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01 XJ 4.0l p0306

wishihaddaxj

NAXJA Forum User
Location
Minnesota
Two days ago my jeep (01, 4.0l, automatic, 122k miles) was running fine, then on the way to work, I looked down, and I swear to god the speedometer went from 0-100-0 in an eerie wave. It started to miss at idle but accelerated fine, and then on the home, it started missing and bucking, with a flashing CEL. The only code it throws is p0306. I've researched extensively on this and other misfire codes, so I think I'm on the right track.

So far, I've:
  • Cleaned the battery terminals and grounds
  • Checked all the fuses under the hood
  • Sprayed out the TB
  • Replaced plugs (copper autolites), correctly gapped, old plugs looked fine
  • Replaced upstream o2 sensors
  • Replaced TPS sensor (from dealership)
  • Checked the cat (no rattles or weird smells, pressure out tail pipe seems okay)
  • Checked the injectors with multimeter, all good
  • Confirmed that #6 cylinder is missing - no change when #6 injector is unplugged
  • Theres no water in the oil, and no bubbles in the radiator when I cranked/started the engine

Open questions/stuff still to try:
  • Is it possible the #6 injector is bad, even though it has the same resistance as the others?
  • I suppose the cat still be could be bad, even though it passed the above checks??
  • Could be a coil rail problem, I've read there's no real way to check.
  • I still need to do a compression test, could be cracked head or bad head gasket, and I'm starting to suspect this is the most likely.
  • Finally, there's a wire tapped onto wiring for the #6 injector for my remote start. I checked, and the factory wire was never severed/broken, just stripped where the remote wire was soldered to it. The connection looks good and isn't corroded. Could that still be a problem?

Anything I'm missing? Any suggestions? Words of encouragement? Random putdowns?

Happy holidays! :cheers:
 
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If OBD-II detects enough missfires (200 in X period of time IIRC) it shuts of fuel to that cylinder and sets that code. You need to remove that code, reset the PCM and let the PCM go through a relearn process to see if number 6 does fire at all. It will not open the injector until you clear the CEL code. They are programed to this in OBD-II to avoid engine and Cat damage if an injector is bad or if the spark is gone.

Do that and a compression test, then post up. Also keep checking plug #6 for oil, coolant. The old plug may have been bad, but you need to clear the CEL code to find out if a new plug, or any work, fixes the problem.
 
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If that code is reset and comes back I would do what I did on my old XJ.
I swapped the #6 to #5, then rest the codes again. If it then thows the code in #6 it might be your coil, plug, wiring, or something other than your injector. If it moves to p0305 then i would suspect the injector.
 
Well, tried a different computer, that didn't help.
Checked compression, 174-185 across the board.

I guess it's injector swapping and/or coil replacement up next. Anything else I should try?
 
Great compression. That is great news!!!!

Have you checked the injector wires on #6 with a noid light yet? To make sure the signal is getting to the injector?

Injector swap would be next, and the noid light test, and just to be 100% sure, swap plugs 4 & 6, while swapping injectors 6 & 5.

Then retest.

Last item is the rail and coil pack, by default I think.
 
Well, I thought that compression was a little high, so I checked the gauge and its "0 point" is actually 18 psi, so those numbers are a little inflated!

Anyway, I swapped the injectors around, and sure enough, cylinder 6 is the bad one regardless. I don't have a noid tester. Is there anything it can tell me if I already know #6 is bad?

So now I need to find out why 6 is bad. Could be the coil I think, but wouldn't 2 cylinders be bad then?? Other than that, I suppose it could be a wiring problem. God I hope not. Anything else I'm missing?
 
Good question on the coil pack, I would agree but I am no expert there. I have never even see the coil pack rig on the later jeeps. I would think there is some point in the HV wiring where #2 and #6 split up, but?

You might drive it to a parts store and borrow their rental Noid light set (if they have one), and test it right there!!!

You left a good clue here!!!!

"Finally, there's a wire tapped onto wiring for the #6 injector for my remote start. I checked, and the factory wire was never severed/broken, just stripped where the remote wire was soldered to it. The connection looks good and isn't corroded. Could that still be a problem?"
 
I really wanted to throw a new coil rail at the problem, but after reading another million threads, I just don't think that could be it. I really think its a wiring issue, which I just detest. I'll try to figure out how to troubleshoot it a little tomorrow...
 
Haven't made any progress on this (cold out side, back at work after the holiday, frustrated, etc.). Still runs like crap. still gives p0306.

My plan of action now is to find a noid light and test #6. I'm thinking there's a wiring issue somewhere leading to the #6 injector, maybe even in the connector.

Other possibilities:
Cam sensor needs indexing
Coil rail or plug boot issue on #6
CPS sensor being weird

Any other ideas, please throw them out there!
 
Morphodite Jeep Gremlins? MJGs!!!! :laugh3:

Put them at the top of the list!:D

I think your on the right path!

I had a rough idle and a number of other problems that I chased for years on my 87 before I got them all nailed down. NO OBD-II to help me. The last idle miss was discovered by accident, turned out to be the female electric connector on one injector that was picky about where it sat. They are a little loose, slack in the clip that locks them in place, old.... and I was wiggle testing them for a new problem when suddenly the last little sometimes miss was gone. It was one of those contacts that worked 80% of the time. A slacker, LMAO!!!

:woohoo:
 
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These are the best options I think.
H

My plan of action now is to find a noid light and test #6. I'm thinking there's a wiring issue somewhere leading to the #6 injector, maybe even in the connector.


Coil rail or plug boot issue on #6
 
Well, I would UN-transmogrify your injector harness by removing the after-market remote starter system.

When in doubt, but it back to OEM and test from there.
 
Looks like it was the coil rail (knock on wood). Took it too a small local shop known for diagnostics and they figured it out and fixed it. I thought 2 cylinders would be bad if it was the coil, but I guess that isn't necessarily the case! I'm just glad it's fixed.
 
Is your '01 still running well after the new coil rail install? My '01 XJ which has already had the head replaced ~ 1 year back started having the same symptoms your XJ had this AM. I haven't had time to work through the typical issues yet, but w/several of the P0306 code threads here not many gave an actual solution to their issues if they fixed them. Thanks
 
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