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Chasing misfire and running out of variables.

D

deloxx

NAXJA Forum User
Location
Massachusetts
Hello all, I have a 1998 Cherokee 4.0 5spd 4x4. I suddenly developed a cylinder #1 misfire and I am having a hell of a time finding the cause. I can feel the misfire at idle, hear it in the exhaust, and am receiving a P0301 code. The following have been tested and accounted for:





  • Compression: #1 150psi #2 165 #3 170 #4 150 #5 160 #6 180 (all normal an within spec)


  • Confirmed spark, also replaced rotor, dist. cap, wires, and plugs to be sure.
  • Ground resistance of installed spark plug is 0.3 ohms, equal to others


  • Tested injector wiring with a noid light, functions normal. Also tested voltage (14v running) and equal resistance to other cylinders wiring.
  • Swapped injector with other cylinders, no change.
  • Ran fuel rail removed from manifold, all 6 spraying equally
  • Fuel pressure normal, 48psi idle
  • Normal vacuum, 21in/hg at idle
  • Mopar crank position sensor installed last year as preventative maintenance
  • Cleaned connector c107 at the suggestion of another thread
  • cleaned alternator grounds at the suggestion of another thread
  • Removed valve cover and measured valve travel, intake and exhaust valves on #1 move equal to other cylinders
  • No intake manifold leak detectable (carb cleaner spray method)
  • Have run seafoam through the intake and MMO hot soak in the cylinder overnight, no effect.


I also have another 98 cherokee/comanche build going on, so I have access to whatever parts for comparison/testing. I will be trying the PCM off of that one tomorrow, though it is an automatic.


I have no idea what else to try.
 
Read Jeep Technical Service Bulletin number: 09-003-03

Then read this http://jeep.blackonyx.net/pdfs/jcss.pdf . Consider the possibility that the timing chain is slightly stretched and the Camshaft Pick-up Sensor in the distributor needs to be re-indexed.
 
Last edited:
The 99s had an issue with weak valve springs and building up carbon on the valve seats.

Best results were using BG 44K per instructions or using the old fashioned water down the throttle body trick. You can find videos on that.
 
Thanks for the replies. That TSB and the cam sensor resync in that second article both appear to b apply to coil rail vehicles (00-01). I did, however, confirm that the distributor was still synced correctly with the oil pump gear, which it was.



Additionally, these seem to refer to random cylinder misfire. Mine is only cylinder #1 and is constant (cold, hot, etc)


While mine is a 98, I will take a look at the valve spring seat for that cylinder. Also, that TSB did mention confirming that the valves are rotating, which it can't hurt to check.



Today I'll try my other PCM, I'll check that the valves rotate, I'll try swapping out my distributor switch plate (aka cam sensor), and I'll give BG44K a try if I can find it locally. I'll report back, thanks.
 
Following. I'm currently having similar misfire issues. I have another thread going on it but haven't made a ton of progress yet.
 
PCM swap did not make a difference.


I did discover that my intake valve on cyl #1 did not appear to be rotating after pulling the rocker arm off. It had a stripe of wear instead of a crosshatch/bullseye pattern on the top of the valves stem. I stuffed the cylinder with weed Wacker line (at TDC) and pulled off the springs, then spun the valve several times to ensure it was loose and able to rotate against the valve seat. Reassembled and there is still. A slight misfire remains, but no codes yet. I will drive some more tomorrow and see what the result is.



I did not have a chance to swap the distributor switch plate yet, but hopefully I won't have to. Additionally, I actually did the distilled water into the intake last week but forgot to mention that on my original post. I ran about 10oz through a spray bottle into the butterfly, it did not make a difference that I could tell.



Lastly, I ran a can of CRC intake cleaner through the manifold vacuum fitting nearest cyl1 (An EVAP line if I recall correctly)


I will take another compression test to see if it's changed at all.






(Also, can I attach pictures directly or do I have to host them on another site and link? I have only a few posts so that may be limiting me)
 
So the misfire has subsided, but is not entirely gone. It seems much lighter until the PCM recognizes it and cuts the cylinder from firing. I am under the impression that the cleaning methods have worked a bit, but not quite enough to completely clear out whatever buildup may be causing the issue.


For stuffing the cylinder, I chose the weed wacker line because it has enough rigidity to push into the cylinder, but will still spaghetti its way into the combustion chamber. The line I used it a heavier duty type, and I had no concern for it breaking off in the cylinder. It is plenty durable to hold up to the valve pushing against it.


At this point I am going to drive the car more, run some more distilled water through it, and hope for the best. Its got 240k on it, and if I need to pull the head next spring for some actual headwork, so be it.
 
deloxx said:
So the misfire has subsided, but is not entirely gone. It seems much lighter until the PCM recognizes it and cuts the cylinder from firing. I am under the impression that the cleaning methods have worked a bit, but not quite enough to completely clear out whatever buildup may be causing the issue.

For stuffing the cylinder, I chose the weed wacker line because it has enough rigidity to push into the cylinder, but will still spaghetti its way into the combustion chamber. The line I used it a heavier duty type, and I had no concern for it breaking off in the cylinder. It is plenty durable to hold up to the valve pushing against it.

At this point I am going to drive the car more, run some more distilled water through it, and hope for the best. Its got 240k on it, and if I need to pull the head next spring for some actual headwork, so be it.
Click to expand...

Pretty fascinating and cool way to support the valves.

My XJ was misfiring at higher rpms and load after I bought it. New spark plugs and coil rail helped, but didn't totally eliminate it. I found that driving it at higher rpm and load slowly resolved the issue, so it was likely carbon buildup on mine.

As a side note, many XJ owners swear by copper electrode plugs, but double iridium plugs are working good for me on my 2000 XJ.
 
How high do you guys rev your 4.0s? I'm curious.

How high were you running the engine when putting the water into the throttle body? What method to introduce the water?
 
I just tried this a few days ago. I got the engine to running temp, then wedged open the throttle to 2500 rpm and started pouring in water, I would pour in enough to hear it start to bog down and when it would even out I added more. 32 ounces total. The only thing that made me scratch my head was that there was never any visible white steam from my tailpipe during this process. Everything I read and videos I watched there was white steam when people did this. Maybe I did it wrong?
 
cruiser54 said:
How high do you guys rev your 4.0s? I'm curious.

How high were you running the engine when putting the water into the throttle body? What method to introduce the water?
Click to expand...

I've not done the water thing, but on the road, I have surpassed 3500 rpm almost every time when passing on a grade. There are a few times when I passed 4000 rpm. Seemed concerning at the time, but in retrospect, high rpm is typically a safe regime as long the rods and rod bolts hold together and valves don't float.
 
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