• NAXJA is having its 18th annual March Membership Drive!!!
    Everyone who joins or renews during March will be entered into a drawing!
    More Information - Join/Renew
  • Welcome to the new NAXJA Forum! If your password does not work, please use "Forgot your password?" link on the log-in page. Please feel free to reach out to [email protected] if we can provide any assistance.

99 4.0 Misfire code, runs fine

Desert4x4

NAXJA Forum User
Location
Phoenix
Ok, so I searched, but there was a lot of conflicting information. Perhaps this thread can turn into a solution to help others eventually.

99 XJ 4.0 AW4 NP242

The codes: always P0300, also random other P0301 or P0302, ect
So, the CEL starts blinking sometimes, highway or city driving. The Jeep drives just as normally as it always does, so I really doubt that there is a misfire.
I've changed the plugs, wires, cap, rotor. TPS crank and cam sensors are reasonably new, (a couple years for the cam sensor, about a year on the crank sensor and less than a year on the TPS).

I would start replacing more things, but I doubt that there really is a misfire, because the jeeps runs wonderfully. Anyone else have this problem with phantom misfire codes?
 
Last edited:
A blinking Check Engine Light usually indicates a severe engine misfire allowing unburned fuel to be dumped into the exhaust system. There it can quickly raise the temperature of the catalytic converter to a point where converter damage is likely.

How are you going to pass smog with a CEL on ? Instead of throwing random new parts at an unknown problem, you should grab your genuine Jeep Factory Service Manual and following the diagnostic procedures for the CEL trouble codes showing.
 
A blinking Check Engine Light usually indicates a severe engine misfire allowing unburned fuel to be dumped into the exhaust system. There it can quickly raise the temperature of the catalytic converter to a point where converter damage is likely.

Yes.

How are you going to pass smog with a CEL on ?
You don't.

Instead of throwing random new parts at an unknown problem, you should grab your genuine Jeep Factory Service Manual and following the diagnostic procedures for the CEL trouble codes showing.
Thanks
 
If the car was really misfiring that severely (multiple cylinders). Wouldn't it be noticeable? It litterally drives the exact same when the light flashes.
 
Similar issue with mine. I just posted my fix. Pin out each injector wire and trace them back to the PCM. My guess is your injector harness has shorted out and that's why you're getting intermittent flashing check engine light.
 
If the OBD-II CEL light is ON, there is a problem. I have never had it go ON for no reason at all. Very rarely the CEL can be ON, and the cause of the problem is not exactly what the code is for, but there still is a problem that should be/needs to be diagnosed and repaired.
 
Hey guys, I am new here. I have a 2001 xj with same problem. my CEL turned on, it doesnt flash but it has same code. the engine runs great, I am curious as to why. I would imagine engine would overheat and lack power not to mention it would probably be knocking. I am not willing to do major engine work unless it is really necessary. any advie is welcome.
 
Make sure all the tune-up parts are fresh. The Jeep 4.0L runs best on plain everyday Champion or NGK sparks plugs. The 2000-01 Cherokee also runs well on AutoLite APP 985 spark plugs. If you buy any engine sensors, genuine Jeep sensors are the only ones you can trust.

As suggested already, testing and inspecting the injector wiring for faults and failures is where to start. Ohms test the injectors too. Try adding a bottle of fuel injector cleaner in the next 2 gas tank fill-ups.
 
Back
Top