Random misfire - Jeep Garage - Jeep Forum

Go Back   Jeep Garage - Jeep Forum > Jeep Technical Discussion > Troubleshooting/Problems Discussion

Join Jeep Garage Today
Reply
 
Thread Tools Display Modes
 
  #1  
Old 08-14-2014, 02:21 PM
Member
My Jeep: 1999 4.0L WJ
 
Join Date: Jun 2014
Posts: 85
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 977
chefjacques is on a distinguished road
Random misfire

I am helping my son with his 1997 wrangler sport. It has the 4.0 straight 6 with 5 speed manual transmission. He bought it a month ago and we were going through general maintenance. It was running fine, but just wanted to teach him how to stay on top of stuff. We Gave it an oil change with filter, spark plugs ( old plugs were warn and not gapped to spec. We replaced with same type of plug as was in it.), and a new set of spark plug wires.
After all was done, fired it up and it ran rougher than before rather than better. I figured it could have been a matter of computer needing to re-adjust to new plugs with different gap than what was in it. Well, 2 days later check engine light came on. I hooked up scan tool and it came up as multiple p030_ there were random misfires coming up on multiple cylinders.
I pulled plugs and 2x checked the gap and torque.
2x checked firing order and that all the new wires were seated properly.
Cleared codes and re-started. It still does not idle smoothly.
Any thoughts???


Sent from my iPhone using JeepGarage

Reply With Quote
Sponsored Links
Advertisement
 
  #2  
Old 08-14-2014, 07:47 PM
Frango100's Avatar
The beast from Brazil

My Jeep: 2004 4.7L WJ
 
Join Date: Dec 2009
Location: Brazil
Posts: 7,264
Thanks: 17
Thanked 366 Times in 353 Posts
Rep Power: 529378
Frango100 has a reputation beyond reputeFrango100 has a reputation beyond reputeFrango100 has a reputation beyond reputeFrango100 has a reputation beyond reputeFrango100 has a reputation beyond reputeFrango100 has a reputation beyond reputeFrango100 has a reputation beyond reputeFrango100 has a reputation beyond reputeFrango100 has a reputation beyond reputeFrango100 has a reputation beyond reputeFrango100 has a reputation beyond repute
Re: Random misfire

Not familiar with the 97 4.0 engine. Does it have O2 sensor(s)? If yes, i would check the fuel trim figures with a scanner. Does your scanner show live data? You will need to look at the STFT (short term fuel trim) figures. They should normally be around the 0 and jump up and down -4 to +4. If single digit figures its ok, but when going into the double digits, then there is something wrong. A faulty O2 sensor could cause a negative STFT (the PCM will remove fuel from the standard schedule) until it becomes that poor that it wil not burn properly anymore, causing a misfire.
Low fuel pressure could also cause this, so would be also on my list to check. You can then expect a double digit positive STFT.
__________________
Frank

JEEP Grand Cherokee 4.7 Lmtd 2004
HID 8000K in low and fog / Quad Squad member
KYB Gas-a-Just shocks / Pirelli Scorpion ATR 245/65/17 - Transgo reprogramming shiftkit.

VW Beetle 2.0 TSI 2013
My new toy
Reply With Quote
  #3  
Old 08-14-2014, 11:36 PM
Member
My Jeep: 1999 4.0L WJ
 
Join Date: Jun 2014
Posts: 85
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 977
chefjacques is on a distinguished road
Re: Random misfire

The 97 is obd 2 compliant. So i believe it would have o2 sensors, but i am not looking at it right now. My scanner does have live data capability as well as snap shot when code is confirmed.
I will check the next time i see my son and his car.

I have a couple of questions following your train of thought, why would it start with installation of new plugs and wires. I have always found that when ever a new behavior starts, i will look closely at what i did last because that is likely the cause of new problem.

Also, my son like to shift early and drive with engine at lower rpms. Could the car just need to be run a little harder for a day or so?


