killbox916

I wanted to start a thread and log my findings as I go. Any recommendations will be welcome as well. I have scoured the forums/internet looking for ideas that I have not tried already.

Background - I personally rebuilt this motor 4000 miles ago top to bottom for a rod knocking. Before and after rebuild it has always had an annoying skip at idle but never gave a CEL code. Other than that a lot of power above idle until this problem occurred.

Now, only code is a flashing CEL - P0306

So far the only thing I've found is very lack luster spark. (I pulled the boot off the coil at #6 and #5 cylinders and arc'd the spark to a good ground.) Spark is very thin and doesn't want to jump very far. Its so thin it hard to even see a color but from what I can see its an orangey/white. Weird because the engine starts very easy.

To this point this is what I have check or replaced with no change.

• -New coil and new NGK plugs.
• -#6 cylinder compression 155psi
• -Swapped fuel injector
• -Check pulse on fuel injector connector
• -Fuel pressure 48 PSI
• -Shot power and continuity from battery thru PCD and to coil connector
• -Checked ground at pin 31 and 32 of PCM
• -Checked entire engine for vacuum leaks
• -pulled valve cover....performed visual insp on valve springs and rocker arms. (engine stationary and at idle)
• -Battery was changed about one week before problem started.
• -No cracks in the head or coolant in the oil from visual inspection.

Only thing I haven't tested yet is the ground driver wires from the PCM to the coil connector and clean the grounds under/rear the PCM. (Got rained out today)

I will end this thread with a solution!! Unless I die, then I will not end the thread.

Stay tuned...

killbox916

So today I had a few hours to shoot some wires and clean some power and ground connections with some results.

1: Shot continuity of driver wires from PCM to coil plug with 0 resistance on all three.
2: I cleaned every ground connection I know of:

• Battery to fender well.
• 4 grounds on the right side of the engine block.
• The ground up to the left of the valve cover that screws into the firewall.
• Two just aft of the PCM.

3: Cleaned the power wire going from the battery to the PDC fuse box.

Honestly I need to upgrade all of these power and grounds running off the battery in the near future.

After cleaning - It runs better and now not throwing any codes at all but I still have a skip once in awhile.

After a closer look, I noticed that the connector that plugs into the coil is worn to poop. The female pins are spread pretty far. I have one on order should be here tomorrow.

killbox916

Final update on this with fix.

Turns out my original issue was a combination of a few things. I would say the biggest being bad ground to the PCM. So when people tell you to check your grounds, do it!!!

Next was a loose/worn S-949 coil connector. I ordered this off amazon for $20 and soldered it to the original harness. This virtually cleared the skip all together but idle was still a little rough and it smelt like I was burning rich.

Since my exhaust was original front to back with 175k miles on it and a little beat up, I decided to take it to and exhaust shop and have that replaced. First I ordered two new NTK O2 sensors off rockauto and had the exhaust shop install them while it was on the lift.








And tada...it's fixed. This thing has never ran better. The only thing I'm going to do now is replace all the heavy grounds and power cables.

So the breakdown is:

1. Cleaned grounds and noticed a major improvement but still had a small miss.
2. Replaced coil connector and miss was fixed.
3. Replaced exhaust including O2 sensors with high flow cat and Dynomax Ultra muffler. Idle cleared up and exhaust smells clean.

Original underlying issue - Weak spark - #6 cylinder misfire

I have seen a lot of #6 misfire threads with no conclusion. Hopefully this can help someone in the future.

Final result -

PatHenry

Very nice! Thanks for providing this info.

JeepingDan

Nice!!
Not properly checking grounds can lead ya on a wild goose chase. Glad you found it.

killbox916

No kidding. Thank you.

Another quick update, went from 11.8 MPG to 15.5

Ian Shaw

I have a 1999 but I hope this can fix my problem. Did your CEL ever flash?

killbox916

Yes it did flash.

Which I have read is a major misfire and can damage the cat. But not sure on that?

mdtobe

Yes, if the CEL is flashing damage is happening. Glad you were able to fix it and thank you for sharing! And thank you for posting that YouTube video - your Jeep sounds AWESOME!!! There is no better sounding engine than a straight six with a good exhaust system (except perhaps a V12 with a good exhaust system - but that's really just double the number of straight sixes anyway). Music to my ears.

I've got a po0306 code popping up, too, but my situation is going to be a simple fix (I hope). It only happens on misty/rainy mornings, and this afternoon I went out and started it with no miss - and when I misted the number six plug wire with a spray bottle of water it started its usual misfire (until the water evaporated). Pretty sure I'll get free new wires because I think I used Autozone Duralast the last time I changed them (around 2009). Love those Limited Lifetime warranties!

Concerning your improvement in mileage, do you think that was from cleaning up the grounds, replacing the O2 sensors, or just from eliminating the misfire, or the combination of all? I definitely need to clean up my grounds - the radio pops in and out with the display lights blinking any time I have other electrical items running (pretty sure that is a result of those two grounds near the PCM being bad). My mileage dropped from around 15 to around 12 last year and I'm hoping cleaning up the grounds will fix that (I've done the spark plugs and the air filter already). I don't think my occasional misfire is causing the terrible fuel economy because it only misfires for the first few minutes of the first drive of the day, and only when it's raining or foggy out. I would suspect the O2 sensor might be causing the bad economy, but I changed that several years ago and never get the CEL code for it.

Best wishes,

John

killbox916

Thank you. I think it was a combination of all. but O2 sensors will really get your MPG out of wack.