Random Cylinder Misfire - JeepForum.com
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post #1 of 2 Old 09-28-2017, 07:48 AM Thread Starter
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Random Cylinder Misfire

Hi guys,

I've been a long time reader but first time poster looking for some help.
I just recently got a 2007 WK Grand Cherokee Laredo 3.7L V6 with 174,000 miles on the engine. I got it from a trusted friend who treated it with meticulous care since he got it new off the lot. It's bone stock (aside from basic tune up and maintenance) and I'm chasing a ghost.
When i got it it had a check engine light with a P0456 (Very Small Evap leak) and a P0300 (Random cylinder misfire). I managed to find the evap leak was caused by a leaky gas cap and leaky purge solenoid, both extremely easy fixes, but cannot for the life of me figure out my misfire issue.

The misfire only really happens bad enough to throw a MIL when getting near or less than 1/4 tank of gas and immediately after refueling if I don't throw half a can of seafoam in the tank. It DOES NOT even throw a pending code until it gets below a quarter tank of gas. Just two days ago, it threw a code under load on the highway with a flashing MIL. I pulled into a gas station and hooked up the code reader to check freeze frame values and everything looked in check except the RPM's were at 300, even though it idles at 600 rpm's consistently. And immediately after refueling, the PCM cleared the pending code on its own with a full tank.

Here is a list of everything I've done diagnostically that looks good:
- Spark plugs changed six months before i got it (only 7000 miles) with Bosch iridium - all looked good
- All the coil packs ohmed out within specs
- All wire harness connections seem solid
- No vacuum leaks under hood (checked with propane and carb cleaner while watching rpm's with reader)
- ASD relay checked per testing procedures found online with voltmeter
- All six injectors have been checked with mechanics stethoscope

Only things I've yet to do is check fuel pressure and drop fuel tank (since it's such a pain in the butt), although, when the key is turned to accessory on but not starting the engine, I can hear the pump kick on from inside the cabin.

Sorry for the rambling post, I just want this thing running as good as it can before I start the cosmetic modifications. Any help is much appreciated,

Last edited by taylorjames; 09-28-2017 at 08:17 AM.
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post #2 of 2 Old 09-28-2017, 03:20 PM
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2000 TJ Wrangler 
Join Date: Oct 2010
Location: Clyde
Posts: 5,744
I agree a fuel pressure check should be next on the list. Something else to consider would be to watch the upstream O2 sensor output to see if it's showing lean during the misfire (lack of fuel), or rich (poor ignition).

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2005 LJ Rubicon
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diagnose , engine , misfire , wk

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