Sent from my iPhone using JeepGarage
Reply With Quote
  #4  
Old 08-15-2014, 12:25 AM
Frango100's Avatar
The beast from Brazil

My Jeep: 2004 4.7L WJ
 
Join Date: Dec 2009
Location: Brazil
Posts: 7,264
Thanks: 17
Thanked 366 Times in 353 Posts
Rep Power: 529378
Frango100 has a reputation beyond reputeFrango100 has a reputation beyond reputeFrango100 has a reputation beyond reputeFrango100 has a reputation beyond reputeFrango100 has a reputation beyond reputeFrango100 has a reputation beyond reputeFrango100 has a reputation beyond reputeFrango100 has a reputation beyond reputeFrango100 has a reputation beyond reputeFrango100 has a reputation beyond reputeFrango100 has a reputation beyond repute
Re: Random misfire

I do understand your way of thinking as well, but you have double checked each replaced part and didnīt find anything wrong. If you had only one cylinder misfire, a faulty plug or wire would be suspected. In this case you have a random misfire. So i would not expect a plug or wire to be the problem. There must be something comon which causes the misfire to happen in multiple cylinders. I do agree that it is strange if now an O2 sensor would give a problem, while it was running ok before. Did you check the distributor cap for any hair cracks? You could try the old wires and see if that makes any difference.
__________________
Frank

JEEP Grand Cherokee 4.7 Lmtd 2004
HID 8000K in low and fog / Quad Squad member
KYB Gas-a-Just shocks / Pirelli Scorpion ATR 245/65/17 - Transgo reprogramming shiftkit.

VW Beetle 2.0 TSI 2013
My new toy
Reply With Quote
  #5  
Old 08-15-2014, 09:49 AM
Member
My Jeep: 1999 4.0L WJ
 
Join Date: Jun 2014
Posts: 85
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 977
chefjacques is on a distinguished road
Re: Random misfire

Old wires are not an option. Reason we bought new is because the clips at end of a couple of the wires remained attached to spark plug when the wire was removed.

I will also check distributor cap. It is possible that it developed a crack while i was replacing wires.


Sent from my iPhone using JeepGarage
Reply With Quote
  #6  
Old 08-15-2014, 10:04 AM
sanjuro77's Avatar
Member
My Jeep: 1996 4.0L ZJ
 
Join Date: Dec 2013
Location: NE Ohio
Posts: 220
Thanks: 3
Thanked 19 Times in 17 Posts
Rep Power: 1479
sanjuro77 has a reputation beyond reputesanjuro77 has a reputation beyond reputesanjuro77 has a reputation beyond reputesanjuro77 has a reputation beyond reputesanjuro77 has a reputation beyond reputesanjuro77 has a reputation beyond reputesanjuro77 has a reputation beyond repute
Out of curiosity, what kind of plugs are in it? I've heard from lots of Jeep guys that the straight 6 really likes copper plugs. I'm not saying that's your problem definitively, but maybe something to consider.
You may also want to inspect the rotor in your distributor for damage or wear while you've got the cap off.
Reply With Quote
  #7  
Old 08-15-2014, 10:48 AM
Member
My Jeep: 1999 4.0L WJ
 
Join Date: Jun 2014
Posts: 85
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 977
chefjacques is on a distinguished road
Re: Random misfire

I put in the autolite copper plugs.
Ironically, it was the exact same plug that was in it before.
And yes, i put platinum plugs in my 1999 grand cherokee with same motor and it actually ran worse than with coppers. Not worth the extra money when the motor likes the cheap ones.


