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Rough idle/running lean

xjAdam97

NAXJA Forum User
Location
Connecticut
What’s up guyses, having an issue with my 98 Xj. Got a running lean code and a pretty bad idle, almost stalls/has stalled @ stop lights and what not. Iv been chasing the problem for a bit now. recently did CPS, MAP sensor, IAC and TPS along with both 02 sensors. Exhaust pushes out a pretty decent amount of soot, so dunno if my cat is clogged bad or what. Any help would be appreciated


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I would check the injectors to see that they are all functioning properly with voltage AND OPERATING IN THE PROPER FIRING SEQUENCE (i.e. color-coded injector wires not switched). All the injectors must have voltage applied to the injectors in the proper firing sequence. Then any cylinders not receiving fuel would be leaner and cooler in temperature. The exhaust header temperatures of each cylinder would be a PITA to measure with a laser thermometer to ID which injector is not opening. But the cylinder, not getting fuel, will be cooler. So, the cooler temperature of the intake manifold (where the injector is mounted) should be measurable with a laser thermometer. Likewise, pulling and examining spark plugs can also help verify/ID a non-working injector. Once you find a non-working injector, just swap it out with a working injector.

Best regards,

CJR
 
Have you hooked it up to an ODB scanner? Would be good to see the fuel trims. Have you pulled any spark plugs to review the color?
 
Have you hooked it up to an ODB scanner? Would be good to see the fuel trims. Have you pulled any spark plugs to review the color?


Yah, their black, all of them, same with the o2 sensors. Everything OBD wise checks out, no codes for any sensors, just throws a running lean code, which could be a million things lol


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I would check the injectors to see that they are all functioning properly with voltage AND OPERATING IN THE PROPER FIRING SEQUENCE (i.e. color-coded injector wires not switched). All the injectors must have voltage applied to the injectors in the proper firing sequence. Then any cylinders not receiving fuel would be leaner and cooler in temperature. The exhaust header temperatures of each cylinder would be a PITA to measure with a laser thermometer to ID which injector is not opening. But the cylinder, not getting fuel, will be cooler. So, the cooler temperature of the intake manifold (where the injector is mounted) should be measurable with a laser thermometer. Likewise, pulling and examining spark plugs can also help verify/ID a non-working injector. Once you find a non-working injector, just swap it out with a working injector.

Best regards,

CJR


Beautiful, I’ll have to give it a shot then, I’d imagine injectors are probably due at this point anyways. I checked the pressure at the fuel rail and everything seemed fine as far as pressure goes


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Lean code with black plugs sounds like an injector except that I'd expect the plug for that cylinder to be light colored. All plugs are black?
 
Lean code with black plugs sounds like an injector except that I'd expect the plug for that cylinder to be light colored. All plugs are black?


Yeah, when I changed them all of them a couple hundred miles ago, they were all black. Ima check the new ones when I get home, I cant imagine they would look any better then before.


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What Check Engine Light trouble codes are active or were download ?

Most 4.0L Cherokees have a cracked exhaust manifold, have you checked ?

What brand and part number spark plugs ?

What brand of O2 sensors ?

Have you tested the O2 sensors and confirmed that their fuses are good ?
 
What Check Engine Light trouble codes are active or were download ?

Most 4.0L Cherokees have a cracked exhaust manifold, have you checked ?

What brand and part number spark plugs ?

What brand of O2 sensors ?

Have you tested the O2 sensors and confirmed that their fuses are good ?


P0171- System too lean, bank 1
No cracker manifold
Champion coppers
Bosch o2s- I read somewhere that those are kinda junk
And fuses are good, actually just replaced one a few weeks ago for the front o2


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There are many threads on different 4x4 forums that say Bosch O2 sensors and the Jeep 4.0 do not play well together. Champion Copper plugs are recommended.

NTK O2 sensors are what Jeep used and so should you.

JEEP P0171 Fuel System 1/1 Lean

Possible Causes

Exhaust manifold leak
Faulty front heated oxygen sensor
Ignition misfiring
Faulty fuel injectors
Faulty Mass Air Flow (MAF) sensor
 
There are many threads on different 4x4 forums that say Bosch O2 sensors and the Jeep 4.0 do not play well together. Champion Copper plugs are recommended.

NTK O2 sensors are what Jeep used and so should you.

JEEP P0171 Fuel System 1/1 Lean

Possible Causes

Exhaust manifold leak
Faulty front heated oxygen sensor
Ignition misfiring
Faulty fuel injectors
Faulty Mass Air Flow (MAF) sensor


Well to rule out injectors, I could just unplug each one at a time and look for any changes in idle. If one of them of shot, I’d imagine it would show no change in how it runs if I unplugged it


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Well to rule out injectors, I could just unplug each one at a time and look for any changes in idle. If one of them of shot, I’d imagine it would show no change in how it runs if I unplugged it


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Yep. That would be a good indicator.
 
The Bosch O2 sensors are probably your most likely suspect, followed by O2 sensor wire harness issues or damage.
 
Last edited:
"Well to rule out injectors, I could just unplug each one at a time and look for any changes in idle. If one of them of shot, I’d imagine it would show no change in how it runs if I unplugged it"

This test is contingent on every injector being properly electrically connected (i.e. proper color-coded wire on proper injector) to insure the injectors open in the proper firing sequence. If the color-coded injector wires are connected to the wrong injectors, the injectors will not open in the proper firing sequence and you will not obtain useful information.

Best regards,

CJR
 
Trace the forward O2 sensor wire harness - from the sensor connector - to where it enters
the plastic fuel injector wire harness rail (near above the #1 exhaust port). Make sure that
the wires aren't melted, or badly spliced, or goofed in any way. Be sure that the wire
harness is routed correctly (above the power steering pump, then down).
 
each injector when unplugged one at a time should give the same reduction in idle speed a completely clogged injector,once disconnected will make no difference however a partailly clogged injector will make a difference in idle when unplugged, but the difference in idle speed will be less than is the injector is not clogged not all

So the change in idle might be still noted with a partial clog, So compare the change in idle between all six, each unplugged injector should yield the SAME change in idle if all six are ok.

of course make sure your spark is not the failure, if the spark is dead, unplugging the injector will make no idle change.
if you i.d. a bad injector, swap it to another cylinder, and redo the test. the problem should move with the injector to the other cylinder, if the problem does not move, then the problem is with the spark, or valve, or compression, intake leak or such.

good luck
 
ps I recently had a bad spark plug, cracked ceramic. this caused intermittent missing as spark shorted to the block. I could see the arcing at night when the boot was pulled back, it also zapped me. the crack was tiny, hairline.
 
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