Constant misfire

[Smokinjohnny1]

My check engine light came on code was p306. I changed the plugs put in champion7042 and an aftermarket coil rail. Light went out. Two days later light came back on with same code. Was told the aftermarket rail was not a good option so I put an OEM rail on it. Light went out two days later light back on. Any tips to figure this out amd fix it are much appreciated

 

[CharlesHS]

Shortly after installing those exact spark plugs in my '01 4.0L I started having numerous random misfires.
I talked to a friend that has 30+ years as a Jeep dealership mechanic; he advised me to drive up to have him connect a factory troubleshooting console. The engine was receiving numerous misfires and he asked why the 7034 plugs were installed instead of the factory recommended NGK ZFR5N. The factory had determined (around 2001) that some Jeeps DO NOT operate well with the Platinum tipped spark plugs,
Once the NGKs were installed he performed a Combustion Chamber cleaning and rechecked the misfires; he found the misfires were gone.
Some members will swear by the platinum plugs and some will swear at them....

EDIT: I rechecked the maintenance book that I maintain and found the plugs were 7034 and not 7042.

 

[Jerry Bransford]

I can't find a 7042 Champion on their website, can you verify that number please? Jeeps new enough to have the coil rail ignition system are VERY fussy about what they'll run well on. There are several plugs that will cause misfires even though some parts catalogs indicate they are suitable... namely platinum plugs that only have their platinum coating on just one side of the gap.

Also please complete your Profile, it's what we use to look up information about your Jeep when answering questions like this. Especially your model year, engine size, and transmission type.

Also, does your engine run any better when cold? As in does it only start misfiring once it starts to warm up?

 

[Jerry Bransford]

Some members will swear by the platinum plugs and some will swear at them....

The only members who will swear at their platinum plugs are those who installed the wrong type of platinum plugs when they have a new enough engine for the type of platinum plug to make a difference.

 

[Smokinjohnny1]

Shortly after installing those exact spark plugs in my '01 4.0L I started having numerous random misfires.
I talked to a friend that has 30+ years as a Jeep dealership mechanic; he advised me to drive up to have him connect a factory troubleshooting console. The engine was receiving numerous misfires and he asked why the 7042 plugs were installed instead of the factory recommended NGK ZFR5N. The factory had determined (around 2001) that some Jeeps DO NOT operate well with the Platinum tipped spark plugs,
Once the NGKs were installed he performed a Combustion Chamber cleaning and rechecked the misfires; he found the misfires were gone.
Some members will swear by the platinum plugs and some will swear at them....

Thank you .. I will replace the plugs with the plugs you suggested

 

[Smokinjohnny1]

I can't find a 7042 Champion on their website, can you verify that number please? Jeeps new enough to have the coil rail ignition system are VERY fussy about what they'll run well on. There are several plugs that will cause misfires even though some parts catalogs indicate they are suitable... namely platinum plugs that only have their platinum coating on just one side of the gap.

Also please complete your Profile, it's what we use to look up information about your Jeep when answering questions like this. Especially your model year, engine size, and transmission type.

Also, does your engine run any better when cold? As in does it only start misfiring once it starts to warm up?

Sorry I thought I did all that in my profile .. going to profile and adding info now .. thanks

 

[Jerry Bransford]

Thank you .. I will replace the plugs with the plugs you suggested

If you are already at that point I strongly recommend any of these three well-proven spark plugs... the Autolite iridium XP-985 good for 200k miles, or either of these two platinum plugs good for 100k miles... the Autolite APP-985 or Champion 7412. Personally I run the XP-985 in my 2004.

All three of those are great plugs and well proven to run reliably. All three utilize fine-wire electrodes which really help provide reliable ignition in difficult combustion situations.

 

[Smokinjohnny1]

Thank you for the tip . I guess I will go thru all the spark plug options until I find the right one that fixes the problem . Also I think I fix my profile and all the info is there for people to read

 

[CharlesHS]

The only members who will swear at their platinum plugs are those who installed the wrong type of platinum plugs when they have a new enough engine for the type of platinum plug to make a difference.

The platinum spark plugs that I installed were the ones that were recommended on the forum for my '01 4.0L.
Once the factory recommended NGK spark plugs were installed; the misfires went away.
Can't argue with results and I will believe what my factory trained Jeep technician recommends.

