Mind boggling miss I just can't solve

[jeffrm20]

Little back story, bought an 04 3.7 KJ a few months ago from a used car lot. Has a little higher mileage (176k) but super clean, very well maintained and has practically every service record done with it in a folder.

Fast forward about a month after purchase, I start noticing a small miss, nothing big enough to throw a code, but definitely noticeable. So I do tune-up on it with factory replacement plugs and throw a bottle of fuel injector cleaner in the tank. A week later miss doesn't change.

About a month later the miss progressively gets worse and now getting a flashing CEL and a random cylinder missfire code. We try running my dads Snapon Modis on the missfire counter, but not getting anything abnormal.

The bad miss comes at weird times. When it's cold and first start driving it, it will miss very bad, and then like a light switch it just goes away. Like instantly runs smooth. But other than that it just feels like it is running rough. Almost makes me wonder if its an injector? It's just hard not having the missfire counter plugged into it 24/7 to catch it when it's actually missing.

I work 90 hours a week so I dont get much time to play with it. I'm close to just throwing a set of injectors at it. :shrug::shrug::shrug:

You must be registered for see images

 

[kage860]

o2 sensor?

 

[jeffrm20]

Forgot to add, new cats and 02 sensors in it too

 

[van462]

I have had a non mopar coil pack that would act funny when it was cold. It created enough electrical noise that the radio picked it up. Once up to temp, it acted fine. Didn't even generate a misfire code.

 

[uss2defiant]

do you know which cylinder is misfiring?
Try taking the coil on that one and switching with one that is good and see whether the problem follows. If so, then you'll need to change that coil.

 

[Birdman330]

I will have to agree, seems like a coil on boot plug is acting up.

 

[libertybob]

Misfire

Tempting but don't replace parts without knowing they are bad. My money would be on an electrical problem. Have you disconnected each coil and cleaned the connectors? The problem going away suddenly sounds like a problem with an electrical connection. When coils fail, they usually don't come back. You can get a LED kit as a loaner from Autozone to test the signal coming to each coil to test the wiring harness. The LED plugs into the connectors and flashes when the coil is signaled to fire. It is puzzling that you get a random misfire code and not a specific coil location.

 

[tjkj2002]

Forgot to add, new cats and 02 sensors in it too

If they are not MOPAR cats or NTK O2 sensors that would be my "next to replace" items.


And yes I've had a few KJ's with that mild misfire issue that was bad cats and yes 1 had brand new aftermarket cats(so that customer got to buy 2 sets).

 

[jeffrm20]

I can't get it to tell me exactly which cylinder is misfiring. Tried multiple scanners and same result. Just telling me random cylinder misfire.

I guess I can pull the coils and check OHM rating on each one of them.

Guess I will start there and let you know :waytogo::waytogo::waytogo::waytogo:

 

[uss2defiant]

just the p0300 code and nothing else?

 

[libertybob]

Clean contacts

I can't get it to tell me exactly which cylinder is misfiring. Tried multiple scanners and same result. Just telling me random cylinder misfire.

I guess I can pull the coils and check OHM rating on each one of them.

Guess I will start there and let you know :waytogo::waytogo::waytogo::waytogo:

When you pull the coils to check them, try cleaning the contact lugs on each coil and clean the connectors for each coil. Also check the boots that cover the plugs for burned areas that can be signs of shorting out.

 

[Birdman330]

If they are not MOPAR cats or NTK O2 sensors that would be my "next to replace" items.


And yes I've had a few KJ's with that mild misfire issue that was bad cats and yes 1 had brand new aftermarket cats(so that customer got to buy 2 sets).

Would this affect Libs of all years, I have an aftermarket Cat/O2 sensor and I have no issues with misfire.

 

[tjkj2002]

Would this affect Libs of all years, I have an aftermarket Cat/O2 sensor and I have no issues with misfire.

It can effect(not all will suffer) any KJ and/or KK with the 3.7 or any Chrysler vehicle with the 3.7/4.7.

 

[Birdman330]

Interesting, I will keep my eye but after year I have had no issues. It still idles the same way as it did before the cat went bad. Though, it is close to tune up time again and I desperately need to do the PCV but I don't have a socket for it.

 

[jeffrm20]

Finally found some time to do compression test. Everything was good on all cylinders. All within 5 psi of each other.

A few times the tank got pretty low on gas and my guess is with it being older Jeep and no fuel filter is it sucked up some sediment from the bottom and clogging the injector. It never ran this bad until the fuel light came on a few times when I let it get a little lower than I should have.

What do you think?

 

[uss2defiant]

Im pretty sure it has a fuel filter.

 

[tjkj2002]

Finally found some time to do compression test. Everything was good on all cylinders. All within 5 psi of each other.

A few times the tank got pretty low on gas and my guess is with it being older Jeep and no fuel filter is it sucked up some sediment from the bottom and clogging the injector. It never ran this bad until the fuel light came on a few times when I let it get a little lower than I should have.

What do you think?

'02-'04 KJ's have 2 fuel filters,'05+ has 1 fuel filter.