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Nuts & Bolts: Troublesome TJ

Posted in How To: Tech Qa on October 19, 2017
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I have a 2002 Jeep TJ with a 4.0L engine, and the check engine light flashes with code P0300 at random, which is a misfire. I have had it at two shops and one dealer, and no one has been able to figure it out. Do you have any ideas or know what the problem might be?

Chad E.
Via nuts@4wheeloffroad.com

With that code we would take a hard look at ignition components, including the coil packs and all ignition sensors, but surely the shops or the dealership would have ruled that stuff out already. We asked around, and Editor Christian Hazel reported that he had a 1999 Cherokee that kept throwing the same code despite his replacing every ignition component. The culprit ended up being weak valve springs. He replaced them with a fresh set of OEM valve springs, and the problem went away. He explained that the weak valve springs were causing the valves to bounce against the seats when closing, which the computer interpreted as a misfire.

Luckily, as engine work goes, replacing valve springs is pretty easy. Take the valve cover off and swap the springs one cylinder at a time. Rotate the engine until the cylinder you are working on is at top dead center (TDC), then use compressed air through the sparkplug hole to hold the valves up while you use a valve spring compressor to remove the keepers and swap the springs.

A similar method involves using a length of rope. Put the cylinder close to TDC, insert the rope into the sparkplug hole, and rotate the engine so the cylinder is at TDC.

Try this and let us know if it fixes the problem in your case.

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