Cylinder Misfire

#1
2002 Sport with 4.0 (97000 miles).....Kept getting OBD II 300, 301 & 302 codes. Eventually, the engine light began flashing and I quit driving. My local shop recommended changing the coil rail. The cost for them to do it was outrageous so I bought one and changed it myself. As long as I had it apart I went ahead and ran compression on all 6 cylinders (152-155 psi on all) which I thought was good and also installed new spark plugs. When I started it up, I could really tell the difference it how it sounded, and a road test showed a marked improvement in power and performance. I drove it for about 25 miles, stopped for awhile, and upon starting up again, the check engine light came on showing a 301 code. I didn't notice any engine miss at all. Any suggestions on what I should check next?
 

mots

TJ Enthusiast
May 11, 2018
674
550
OH, USA
#2
Hey, what spark plug brand/part number are you using now?

One thing to consider. If misfires are only ever in cyls 1 and 2, this could be a clue. The ignition coil assembly has 3 coil-packs that each service two cylinders (for the direct ignition new TJ's). Coil-pack 1 services cyls 1 and 2, Pack 2 services cyls 3 and 4, and Pack 3 services 5 and 6. If the misfire is always for cyls 1 and 2 (p0301 and p0302), then it is possible the coil-pack 1 is causing the issue. If you try a new coil-pack assembly, I'd stick with OEM.
 
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YooperSkipper
#3
Hey, what spark plug brand/part number are you using now?

One thing to consider. If misfires are only ever in cyls 1 and 2, this could be a clue. The ignition coil assembly has 3 coil-packs that each service two cylinders (for the direct ignition new TJ's). Coil-pack 1 services cyls 1 and 2, Pack 2 services cyls 3 and 4, and Pack 3 services 5 and 6. If the misfire is always for cyls 1 and 2 (p0301 and p0302), then it is possible the coil-pack 1 is causing the issue. If you try a new coil-pack assembly, I'd stick with OEM.
Autolite APP 985's........and I just installed a new coil rail? The original misfires were on cyls 1 & 2 along with po300 random misfires. That's why I thought it was the coil rail. Now only po301 is present.
 

mots

TJ Enthusiast
May 11, 2018
674
550
OH, USA
#4
Plug model is great. Sorry, are you using a Mopar brand of coil rail? Right after I got my Jeep, I tried a BWD coil rail from Advance Auto and was going to keep my Mopar coil rail as a backup. Got it for $80 with coupon but then returned it because the Jeep ran like crap on it. Ran better with the Mopar coils with 102k!
 
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YooperSkipper
#5
Plug model is great. Sorry, are you using a Mopar brand of coil rail? Right after I got my Jeep, I tried a BWD coil rail from Advance Auto and was going to keep my Mopar coil rail as a backup. Got it for $80 with coupon but then returned it because the Jeep ran like crap on it. Ran better with the Mopar coils with 102k!
Correction....I put the Autolite AP985's in rather than the dual tip APP985's! Somehow I didn't check the numbers before leaving the auto store. I didn't want to wait for a Mopar rail and bought one from NAPA instead. Think I should return it along with getting the right plugs?
 

Jerry Bransford

TJ Guru
Supporting Member
Nov 9, 2015
8,822
9,776
Escondido California
#6
Correction....I put the Autolite AP985's in rather than the dual tip APP985's! Somehow I didn't check the numbers before leaving the auto store. I didn't want to wait for a Mopar rail and bought one from NAPA instead. Think I should return it along with getting the right plugs?
Whoops there's the issue, at least you have an issue, the AP-985. Single-tipped platinum plugs like the AP-985 or Champion 3034 should never be used in a newer 4.0L distributorless engine, the kind with the coil rail. They will cause hard to diagnose misfires, bad engine idle, etc.

Switch that AP-985 out for an APP-985, Champion 7412, or the iridium-tipped Autolite XP-985 which will perform well for no less than 200k miles.
 

Mr. Bills

TJ Enthusiast
Nov 24, 2017
793
1,009
Area Code 530
#7
. . . Switch that AP-985 out for an APP-985, Champion 7412, or the iridium-tipped Autolite XP-985 which will perform well for no less than 200k miles.
@Jerry Bransford:

I call bullshit.

Autolite does not claim that their XP-985 plugs "will perform well for no less than 200k miles" nor does anyone other than you. If you have any proof to the contrary then show us.

I recall the post on another jeep forum where you made this claim for the first time. After some prodding you finally admitted that a videographer employed by Autolite who was at a sales seminar you attended told you that his iridium XP-985 plugs looked unworn after more than 100,000 miles. From that unsubstantiated statement by some unidentified third party you alone extrapolated the 200,000 mile service life.
 
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mots

TJ Enthusiast
May 11, 2018
674
550
OH, USA
#9
Correction....I put the Autolite AP985's in rather than the dual tip APP985's! Somehow I didn't check the numbers before leaving the auto store. I didn't want to wait for a Mopar rail and bought one from NAPA instead. Think I should return it along with getting the right plugs?
I would try the right plugs with the original Mopar coil rail and see if all is well. If it is, then you can return the rail and say it was giving misfire codes and CEL. Otherwise, give the NAPA rail a shot if you have to. I’ve heard better things about NAPA parts in general, but I would still choose OEM if possible.
 

Mike_H

Rust Belt Heavyweight
Supporting Member
Feb 28, 2017
3,587
3,808
Grand Rapids, MI, United States
#11
I had a misfire code intermittently on my jeep too, 04 4.0L. I tried changing plugs, which fixed it for a while. I was also having hard start issues as I was losing the pressure charge in my fuel system. I replaced my fuel pump assembly and the misfire codes went away. Its been 6 months now without setting a code. It was only ever P0301...doesn't make sense to me, but there it is.
 
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YooperSkipper
#12
I would try the right plugs with the original Mopar coil rail and see if all is well. If it is, then you can return the rail and say it was giving misfire codes and CEL. Otherwise, give the NAPA rail a shot if you have to. I’ve heard better things about NAPA parts in general, but I would still choose OEM if possible.
OK.....got the right plugs (APP985), returned the NAPA rail and my new MOPAR coil pack will be here tomorrow. The plugs are pregapped @ .032. Manual calls for .035. What is recommended for these in an 02' 4.0L engine? I really appreciate all the tech tips......thanks!