeep Club of Florida 
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Jeep Tech— 4.0 Exhaust Manifold Upgrade: by Ray Woo Universally, every 4.0 liter engine in Jeeps from 1991-2000 had problems with a failure of the exhaust manifold (the red circle in figure below) where the two collectors join into the downpipe. Poor design, engine vibration, and aging motor and transmission mounts all contribute to this problem. Jeep attempted to solve the problem in 2000 by adding flexible couplers, with variable success, and welding of the cracks is only a temporary solution. Replacement is the only option, and is rather difficult because the exhaust manifold shares a common gasket with the intake manifold, and both manifolds and the power steering must be removed. Popular replacement/upgrade options include a variety of aftermarket headers. Although this article is written for an XJ, it’s essentially the same for all other models.
The cheapest header I found was a $120 direct copy of what Banks wants over $400
for their Torque Tube. It fitted perfectly and came with a new gasket and O-ring
for the collector.
Fuel injectors can be swapped with OEM units from any 5.0 Mustang motor. I bought new Denso Mean Greenie injectors from http://www.fiveomotorsport.com. Although this job took a whole day, the results were great, with better mileage, more power, and no more exhaust leak!
Injector Removal Remove the throttle cable and cruise control assembly by removing the three bolts holding that in place. Lay it over the valve cover. Depressurize fuel system using the Schraeder valve on the fuel rail. Disconnect quick-disconnect fitting on fuel lines by depressing the white nylon clips each side and sliding it back. The high pressure and return lines have different diameter fittings to avoid confusion when reconnecting them. DO NOT bend the fuel supply line. It's a flexible plastic piece, but take care not to flip it around too much or you may pinch the line. Label the electric connector for each injector (1-6) and tuck it safely out of the way. Don't screw up the numbering! Unplug injector wires from the injectors. On earlier models (ie. pre 95) you may need to carefully pry off the u-shaped spring clip that retains the plug onto the injector. Check to be sure that the rectangular rubber gasket inside each connector is retained. Unbolt the four bolts that hold the fuel rail on the manifold. The rear most bolt has a stud that holds a clip for the wire from the Crank Position Sensor. Label this wire and keep the clip. Tug and wiggle the rail upwards at each end and along the rail pulling back hard until it pulls free (with injectors attached). NOTE: if you are not replacing the injectors this method keeps the upper O rings on the injectors intact on the fuel rail, saving you worry about possible leaks, You can clean up the bottom ones by removing them and soaking them in diesel for a while and giving them a good rub if they are still good. Otherwise you will have to buy a complete kit to replace upper and lower ones. If you are replacing the injectors, using a flat screwdriver to carefully pry the clips off the rail-end of the injectors and remove the injectors from the rail. They are retained only by o-rings so they need a straight pull to remove each. Remove any o-rings that got left behind in the manifold or the fuel rail. Use a small screwdriver to clean gunk out of injector holes in manifold - carefully as to avoid dumping gunk into cylinder. Manifold Removal Remove the auxiliary fan, intake air hose and airbox. On pre-95’s you do not have to remove the fan because it is smaller than that found on later models. You can do almost everything from up top (ie. reach all but 1 or 2 of the lower bolts) if you remove the intake air box The two toughest items to get off will be the power steering pump and the catalytic converter exhaust connector. Loosen and remove the serpentine belt. Remove the two slider bolts on the rear of the power steering reservoir, and remove the top bolt from the steel PS reservoir bracket in front. Remove the four bolts holding the larger cast aluminum PS bracket to the front of the manifold and water pump housing.. The key to getting the power steering pump off is the center bracket bolt off. This bolt is in the front hidden behind the pump itself. You have to take the slider bolts off and rotate the pump out of the way to get to the center bolt. This center bolt connects the power steering pump bracket directly to the intake. Disconnect the exhaust downpipe from the exhaust manifold; there are two nuts on the exhaust side that are probably rusted in place. Soak them with PB Blaster early on and try it again now before pulling the manifolds off. Disconnect and label the electrical connections on the throttle body (throttle body position sensor, idle air bypass control). Pop off the cruise control, transmission kickdown, and throttle cables from the throttle body. Unbolt and remove the throttle body. (Now’s a good time to spend a little time cleaning the throttle body, butterfly plate, and idle air bypass solenoid passages.) Remove and mark (if necessary) the vacuum hoses from the intake manifold, the electrical connector for the Intake Air Temp sensor, and