NHXJ

98 xj intermittent misfire under load. seems to be more mostly in 2nd gear (auto trans) around 20-30 mph (according to my speedo, but im on 35's)

had an old code for a tps issue so i tested that out, cleared the code, passed the test perfect, and code has not come back on with the issue going on still at least 6 trips.

when it happens it has zero power, pops out the exhaust loud ( magna flow)

know issues so far,

-fuel gauge code as well as stored code for gauge voltage
-incorrect speed gear
-flex plat bolt had gone loose and made slight noise but were re tourqed and inspeded flywheel for zero cracks ( flywheel was replaced as well was the trans around 55k ago )
-cracked exhaust manifold
-exhaust bent about 6-8 inches after 1st o2
-jeep has been wheeled haaard and has had a decent amount of electrical issues from mud
-valve cover leak, rear main leak ( not major)

not sure where to go. the last month iv been driving it about 120 miles a day 5 days a week, but it never happens on the highway or off ramp. only at low speeds.

adjusted the kick down cable off the throttle and it went away for as while, however my driving may have been a little harder doing those times.

few specs on the truck,
98 Cherokee 167xxx auto, 6.5 lift, 35's

when it happens people around me jump but if i smash the throttle it downshifts and its like nothing happened until i get to the same load again.

also had an issue with the temp gauge before, replaced sensor to fix.

Radi

Load-dependent misfire and exhaust backfire? Just for giggles, disconnect your MAP sensor and see if anything changes.

tjwalker

You still could have a throttle position sensor issue.

Also need to verify fuel pressure. For a 98, you should have 49 psi, plus or minus 5 psi. Must use a gauge; no shortcuts.

TheJerm

Co-worker had access to All-Data and pulled alot of factory service bulletins. Well for the 2000 XJ (thats the file he sent me and could relate to other years) have an issues with intermediate misfire under load due to exhaust valves not spinning, crazy i know. But I guess carbon can build up on the valves and cause them not to spin, which in turn cause the valves to be slow and cause misfire. I dont know if this is the same thing but I just read thru the bulletin and i kinda had to laugh

NHXJ

interesting. not something i would of expected. whats the fix in the tsb?

NHXJ

tbs checked out 100%

psi is at 50 on the dot

NHXJ

wa thinking same thing yesterday. did a full map test and swithced it with a know working part and same issue.

however now after doing all these test it only happens now and then. i can get it to the same speed and same load and it wont happen.. happens maybe one out of every 10 times. very confused.

its turing into a dart board of parts.. only thing i can think of is the exhaust leak and bend giving it funky o2 readings every now and then and it not taking the load. but then when the exhaust flow increases it goes away

