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Unread 12-22-2013, 07:43 AM   #1
specia1k
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2004 WJ 
 
Join Date: Sep 2011
Location: Jacksonville, FL
Posts: 323
Wj '04 WJ Runs Rough at Startup

My daughters '04 WJ Sport 4.0 occasionally when started will run super rough. She says it feels like it's going to explode. Most of the time you can turn it off, re-start it and it goes away. I suggested she put a can of Seafoam in the fuel to clean injectors. The spark plugs were replaced a couple years ago. Otherwise the WJ runs well. She just came back from college for XMAS and got 400+ miles to a tankful, so the engine is running efficiently as it has in the past.

Thoughts?

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2004 Jeep Grand Cherokee Limited 2x4 4.0L
2002 Jeep Grand Cherokee Sport 2x4 4.0L
1998 Jeep Grand Cherokee Laredo 2x4 4.0L
"Three daughters, three Jeeps"
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Unread 12-22-2013, 08:38 AM   #2
doityourselfMM
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1999 WJ 
 
Join Date: Dec 2010
Location: manassas, va
Posts: 749
I suspect the problem is electrical. If it was mechanical, the problem would most likely persist. Since turning it off and restarting seems to 'clear' the problem, I don't think it's plugs.

I had a similar idle fluctuation issue. Turning it off and restarting always smoothed the idle out, to a nice flat 550 rpm. I removed the throttle assembly and cleaned. Particularly the IAC valve. So far, so good.

Try the simple (and inexpensive) remedies first. It would probably be wise to install a new throttle gasket after cleaning. Don't over torque (and snap) the 4 bolts.

And don't rule out the Crank Sensor.
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Unread 12-22-2013, 08:50 AM   #3
Upside
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2002 WJ 
 
Join Date: Dec 2004
Location: Irvine, CA
Posts: 805
My 2002 4.0 does this on occasion. I've replaced my battery cables to solve a slow cranking issue and cleaned the intake. When the misfire happens it seems like either an injector or a plug isn't firing. I haven't checked the resistance on the coil or injectors since it hasn't happened in a while. I do run MMO in the tank now for better fuel mileage so maybe it was a sticking injector.

My driving is very consistent and I get 2 MPG better with the MMO. The mileage is apparently dependent on your specific Jeep and the fuel available in your area, some see an increase and others do not. I think the MMO just allows the computer to run more timing on our California fuel.
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