misfire at highway speeds

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06-03-2019, 03:12 PM

Bram

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Join Date: Mar 2019

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Model: Cherokee (XJ)

misfire at highway speeds

Hello. I have a 2001 Jeep grand cherokee 4.0

My problem comes at 75 mph The check engine light begins to flash and If I continue it will start to buck and shake.
If I pull over and turn the car off, It will start without a problem and drive smoothly.

the check engine light stays on.
the code comes back as a random misfire.
I have changed the plugs and the coil pack.

to be honest I am not a "car guy" but I have an honest mechanic who checks my codes for free.

His next idea ia a new computer.
Is this truly the only option?
the fact that the car drives fine after I turn ift off and turn it on is his reasoning...

all ideas would be appreciated .
thanks

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06-06-2019, 02:57 PM

dave1123

Old fart with a wrench

Join Date: Nov 2010

Location: Manlius, east of Syracuse, NY

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Year: 2000 XJ Sport & WJ Laredo

Model: Grand Cherokee (WJ)

Engine: 4.0L

What spark plugs are you running? They may be getting hot and detonating. Try NGK ZFR5N (3459). If they get too hot, any carbon will melt and cause a misfire. I had that problem with Champion until I went to NGK. Mine were shorting out when pulling up a hill at 70.