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Braffail 02-12-2020 10:48 AM

Cylinder 8 misfire
Hey all, so I’m completely lost with this misfire. I have a 2002 Jeep Grand Cherokee overland 4.7. HO. It threw a cylinder 8 misfire and then a radon cylinder misfire after a couple months of light coming on and off. I finally went out to a shop and they said compression test was good, but I need sparks, coils, and injectors. Which we did ourselves and saved $1300 doing so. Drove truck 250 ish miles and now again, cylinder 8 misfire with random. Threw a vacuum leak but we detected that and that didnt come on when the misfire came on. Very confused. Any help? Thanks!

Mattyjm 02-12-2020 01:24 PM

What codes are you getting? Please be specific.

Bigrigr 02-12-2020 10:58 PM

Double check all your wiring connections, these Jeeps have a real problem with them getting brittle, and breaking the lock tab off the connector. If they do break, the connector then just works its way off until it causes a misfire. Also, what does the spark plug in #8 look like now, is it getting contaminated with burning oil, or coolant from a head gasket leak?

Braffail 02-13-2020 01:43 PM

P0302 cylinder #2 misfire. And to me the spark plugs look good. I typed in through rockauto the spark plugs I need and they sent them out. Ac delco 41-602. I checked the book and it says champion rc7pycb4 or similar. Hoping these are the right ones.

Bigrigr 02-14-2020 12:09 AM

Several different auto parts stores online say that the 41-602's are the correct basic plug for you. I just wonder if basic plugs are OEM equivalent? I cant remember what i put in mine while doing a tune-up, but it seems to me that they were recommending platinum plugs?

I double checked the Champion # you gave earlier and it shows its a platinum plug....

Braffail 02-15-2020 12:05 AM

So is that recommended to go platinum? It’s just weird that the two cylinders that started all this are the ones throwing codes now. It also idles at 400-ish rpms now instead of its normal 620-ish after the light came on. I’m lost. Lol The shop said good compression as well.

Bigrigr 02-15-2020 09:23 AM

I would be changing the plugs to at least OEM equivalent. These modern ignition systems are designed around very specific spark plugs, and they don't work well with low grade replacements. Many times its best to go directly to the dealer, just to make sure you get the correct ones to keep the system happy.

Every internal combustion engine needs three things to run well. Sufficient Fuel, sufficient spark to ignite that fuel, and sufficient Mechanical ability to draw in air and squeeze it down along with the fuel for proper burning and power production. Simply put- fuel, spark, compression. To diagnose misfires and check engine lights, these need to be kept in mind. Assuming the shop you used did a proper compression test, you can eliminate the Mechanical part of the equation. The engine must be able to draw in proper amounts of air through the air filter, intake and valvetrain, compress it mechanically, and expel it through the exhaust system. Eliminate this part of the trio. This leaves spark and fuel. You appear to have the improper spark plugs for your application, this means you more than likely have insufficient spark to ignite the air/fuel mixture. Replace spark plugs with OEM equivalent-just to get you on even ground to base your diagnostics from. You say you changed all the ignition coils, with the spark plugs correct? Assuming that the coils are all functioning correctly because they are new, double check the Human factor here, and make sure that ALL wiring connections are locked down and tight on the coils. If nothing obvious shows up, it's time to consider the last part of the trio. Fuel pump, fuel injectors, and fuel filter. Get a proper fuel Pressure gauge, and check the pressure for proper spec. Any discrepancy here would indicate fuel pump and fuel filter/regulator need replaced. If still having issues, remember, you have a individual fuel injector on every cylinder-just like the spark plugs and coils. You could have a couple of injectors that are not supplying proper fuel that would cause misfires as well. I had one injector on my WJ when i got it home that was setting a check engine light. It wasn't running poorly(no obvious miss) but the internal resistance in the injector itself was out of spec and causing the PCM to not be happy. I swapped the injector and all is well again. Eliminate your obvious spark plug problem by installing the correct plug for your application, then move on to the fuel system, and start eliminating items over there. Im betting you will find your problem then.

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