Repeated Misfire Codes

YooperSkipper

Member
Joined
Jan 31, 2018
Messages
52
Location
Michigan, United States
A while back I posted a problem with misfire codes.....mainly PO300, 01 & 02 on my 2002' 4.0. It was suggested I needed to replace the coil rail which I did. I bought an OEM Mopar unit as was suggested and put in new APP 985 plugs as well. All was well for a while then I started getting just a PO301 code. I cleared to see if it would return and it did. A couple of times I got a bad engine miss with the CEL flashing. The codes were PO300 & PO301. The thing that puzzles me is that when I turn off the engine while it's missing, clear the codes, and start it back up again it purrs like a kitten and runs well for awhile until the PO301 pops up again! I had a PO122 just once and never returned. I just can't help but feel If something was really wrong it would be there constantly until corrected? I replaced the ECM last summer when the engine would not start at all! I'm beginning to wonder if this could be the problem? I'm located in the UP of Michigan and knowledgeable Jeep Wrangler mechanics are scarce! The closest Jeep dealership is 60 miles away so I'm pretty much relying on help from this forum before I give up and take it to the dealer! Any suggestions?
 

danof76

TJ Enthusiast
Supporting Member
Joined
Nov 21, 2018
Messages
317
Location
San Antonio, TX
Ouch, that's a tough break. I was thinking ECM myself, could it possibly be bad O2 sensors as well? Maybe bad wiring somewhere?

Where did you get the new ECM from? Does it have a warranty?
 

KCsTJ

TJ student
Supporting Member
Joined
Jan 9, 2018
Messages
2,851
Location
Farmersville, TX, United States
I would focus on cylinder #1 first, and all the wiring to it as well.

Do you happen to know if the engine has ever overheated possibly?

Is it holding antifreeze?
 

Alex0413

TJ Enthusiast
Joined
Oct 26, 2018
Messages
184
Location
Baton Rouge, LA
I bet you have cracked fuel injectors that are intermittently going out, pull them and check if they're cracked. I'll bet new injectors fix it!
 
  • Like
Reactions: mots

mots

Goldilocks Jeep Enthusiast
Supporting Member
Joined
May 11, 2018
Messages
1,249
Location
OH, USA
I bet you have cracked fuel injectors that are intermittently going out, pull them and check if they're cracked. I'll bet new injectors fix it!
It could be, but likely only #3 is affected per the codes. If I were to swap any of them, I'd do #3 first.
 
  • Like
Reactions: Alex0413

Alex0413

TJ Enthusiast
Joined
Oct 26, 2018
Messages
184
Location
Baton Rouge, LA
It could be, but likely only #3 is affected per the codes. If I were to swap any of them, I'd do #3 first.
absolutely, swapping 3 with 4 and seeing if the code moves to 4 will confirm it. They're so cheap to change, and a maintenance item, I'd replace them all.
 

KCsTJ

TJ student
Supporting Member
Joined
Jan 9, 2018
Messages
2,851
Location
Farmersville, TX, United States
The recurring code is P301 which is a misfire cyl #1, if my engine code book is right.
How many miles on the motor?
Had it ever overheated?
Generally speaking, most misfires are electrical in nature so I would start there.
 
  • Like
Reactions: mots
OP
YooperSkipper

YooperSkipper

Member
Joined
Jan 31, 2018
Messages
52
Location
Michigan, United States
Ouch, that's a tough break. I was thinking ECM myself, could it possibly be bad O2 sensors as well? Maybe bad wiring somewhere?

Where did you get the new ECM from? Does it have a warranty?[/QUOTE

When it wouldn't start I had to have it towed in to a local shop. They diagnosed the problem to be the ECM. I'm not sure where they got it but the unit itself was $400! I do believe it has a warranty though. I'll get that info from that shop and pursue myself. If I knew then what I know now, I would have done the work myself. Changing the ECM is just plug & play right?
 

