Random Misfire #1cyl only - JeepForum.com
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post #1 of 2 Old 03-17-2013, 01:46 PM Thread Starter
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1999 WJ 
Join Date: Mar 2013
Location: Glover
Posts: 2
Random Misfire #1cyl only

1999 Grand Cherokee Laredo 4L random misfire #1cyl only. Coded P0301. Changed spark plugs twice and coils once. Ran fine for a couple of days or weeks after doing this. Then back to the same problem. Starts fine but once the engine has warmed up, the misfire starts when idling in park. The check engine light starts to flash. Can shut it off and it starts up fine again and if you start driving, it drives fine. It will go into the misfire while driving when I am coasting down a long steep hill using the brake. But in the same scenario, if I hit the gas for a quick spurt, it won't go into the misfire. Any thoughts or advice would be great!! Love the old gal and would love to keep her!!

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post #2 of 2 Old 03-17-2013, 05:57 PM
It's the crank sensor!
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1999 XJ Cherokee 
Join Date: Nov 2007
Location: Central Minnesota
Posts: 8,850
Here are some possibilities. Troubleshooting is a process of elimination; these are suspects.

*Verify fuel pressure. Verify, but for a 99 Cherokee the spec is 49 psi, plus or minus 5 psi. Only takes a few minutes and a fuel pressure gauge can often be rented from many parts stores; call around.

*Swap the fuel injector from cylinder #1 to a different cylinder. If the misfire follows with the swap, you have found the root cause of your misfire.

*Test compression. Not as likely, but can't rule it out. The compression spec. for the 4.0 is 120-150, with no more than a 30 psi variation between cylinders.

*Test your "throttle position sensor".

99 Cherokee, 4.0 AW4, NP242
Past Jeeps: 49 Willys, 81 Scrambler, 88 Comanche
Without "data", all you have is an opinion!
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check engine light , misfire cyl

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