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Unread 03-03-2011, 12:56 PM   #1
VTZJ
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96 ZJ 4.0 Multiple Random Misfire -- SOLVED!

So my normally smooth running high-miles ZJ suddenly developed a driveability problem two weeks ago that had all the hallmarks of a classic cap/rotor/wires ignition problem:
  • started well
  • idled a little lumpy but okay
  • stumbled under accelleration
  • stumbled and bucked in top gear at highway speeds
  • fuel consumption very high -- about 12 mpg (18 to 22 normally)
  • check engine codes for misfires on cyl 6
  • check engine codes for multiple random misfires
A check of spark plugs revealed cyl 6 plug wet with fuel, but all others looked okay. Compression checked out okay. I put new Mopar cap/rotor/wires and new Champion plugs in, with no real improvement in driveability. A new coil improved things somewhat, but there was now a definite miss in one cylinder at idle, and highway driving was slightly smoother but still generating codes for multiple random misfires. I changed the fuel filter, cut open the old one, and found it very clean inside the pleats.

I was done throwing parts at it, so I consulted my Jeep guru, AlteredXJ. Under his advice, we ordered one new fuel injector for cyl 6, plus all new o-rings for the injectors. We pulled the fuel rail and found the fuel it contained rust colored and full of sand-like debris, probably rust particles from 15 years/195,000 miles of service. This discolored and dirty fuel was especially concentrated at the number six position in the rail. After cleaning this out, cleaning the injectors, and reinstalling with a new injector in the six position, and new o-rings, the old girl started and ran like silk. Mileage and power are back to normal. Problem solved.

Analysis: It may have been bad o-rings; they were clearly overdue for changing. It may have been a bad injector on six, we didnt test it, just replaced it. Most likely, though, it was a starved injector on six due to a build up of debris. The number six injector is at the rearward end of the fuel rail, which is fed at the forward end by the fuel supply line. It stands to reason that the injector at this "downstream" position in the fuel rail may be more vulnerable to an accumulation of debris, as fuel flows past the other five injector positions, washing them somewhat.

Conclusion: Pulling the fuel rail for inspection of the fuel quality in the rail and manual cleaning of the injectors is a low cost diagnostic step that would have saved me time and money in prematurely replacing ignition components. Given the fuel quality of today, and the age of my vehicle, going foreward I will probably pull the rail for inspection at 25,000 mile intervals.

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Unread 03-03-2011, 01:12 PM   #2
melk
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Good information! And gives me another project

How hard was it to pull the rail? How do you go about cleaning it?
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Unread 03-03-2011, 01:27 PM   #3
VTZJ
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Quote:
Originally Posted by melk View Post
Good information! And gives me another project

How hard was it to pull the rail? How do you go about cleaning it?
It's kind of fussy work with delicate parts, but overall pretty easy. Disconnect throttle cables, remove a few bolts, pull the electrical connectors, then pull the rail off. Injectors come with it as they are mounted in the rail. Injectors are held in the rail by friction of o-rings and a metal retaining clip. After removing the injectors for cleaning, the six "ports" in which the injectors seat in the rail give access to clean the fuel rail.

We left the fuel supply attached, sprayed a little carb & choke cleaner through the rail, then used the vehicle fuel pump to flush the rail (probably not a recommended procedure), catching the waste fuel in a pail. Amazingly, it took several cycles to flush all the junk out of the rail. We chose not to use any mechanical means to to brush or scrub the interior of the rail, for fear of waking sleeping dogs.
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Unread 03-03-2011, 04:07 PM   #4
coralman
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This will become increasingly needed as our government ups the allowable "corn sqeezins" to be used in gasoline.The ethanol has agreat affinity for moisture.
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Unread 03-03-2011, 04:49 PM   #5
ZJPunk98
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My 98zj is starting to lack power, and i already plan to do the full tune up this spring. After reading this, i may clean/install new injectors.
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Unread 03-03-2011, 07:00 PM   #6
VTZJ
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Quote:
Originally Posted by ZJPunk98 View Post
My 98zj is starting to lack power, and i already plan to do the full tune up this spring. After reading this, i may clean/install new injectors.
Not a bad idea. You may be interested to know a few more details. I had a little spare time this evening so I inspected the used injector pulled from the number six position as described in the OP.

First thing to note is that the injector is an OEM unit, branded with the Chrysler pentastar and made by Siemens, the "GE" of Germany. It's also date stamped "95" so is very likely original to the vehicle with 15 years of service over 195,000 miles. I am amazed the thing lasted this long, really.

The second thing to note is that on the intake side of the injector body, where fuel flows into the unit, there is a filter which is practically invisible to the unaided eye. The filter is a fine mesh shaped like a bucket and woven by magical German elves using an extremely fine thread, probably the stolen **** hairs of sleeping Bavarian Princesses, so incredibly fine that I could not see the mesh until I used a 20x magnifying glass. Once I brought the 20x glass into play, I could see that this filter was absolutely full of junk, which looked like hideous boulders of lava under my powerful magnifier.

As an experiment, I tried and found that I could remove most of this junk by gently using one fine brass bristle from a brass brush. I succeeded in excavating about 90 percent of the crap filling up the intake port of the injector, but it took a good 30 minutes to do so. Why do I mention this? Trail fix knowledge, I guess. And I would like 4.0 users to know that this crap is mostly rust particles and cannot be disolved by Techron or Gumout or any other of your favorite magic elixers. You gotta get in there and ream this stuff out mechanically, or woe betide you.
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Unread 03-03-2011, 07:35 PM   #7
ZJandI
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Thanks for the info, I may start on this soon since I just did a full tune up.
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Unread 03-04-2011, 04:47 AM   #8
coralman
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As far as the screens it would be abetter option to just replace them. I think you just find a sheet metal screw that screws into the plastic ring of the filter tightly. Clamp the head of the screw in a vise and gently pull the screenout.

There are plenty of injector places that sell the orings, pintle caps, and screens and they are pretty cheap. There is also a "how to clean your own injector" video's on you boob. One guy has a rig where you can pulse the injector while flushing it .
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Unread 07-13-2012, 03:58 PM   #9
mk1926
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this was wonderful information. i have been replacing componets with no results, ahve the same random misfires code 300,303,306 etc.

thank you
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Unread 07-13-2012, 08:04 PM   #10
MoparCharlie
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Those fine mesh screens don't stop all of the debris from entering the injector.
From your description I would say that the injector was not clogged, but rather stuck slightly open ( debris not allowing the pintle to seat ).
That would explain the wet spark plug...and if it were dumping fuel, the computer would try to compensate for the one rich cylinder by leaning out the cylinders giving you the stumbling and bucking.

I would check your oil....smell it... does it smell like gas?
If so change it.
The last thing you want in the warm weather is comprimised oil.
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Unread 04-19-2013, 02:11 PM   #11
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Unread 04-19-2013, 02:17 PM   #12
ponytail
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Thanks for posting the fix. My wifes XJ had the same issue. #4 misfire and random misfire. I removed all injectors and cleaned them as best I could. new o-rings. runs the best it has in the last year. It had lots of what looked like fine sand it the rail. When it had fuel on it, it was like paste.

Thanks again, Chris
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