NOT FIXED P0306 - 2002 Sahara 4.0 automatic (at wits end!)

S

Smokin151

New Member
Original poster
Supporting Member
Joined
Jun 4, 2024
Messages
23
Location
Gastonia, NC
Good afternoon,
I have the dreaded P0306 and can not for the life of me get it to go away. I have read through the previous threads and do not see what I should look at.

Here's where I am at: New rebuilt engine 5k miles ago(not because of this, it had a knock), New plugs(OEM), New Coil pack, New cam sensor(Echlin, OEM is discontinued), New cam sensor and sync shaft, fuel rail, swapped injectors, replaced all four o2 sensors. Compression check 175 psi per cylinder all 6, fuel pressure at 56psi.

If its idling and I unplug each injector the engine reacts.

When I am on the highway for extended period of time I get the "flash" then it goes in limp mode. If I turn off the key and restart runs fine with a hard fault P0306. Also when I WOT it will also flash and I can feel the misfire. I restart engine and its gone. I have moved the harness, wiggled, checked for breaks NOTHING.

I am leaning towards and ECM/PCM but hate to keep throwing parts. Any help is and will be much appreciated.

TIA
 
  • Like
Reactions: Wranglerfix
Do you have any heat protection on your fuel rail? Usually that's more like cylinder 3-4.

Could try swapping the #6 injector and spark plug with #5 and see if the code changes. Same with the coil packs. I'd do one change at a time.

@Wranglerfix www.wranglerfix.com can test your PCM if you want to rule that out. But of all the symptoms Mark has listed for bad PCM I haven't seen one for P306.

-Mac
 
  • Like
Reactions: Wranglerfix
macleanflood said:
Do you have any heat protection on your fuel rail? Usually that's more like cylinder 3-4.

Could try swapping the #6 injector and spark plug with #5 and see if the code changes. Same with the coil packs. I'd do one change at a time.

@Wranglerfix www.wranglerfix.com can test your PCM if you want to rule that out. But of all the symptoms Mark has listed for bad PCM I haven't seen one for P306.

-Mac
Click to expand...

I did add the heat shield and have swapped the injector 3 weeks ago and the plugs this past weekend. Code still comes to cylinder 6 only. Prior to changing the crank sensor out I had an O'Reilly's one and used to get Cyl 1-6 and 3-5 now that I changed to an echlin only cylinder 6 misfire. The only thing that keeps me going to electrical is that as soon as I restart even coasting the misfire/limp mode goes away.
 
  • Like
Reactions: macleanflood
macleanflood said:
Try swapping the coils?

Got a bore scope to look at the valves through the spark plug hole?

Leakdown test on 6?

-Mac
Click to expand...

I will grab a bore scope and look down for the valves. Coil has been swapped, same result. I have yet to do a leakdown as I figured dry compression was relative to all the other cylinders and man I hope with a fresh rebuild its not mechanical. Thankfully I have warranty.
 
  • Like
Reactions: Rygrego and macleanflood
UPDATE:
Checked cam/crank sync, fluctuats 2/3 adjusted to 0/1. Took for a ride sure as shit misfire cylinder 6. Scan tool confirms. I run back to the house grab a noid, and took it for another rip. Misfires cyl 6 hard and staying. Plug the noid tester and its not flashing. Restart engine back to flashing. I checked resistance from injector harness to pcm and good, relative to cyl 5. I think I confirmed I am loosing the driver from the pcm.
 
Mark at Wranglerfix will help you- he is a good man -

He might even let you return it if it does not solve the issue. That is his call, not mine.

He is out to help people, not push PCMs.
 
  • Like
Reactions: tr21triton
I'd be pulling the plugs on your PCM at this point and checking for corrosion.

I'd also deloom the harness from the PCM connector to injector 6...or at least ohming out continuity...and verify you don't have a harness issue.

They love to break in the back where it turns on the valve cover.

-Mac
 
AndyG said:
Mark at Wranglerfix will help you- he is a good man -

He might even let you return it if it does not solve the issue. That is his call, not mine.

He is out to help people, not push PCMs.
Click to expand...

I spoke to Mark the other day. What a great guy. He mentioned to send the pcm down to test it. After last nights findings, I will just buy a new PCM.
 
  • Like
Reactions: Rygrego, macleanflood and AndyG
macleanflood said:
I'd be pulling the plugs on your PCM at this point and checking for corrosion.

I'd also deloom the harness from the PCM connector to injector 6...or at least ohming out continuity...and verify you don't have a harness issue.

They love to break in the back where it turns on the valve cover.

-Mac
Click to expand...

I checked the resistance from injector to PCM and no open circuit. I have always suspected PCM since I can restart it when this happens and the misfire goes away.
 
  • Like
Reactions: macleanflood
Moral of the story.. stop overthinking. I get the news that the pcm was good which put me back at the beginning. So, back to basics right? Checked the complete harness for the injectors, no breaks. Start testing resistance at the 12v lead from fuse and O/L. I'm like wtf. So it hits me. I replaced the o2 sensors however bank 2/1 connector was different when I bought it. What do I do? I swap the connector and go with it. Now, no o2 sensor codes so I think nothing of until...tonight! I can't get the wire to tone out for any injector. So I unplug bank 2/1 o2 and wouldn't you know, no more open circuit. I wire in the original o2 sensor and we still tone out. As I look back and think, running rich, shit mileage, and only p0306 when the o2 sensor really needed to work. So kids remember, sometimes we create our own gremlins!
I do want to thank everyone who attempted to help me. Yall really did make an old man think!
 
Last edited:
  • Like
Reactions: macleanflood
macleanflood said:
For posterity what brand O2 was bad and where did it come from?

-Mac
Click to expand...

It was an NGK, the o2 wasn't the problem, I was. I was trying to use a downstream in place of an upstream. I wired the o2 to match the one I took out chasing other codes in the system. I never thought to check the 12v going to the injector.
 
  • Like
Reactions: frank1, macleanflood and Jamison C
Smokin151 said:
It was an NGK, the o2 wasn't the problem, I was. I was trying to use a downstream in place of an upstream. I wired the o2 to match the one I took out chasing other codes in the system. I never thought to check the 12v going to the injector.
Click to expand...

Good catch, thanks for the update.
 
Disregard anything I just said. PCM came back with no problems. Installed o2 I was thinking was the problem test drive and same issue. Came home, swapped injectors same thing. I lose any pulse to injector 6. What's next? Overlay wires? I only lose cyl 6 injector.
 
Chris said:
Without reading the entire thread, have you inspected the wiring harness thoroughly?
Click to expand...

Yes, I stripped it down the other day. No signs of any breaks. All wires ohm out pertaining to #6 injector. I am getting injector pulse when restarting engine. Only lose it under hard load. I can duplicate the issue everytime
 
Top Bottom