KCsTJ

TJ student
Supporting Member
Joined
Jan 9, 2018
Messages
2,851
Location
Farmersville, TX, United States
I found a cyl #3 misfire I have been hunting for a year, turns out it's a "tuliped" valve in cyl #3.
The head comes off and new valves go in for the fix.
 

mots

Goldilocks Jeep Enthusiast
Supporting Member
Joined
May 11, 2018
Messages
1,249
Location
OH, USA
The recurring code is P301 which is a misfire cyl #1, if my engine code book is right.
How many miles on the motor?
Had it ever overheated?
Generally speaking, most misfires are electrical in nature so I would start there.
Good catch I read the code wrong.

Edit: that realized, definitely concentrate on cylinder #1, check the wiring and check the injector connector for corrosion etc. Replace injector #1 if all else fails. If the injector circuit is not sound, it will typically throw a code. In this case, it could be intermittent failure of the injector. Keep pulling codes often to make sure other cylinders are never affected. If you still have the PO122, make sure you are using a Mopar TPS sensor, not a cheap aftermarket one.
 
Last edited:
  • Like
Reactions: KCsTJ
OP
YooperSkipper

YooperSkipper

Member
Joined
Jan 31, 2018
Messages
52
Location
Michigan, United States
The recurring code is P301 which is a misfire cyl #1, if my engine code book is right.
How many miles on the motor?
Had it ever overheated?
Generally speaking, most misfires are electrical in nature so I would start there.
96800 miles. I don't know if it was ever overheated....anything possible buying used though! I've been searching for any wiring issues and so far have found none!
 
  • Like
Reactions: KCsTJ

KCsTJ

TJ student
Supporting Member
Joined
Jan 9, 2018
Messages
2,851
Location
Farmersville, TX, United States
Nice jeep btw, i would carefully examine the wires @ the #1 fuel injector, make sure they are not touching, check condition of the insulation..if it looks like this, it could cause a misfire...
20181222_080213.jpg
 
  • Like
Reactions: Alex0413

danof76

TJ Enthusiast
Supporting Member
Joined
Nov 21, 2018
Messages
317
Location
San Antonio, TX
When it wouldn't start I had to have it towed in to a local shop. They diagnosed the problem to be the ECM. I'm not sure where they got it but the unit itself was $400! I do believe it has a warranty though. I'll get that info from that shop and pursue myself. If I knew then what I know now, I would have done the work myself. Changing the ECM is just plug & play right?
Yes it should just be plug and play, that's a high charge though. Flagship One (FS1) is who @Chris stands by and always mentions @wrangler6035 who works for them charges only about $160 on their site. Not sure if that's with a core or what though.

In either case if it ends up being the ECM maybe talk to the shop. If not go with FS1 if need be, they have a warranty as well, not sure what it's length or conditions are though.
 

mots

Goldilocks Jeep Enthusiast
Supporting Member
Joined
May 11, 2018
Messages
1,249
Location
OH, USA
In addition to my "Update" in the last post. If you have a multi-meter, you can test the injector electrical circuit to see if it is in spec. However, if resistance is out of spec, it should throw a CEL. Just wanted to throw this out there in case it is useful during troubleshooting.

Disconnect the fuel injector connector
Place an ohmmeter across the two terminals on the injector
Resistance should be 12 ohms ±1.2 ohms at 20°C (68°F).
Compare to a new injector or known working injector to prove it out.
 
  • Like
Reactions: KCsTJ

wrangler6035

New Member
Joined
Nov 8, 2018
Messages
22
Location
New York
Yes it should just be plug and play, that's a high charge though. Flagship One (FS1) is who @Chris stands by and always mentions @wrangler6035 who works for them charges only about $160 on their site. Not sure if that's with a core or what though.

In either case if it ends up being the ECM maybe talk to the shop. If not go with FS1 if need be, they have a warranty as well, not sure what it's length or conditions are though.
We really appreciate the recommendation! If you would like to reach out through private messages and I can run your VIN through my Chrysler license.
 
  • Like
Reactions: danof76