TheJerm

INSPECTION:
VALVE ROTATION:
NOTE :IT IS IMPORTANT THAT THE VALVE ROTATION SECTION OF THIS
REPAIR PROCEDURE BE PERFORMED.
1. Raise vehicle hood.
2. Remove the engine valve cover and all six exhaust valve rocker arms (the
intake rocker arms are also removed during this step). Refer to the appropriate
vehicle Service Manual for detailed removal instructions. The valve cover
gasket is reuseable. Keep each pair of rocker arms matched to their respective
valve and cylinder.
3. Inspect the end, or tip, of each exhaust valve stem where it makes contact with
the respective rocker arm.
4. Determine if each exhaust valve is rotating within its respective valve guide. An
exhaust valve that is rotating will have a "bulls eye" or circular wear pattern on
the face of the valve stem tip. If the exhaust valve is not rotating a straight
mark-like pattern will be present across the face of the valve stem tip.
5. If there are exhaust valves which are not rotating then proceed to the VALVE
ROTATION section of this Repair Procedure.
6. If all exhaust valves are rotating, then this bulletin does not apply and further
diagnosis is required. Install the engine rocker arms and valve cover. Refer to
the appropriate vehicle Service Manual for detailed installation instructions.
1. If one or more engine exhaust valves are not rotating, perform the valve
rotation procedure to all six (6) exhaust valves.
2. Clean and mark the tip of each exhaust valve stem with a paint marker. The
paint mark will be used later to assist with determining if the exhaust valve has
been rotated 90°.
3. Bring number one ( # 1) cylinder piston to top dead center using the mark on
the crankshaft front dampener/pulley. This step is important to prevent the
possibility of the exhaust valve from falling completely into the cylinder.
4. Install the essential service tool valve spring compressor, MD-998772A, to the
ALLDATA Online - 2000 Jeep Truck Cherokee 2WD L6-4.0L VIN S - Repair Procedure Page 1 of 3
http://library.alldatapro.com/alldat...6/78219301/782... 11/29/2011
NOTE : THE FOLLOWING VALVE ROTATION PROCEDURE WAS DEVELOPED
WITH THE USE OF VALVE SPRING COMPRESSOR TOOL MD-998772A. THE
MD-998772A IS AN ESSENTIAL SERVICE TOOL AND PART OF THE
DEALERSHIP REQUIRED TOOLS. THE MD-998772A COMPRESSOR
ENGAGES THE ENTIRE PERIMETER OF THE VALVE SPRING RETAINER,
UNLIKE OTHER MAKE VALVE SPRING COMPRESSORS.
DECARBONING COMBUSTION CHAMBER AND VALVES:
# 1 cylinder exhaust valve spring.
5. Compress the # 1 cylinder exhaust valve spring enough to gain access so
that the exhaust valve can be rotated 90°. Rotate the exhaust valve 90°.
Slowly remove the compression on the exhaust valve spring. Verify that the
valve keeper is properly seated to the valve stem and valve spring retainer.
6. The 4.0L firing order is 1 - 5 - 3 - 6 - 2 - 4. Without rotating the engine
crankshaft repeat steps 3 through 5 to cylinder # 6.
7. Rotate the engine crankshaft 1200 and repeat steps 3 through 5 to cylinders #
5 and # 2.
8. Rotate the engine crankshaft another 1200 and repeat steps 3 through 5 to
cylinders # 3 and # 4.
9. Install all cylinder rocker arms (intake and exhausts) and retaining bridge.
Make certain that the push rods are properly seated to their respective rocker
arm and lifter. Tighten the respective cylinder bridge/rocker arm cap screws to
30 Nm (21 ft. lbs.) when each cylinder piston is at top dead center (cylinder
intake and exhaust valves are closed).
10. Install the engine cylinder head valve cover. Tighten the valve cover bolts to
10 Nm (85 in. lbs.). Refer to the appropriate vehicle Service Manual for
detailed assembly instructions.
1. Start the vehicle engine and allow the engine to reach normal operating
temperature.
2. Remove the air tube from the engine throttle body.
3. With the engine at idle, spray the entire contents of Mopar Combustion
Chamber Cleaner, p/n 04318001AB, directly into the throttle body. As the
cleaner is being ingested, allow the vehicle to "load up" with the cleaner to the
point that the engine is almost stalling out. Maintain this condition until all of
the cleaner is used/ingested.
4. Stop the engine once the entire can of cleaner has been ingested.
5. Install the air tube to the throttle body.
6. With the hood closed and the vehicle parked inside the garage, allow the
vehicle engine to heat soak for two to three hours. This will ensure that the
engine will maintain its temperature and will allow proper solvent penetration.
7. After engine soak, start the engine and drive the vehicle until the engine is
has reached normal engine operating temperatures.
8. If the vehicle is equipped with an automatic transmission, place the gear
selector into "L" (low). If the vehicle is equipped with a manual transmission,
place the transmission into first gear.
9. In a safe vehicle operating location that will allow the vehicle to be driven
safely and at the posted speed limit, accelerate the vehicle until the engine
reaches 4500 RPM.
10. Hold the engine speed at this RPM for 15 seconds.
11. Slow down and in a safe location pull to the side of the road. Allow the engine
to idle for five seconds.
12. Repeat steps 9 through 11 five more times.
13. With the vehicle at operating temperature and using any available Freeze
Frame data recorded when the misfire DTC occurred, verify that the misfire
condition has been corrected.
14. Erase any engine DTC's once the misfire condition has been corrected.