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Unread Yesterday, 10:46 AM   #1
xtrattitude4u
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1996 ZJ 
 
Join Date: May 2013
Location: Vancouver, WA
Posts: 18
96 GC Random Cylinder Misfire

96 GC Laredo 6cyl, 4wd w/ Select-trac. 140k miles.

So, we've been through quite a bit with My Fiance's Grand Cherokee since she bought it last year, but this last problem has got me stumped.
It's throwing a random cylinder misfire code along with cylinder 1, and were running out of ideas. The symptom it shows is an occasional jerk when getting on the on-ramp, so I think it only happens under heavy load.

While trying to fix an issue that ended up being a faulty CPS, we replaced the entire ignition system from harness to plugs: Coil, plug wires, plugs, cap, rotor and and the distributor pickup. We use Echlin brand parts from Napa (except the CPS, which was Mopar). We've also replaced the IAC, TPS and the fuel filter. I cleaned the connectors for the PCM and the CPS, also installed lock washers under the PCM screws in case we had the infamous PCM short going on. After that last garage day I cleared the codes, but they just came back yesterday.

At this point I'm thinking it's an intermittent fuel issue, or possibly a slight wiring issue I haven't found. When I checked the fuel pressure, it was a solid 50psi at idle and at high rpm. While raising the rpm the needle was bouncing rapidly between 40 and 50, but I couldn't tell if that was really the pressure changing or just the vibration from the motor shaking the gauge. Once the RPM was steady the pressure held at 50 psi.

I also took the fuel injectors out and replaced the o-rings that were shot, along with cleaning a lot of dirt and grease buildup that were on the engine side of the O rings.

Has anyone experienced the fuel pump going out in such an intermittent fashion? Dropping the tank scares me a little, never done that before.
I have an OBD2 reader that reads live data, but fuel pressure isn't supported and I'm not sure of how to monitor that while on the road.

I'm thinking dropping the tank is where we're headed, so I guess I'm wondering if anyone knows something easier to check or of a test I can perform to further isolate this rather than poking around.

Thanks All!

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Unread Yesterday, 06:56 PM   #2
ZeeJay1997
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did you swap places with the injectors or move them to another position?
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Unread Today, 01:29 AM   #3
blairwill4
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Join Date: Jun 2014
Location: bluffton, Indiana
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do what ZeeJay said. A little experience. I was getting a random misfire code on my jeep and did just about everything i could to make it stop. Of course it didnt go away just like in your situation.... Kept driving for a week or so and then it happened.... The head gasket finally gave out on my. I tore it all down last week and found a big crack in the head between the valves in cylinder 6, and another small crack in cylinder 5. Its hard to fire properly when there is coolant in the cylinder. I would check your compression. Im not sure the exact numbers youre looking for, but I know you are just wanting them to be very similar and not have 5 cylinders at 140 and one cylinder at 80. Personally I would test compression first becaue it will be the easiet, and then try what zeejay stated with moving the injectors.
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Unread Today, 02:03 AM   #4
Uniblurb
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Location: Central, Ohio
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If you want to monitor your fuel pressure while driving duct tape the gauge face to the windshield where you can read it while it's connected to the fuel rail. An old ZeeJay trick and I mean the trick is old not Zee!

There was a TSB (technical service bulletin) on the 96 ZJ fuel pumps in particular where overheating causes them to fail. I know it's cool out now but maybe the constant overheating is starting to make if hiccup if it's never been changed out before. Do the fuel pressure test while driving first.

But Zee does have a good point if the injectors were shifted around.
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96 4.0 ZJ Laredo, 2004 4.7L WJ Limited, 93 4.0 XJ (spare), 96 4.0 XJ (son's)

-Stalling ZJ? 12 things to check before replacing a sensor; the Dirty Dozen
-Crankshaft position sensor multimeter test. & video of testing.
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Unread Today, 03:03 AM   #5
dflewelling
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An old ZeeJay trick and I mean the trick is old not Zee! Lol
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Unread Today, 03:03 AM   #6
dflewelling
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Darn quote doesn't work on mobile! Anyway...
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Unread Today, 08:01 AM   #7
ZeeJay1997
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hey, if the shoe fits...
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Unread Today, 10:46 AM   #8
Uniblurb
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Location: Central, Ohio
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Or maybe it's if the slippers fit?? Fine one to be calling the kettle black since I be older than you!
__________________
96 4.0 ZJ Laredo, 2004 4.7L WJ Limited, 93 4.0 XJ (spare), 96 4.0 XJ (son's)

-Stalling ZJ? 12 things to check before replacing a sensor; the Dirty Dozen
-Crankshaft position sensor multimeter test. & video of testing.
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