Sent from my iPhone using JeepGarage
Reply With Quote
  #8  
Old 08-15-2014, 01:27 PM
sanjuro77's Avatar
Member
My Jeep: 1996 4.0L ZJ
 
Join Date: Dec 2013
Location: NE Ohio
Posts: 220
Thanks: 3
Thanked 19 Times in 17 Posts
Rep Power: 1479
sanjuro77 has a reputation beyond reputesanjuro77 has a reputation beyond reputesanjuro77 has a reputation beyond reputesanjuro77 has a reputation beyond reputesanjuro77 has a reputation beyond reputesanjuro77 has a reputation beyond reputesanjuro77 has a reputation beyond repute
Gap is .035", right?
Reply With Quote
  #9  
Old 08-15-2014, 01:47 PM
Member
My Jeep: 1999 4.0L WJ
 
Join Date: Jun 2014
Posts: 85
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 977
chefjacques is on a distinguished road
Re: Random misfire

Yep


Sent from my iPhone using JeepGarage
Reply With Quote
  #10  
Old 08-15-2014, 01:49 PM
Member
My Jeep: 1999 4.0L WJ
 
Join Date: Jun 2014
Posts: 85
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 977
chefjacques is on a distinguished road
Re: Random misfire

Plugs that came out were worn to 2x that. It was running fine aside from being a little slow to start when cold.


Sent from my iPhone using JeepGarage
Reply With Quote
  #11  
Old 09-16-2014, 09:11 PM
Member
My Jeep: 1999 4.0L WJ
 
Join Date: Jun 2014
Posts: 85
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 977
chefjacques is on a distinguished road
Re: Random misfire

At long last, a follow up. I re checked the plugs and found 2 that must have been bumped while my son was installing them. Gap was wrong. Since then, no issues.


Sent from my iPhone using JeepGarage
Reply With Quote
  #12  
Old 09-17-2014, 08:31 AM
Frango100's Avatar
The beast from Brazil

My Jeep: 2004 4.7L WJ
 
Join Date: Dec 2009
Location: Brazil
Posts: 7,264
Thanks: 17
Thanked 366 Times in 353 Posts
Rep Power: 529378
Frango100 has a reputation beyond reputeFrango100 has a reputation beyond reputeFrango100 has a reputation beyond reputeFrango100 has a reputation beyond reputeFrango100 has a reputation beyond reputeFrango100 has a reputation beyond reputeFrango100 has a reputation beyond reputeFrango100 has a reputation beyond reputeFrango100 has a reputation beyond reputeFrango100 has a reputation beyond reputeFrango100 has a reputation beyond repute
Re: Random misfire

Thought that you had said that you already twice had checked the plugs before?
There are only three coils for the 6 cylinders, which means that always two cylinders at a time will have a spark, one which is used for the ignition in one cylinder, while the other spark is waisted. A too close gap on one plug, will result in a too weak spark in the paired other plug and can cause misfires.Good to hear that it is solved.
__________________
Frank

JEEP Grand Cherokee 4.7 Lmtd 2004
HID 8000K in low and fog / Quad Squad member
KYB Gas-a-Just shocks / Pirelli Scorpion ATR 245/65/17 - Transgo reprogramming shiftkit.

VW Beetle 2.0 TSI 2013
My new toy
Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
random misfire jeepnuts Troubleshooting/Problems 0 01-04-2013 06:03 PM
Help! P0304 cylinder 4 misfire SvenskaJeepGuy Troubleshooting/Problems 16 09-16-2012 02:05 PM
Cold start misfire carldieselgc Export & Diesel Powered Jeep Models 3 10-20-2011 12:49 PM
Mixed plugs = Misfire & 10mpg ! NeilF Engine Performance/Intake/Exhaust 10 01-04-2011 05:56 AM
CYL#4 misfire violant00 Troubleshooting/Problems 5 12-06-2010 12:10 PM

Powered by vBadvanced CMPS v3.2.3

All times are GMT -5. The time now is 07:11 AM.


Powered by vBulletin® Version 3.8.8 Beta 4
Copyright ©2000 - 2016, vBulletin Solutions, Inc.
Copyright 2012 - JeepGarage.Org
The Jeep Grand Cherokee Owners Community

JeepGarage.org is in no way associated with or endorsed by FCA US LLC. Chrysler, Dodge, Jeep, Ram, Mopar and SRT are registered trademarks of FCA US LLC.