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96 xj runs really rough and bucks at lower RPM

J

juicebox

NAXJA Forum User
Location
north carolina
hey guys i bought a 96 xj with a cracked head. i replaced the head with a resurfaced one. Jeep drove fine for about a week then developed a muliple miss fire code.

the jeep runs really really rough at idle and at lower rpms now. it almost feels like the timing is off or something. it bucks and bounces and has low power.

i have replaced every part of the ignition system minus the crank sensor and cam sensor.

does anybody have any clues to what my problem might be. im about out of ideas minus scraping the motor and getting another one.
 
Re: 96 xj rouns really rough and bucks at lower RPM

If the Check Engine Light is ON, the Jeep is trying to tell you what is wrong.

What CEL trouble codes numbers did you download with the OBD-II trouble code reader ?
 
Re: 96 xj rouns really rough and bucks at lower RPM

im not sure exactly what the code numbers were as it was a few weeks ago when i scaned it. but i do remember it was one code or multiple missfire and individual miss fire codes for atleast 5 (possibly all 6) cylinders
 
Re: 96 xj rouns really rough and bucks at lower RPM

Knowing the exact Trouble Code numbers gives you important clues about where and how to start your diagnostics.

Are all tune-up parts fresh ? Rechecked all the wire plugs and ground wire connections ? Have you tested the coil, the crank sensor, cam sensor, coolant sensor, throttle position sensor, O2 sensors, fuel injectors, and done a continuity check on their wire harnesses ? Battery good, and alternator charging ?
 
I no that the wires, plugs, cap, rotor button and coil are good as they are all brand new.
i am not shure of the cam and crank sensor though, i was thinking about those two sensors being the problems as well. how do i check to sensors. is there a way to bench test them or test them on the vehicle?
 
Trouble shooting steps and bench tests have been posted lots, using the Search function or a Google search restricted to NAXJA.org will find lots of great info.

Be sure to see the FAQ's also -> Frequently Asked XJ Tech Questions

I suggest spending $7.95 at PacificCoastManuals for a genuine Jeep Factory Service Manual with detailed diagnostic steps and test procedures.

Try this:

CPS Testing
TESTING PROCEDURE 1991 –2001 4.0L H.O. engines
1. Near the rear of intake manifold, disconnect sensor pigtail harness connector from main wiring harness.
2. Place an ohmmeter across terminals B and C (See Image). Ohmmeter should be set to the 10K-or-2OK scale for this test.
3. The meter reading should be open (infinite resistance). Replace sensor if a low resistance is indicated.


Crankshaft Position Sensor Connector (CPS/CKP)
.
standard.jpg

.
standard.jpg
.
 
Been in a similar situation before. Mine was running at 35 psi fuel pressure. Run a fuel pressure test. New fuel pump and it ran better than before the old one went. Also as for the idle, clean your IAC. You can do both of those for free and rule out some culprits. If you don't have a fuel pressure tester rent one from autozone.
 
I too would suspect a fuel problem, because of the multiple
misfires.

You can swap out the fuel pump relay with a similar one in
the relay box; see if that helps...

Probably a weak fuel pump though...
 
so i guys i just checked the fuel pressure and unless my gauge is off witch i dont believe it its i am running 50 psi at idle.

the gauge never moved with with the key at koeo. but when cranked and idleing the pressure was at 50 psi. isnt this really high for a 4.0. i thought they were ment to be abround 35 psi.
 
That's the correct fuel pressure (about 50 psi) for a
96 model. It has a returnless fuel system with the
regulator in the pump assembly, and runs higher
pressure than the earlier model years.

If it still runs rough at idle, then you can rule out the
fuel pump as the cause.

I think this is something simple that's been overlooked.
When you replaced ignition components, did you
replace the coil to distributor wire? If it's bad or has
poor contacts, that would possibly give multiple
misfires...
 
i have already replaced all plugs with champion plugs and replaced the coil, all wires including coil to dist. wire, dist. cap and rotor with borg warner parts.

after replacing the ignition parts it did smooth the engine out some but it still runs rough in lower rpm's. when the engine gets over say 2500 rpm it seems to smooth out.

Could the timing be an issue on these motors. thats one thing i have thought of but have not checked yet as i dont have a timing light.
 
Could it be that the IAC motor is bad and not able to smoothly adjust the idle mixture?
I had a bad one on a 98 and it threw misfire codes and smelled like rotten eggs.
One you got it up to speed it smoothed out.

-Ron
 
juicebox said:
i.....after replacing the ignition parts it did smooth the engine out some but it still runs rough in lower rpm's. when the engine gets over say 2500 rpm it seems to smooth out.

Could the timing be an issue on these motors. thats one thing i have thought of but have not checked yet as i dont have a timing light.
Click to expand...
The timing is not adjustable, but it can be thrown off if the
distributor has been installed 'one tooth off' with the cam
gear. That's not likely since the engine ran fine for a week
after the head work.

However, if the wires were installed one terminal off in the
distributor cap, that would indeed cause firing problems.
You can determine if the timing is correct by setting the
engine on TDC (compression stroke), and observe the rotor
location in relation to the number 1 cap terminal. The rotor
should be at the #1 wire position on the cap, but slightly
biased to the CW side.
 
winter beater how do u set the timing on it. i just figured if it was off it would need to be replaced.


i did check the crank sensor again with a better volt meter (fluke) and the ohms between teminals 2 and 3 according to that diagram posted earlier read 6.2 ohms.
im assuming this means a bad sensor right.
i am waiting untill payday to buy the sensor and try that. hopefully this will fix it.
 
Tested/replaced map sensor yet? Checked vac line from the sensor?
 
You show the classic symptoms of a bad Throttle Position Sensor. Don't just throw a
new part at it. Test it first. You'll need an analog ohmmeter (not digital) to do this.
It's very easy to do. There are plenty of write ups, and videos, online on how to do this.
 
There is a sensor in the distributor also, replacing my distributor helped clear misfire codes on mine
 
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