 

[Smokinjohnny1]

I can't find a 7042 Champion on their website, can you verify that number please? Jeeps new enough to have the coil rail ignition system are VERY fussy about what they'll run well on. There are several plugs that will cause misfires even though some parts catalogs indicate they are suitable... namely platinum plugs that only have their platinum coating on just one side of the gap.

Also please complete your Profile, it's what we use to look up information about your Jeep when answering questions like this. Especially your model year, engine size, and transmission type.

Also, does your engine run any better when cold? As in does it only start misfiring once it starts to warm up?

Sorry I didn’t see the last part about running cold . Yes it runs better cold problem gets worse as the engine warms up

 

[Jerry Bransford]

Sorry I didn’t see the last part about running cold . Yes it runs better cold problem gets worse as the engine warms up

That sounds like it could be caused by a bad O2 sensor then. What year is your TJ and what engine? Your updated Profile wasn't showing up when I last checked it. A bad O2 sensor can really screw up the fuel mixture and cause bad misfires.

The computer only uses the upstream O2 sensors for setting the fuel mixture once the engine has started to warm up.

If you end up replacing your upstream O2 sensor(s) be sure to ONLY install NTK, avoid Bosch or other brands. The downstream O2 sensors will not cause misfires.

Also your Profile is still not showing up, what year TJ and what engine/transmission do you have?

@Chris is there a problem with our Profiles? I click on 'View Profile' and not even my Profile is showing up.

 

[Rubicon88]

NGK plug ZFR5N is what you need to run.

You very well might have another issue
(Runs good for 2 days then misfire)
but you need to run the above plug.

Moreover do you have a scan tool? If so you need to watch the behavior of your o2 sensors. The upstream o2 is the one of most importance. If its behaving properly...once the high rpm drops from the cold start and your engine comes down to normal idle rpm...the o2 should be going reading something like .782 then immediately go to like a .086, (as example)

In other words in really fast time you should see it behaving like hi,lo,hi,lo, in one second. It should be a constant hi,lo hi,lo,hi,lo,hi,lo
Typically at idle the hi will be around .788-.845

The low will be around .078-.134

Just remember the exact numbers aren't really important but the behavior of hi,lo constantly is of the utmost importance

Good luck.

 

[Smokinjohnny1]

That sounds like it could be caused by a bad O2 sensor then. What year is your TJ and what engine? Your updated Profile wasn't showing up when I last checked it. A bad O2 sensor can really screw up the fuel mixture and cause bad misfires.

The computer only uses the upstream O2 sensors for setting the fuel mixture once the engine has started to warm up.

If you end up replacing your upstream O2 sensor(s) be sure to ONLY install NTK, avoid Bosch or other brands. The downstream O2 sensors will not cause misfires.

Also your Profile is still not showing up, what year TJ and what engine/transmission do you have?

@Chris is there a problem with our Profiles? I click on 'View Profile' and not even my Profile is showing up.

2003 Tj

 

[Smokinjohnny1]

2003 Tj 4.0L it’s a manual transmission not sure exactly what teams it is though . New to this jeep world amd still learning all about the Jeep .

 

[Rubicon88]

Nv3550

 

[Jerry Bransford]

NGK plug ZFR5N is what you need to run.

No, he does not need to run that exact plug. Period. If we needed to run that plug to avoid problems we'd all be running it... and we're not. Just because it works for you does not need everyone needs to run it.

 

[Jerry Bransford]

2003 Tj

No guarantee it's not an inappropriate spark plug but that your misfires only occurring once the engine has started to warm up has me leaning more towards a bad upstream O2 sensor and not the incorrect plugs. Your TJ has two upstream O2 sensors. Do you know if your engine is a California compliant or a 50 states version?

 

[Rubicon88]

No, he does not need to run that exact plug. Period. If we needed to run that plug to avoid problems we'd all be running it... and we're not. Just because it works for you does not need everyone needs to run it.

Ok Jerry.
We deal with Chrysler shit every day. There is a reason the Original Equipment Manufacturer recommends certain components. These mfg's don't care one iota what you want to run...in particular once your in service date and mileage have expired. They do however care about paying warranty claims and invest a lot of time, money and research into this stuff to avoid such.

From what I've read the OP is having issues. The OP isn't using the recommended plug by Jeep. OP may have more than one problem happening but getting back to ground zero is the goal especially when they probably don't have access for a proper diagnosis.

Hence posting the question on this forum.

Thx