the connector from the Crank Position Sensor. You can see from the picture the overlap of the exhaust and intake manifolds. Since they employ the same bolts replacement can be a bit of a bear on the bottom. The intake and exhaust manifolds are held by five upper bolts above and four underneath the intake manifold. In addition, there is one stud and an alignment dowel on each end. It’s easiest to remove the intake manifold first before tackling the exhaust manifold. Remove all the bolts and except at locations #6.7 and 3. I was able to get the all the bolts off with the 9/16” Gearwrench and socket with a 12” wobble extension. Remove the intake manifold. Spend some time cleaning the carbon off the intake and exhaust ports and the gunk in the injector ports with engine degreaser, a flat screwdriver and a rag. Use the wire brush and drill to clean any gasket residue on the mating surfaces of the manifold. Remove the nuts from the studs at #6 and 7, and the bolt at #3. NOTE: The stud at #7 is VERY COMMONLY broken. Use lots of PB Blaster penetrant and go slowly. Be prepared to drill out and replace the stud if it breaks. Remove and discard the exhaust manifold. Clean any gasket residue from the cylinder head with the wire brush. Clean the bolt holes with a tap. Consider replacing the Crank Position Sensor at this time. It is located on the top of the transmission bellhousing at the driver side of the transmission tunnel. It’s very hard to replace from under the Jeep and relatively easy with the exhaust manifolds removed. A bad CPS is a very common reason for starting/running issues on the 4.0 engine and is known to crap out at the most inopportune time. It is held by two 7/16” bolts. Be sure to reuse the rubber plastic cover that covers the access hole in the bellhousing around the CPS. Save your old CPS. Inspect the freeze plugs on the engine block for leaking or corrosion. Jeep used steel plugs which corrode easily. It’s a lot easier to replace them now with brass ones rather than discovering a leak after the manifolds have been replaced (don’t ask me how I know that!).
Reinstallation - Manifolds
Position the exhaust manifold on the mounting studs and dowels hold in place with a finger-tight bolts ands nuts at # 6,3, and 7. Use LocTite #272 (or equivalent) to retain them (any other formula will break loose because of the exhaust heat.) Install the intake manifold and secure with the upper bolts, Install the lower bolts. This is a royal PITA!. Actually, the only one that posed a real problem was bolt 1. The pipes obscure a direct attack on the bolt from the bottom and the intake from the top. Bolts #4 and 1 can be accessed from the front, and #2 from the side. You will need to get under the Jeep to get #5, Use the swivel sockets, wobble extensions, and gear wrenches to start. After starting all of the nuts and bolts its important to torque every one in order.(See below) This will keep the manifolds from torque warping, which could cause a leak. It's always a good idea to initially torque all bolts to a value lower than the final figure and then retorque everything once the first set has been finished. It was impossible to use the torque wrench on the bolts underneath, so do you beast with the wrenches.
Reconnect the exhaust at the downpipe. Be sure that the mounting flanges are parallel to each other. I had to install the upper bolt from the bottom. Assembly - Injectors Each injector is held in place by the fuel rail, and pressurized fuel is held in by the use of two o-rings per injector. Use the old injectors as a reference, but the basic idea is to lube the o-ring to prevent tearing and then install on each injector. We used the Valvoline Synth lube, but a small dish of clean motor oil is recommended. Make sure the injectors are inserted the correct direction with the electric connector facing up. Lightly coat the outside of the new o-rings on the new injectors with oil. I found that just enough to shine the surface without globs hanging off was sufficient. Press injectors into fuel rail. If resistance seems to be risking damaging an o-ring, add more lube and try again. Reapply injector clips. Some aftermarket injectors do not have the same slots found in the OEM injectors for these clips. If so, then discard the clips. Line up fuel rail with manifold, and insert injectors into manifold. Bolt down fuel rail. Reattach fuel line at quick-disconnect fitting. Be sure to attach the high pressure line to the fitting at the very end of the fuel rail. Reattach the throttle body cable bracket. Plug injector wires into appropriate injectors. Reconnect everything else. The last obstacle is reinstallation of the power steering brackets. It is difficult to align the bracket holes. This is solved by starting whatever bolts you can first, keep the bolts loose, and start the last bolts with someone pulling up on the pump assembly. Restarting the car When starting the engine, pay attention for possible gas leaks at the injectors as well as exhaust leaks from the manifold and exhaust connector. There will be some smoke as the oil and grease on the manifolds burn off. Retorque the manifold bolts again after 2 weeks of driving.
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