#6 missfire and rough Idle

[jthoman]

Hi everyone, I'm new to the site and I just bought my first Jeep Liberty. Its an '07 Sport and it's having some trouble idling. It has thrown a code and it came up as a #6 misfire. I ran a bottle of injector cleaner through it and the dealership did a treatment to clean carbon out of the valves. The technician said it was an "Idle only misfire". It seams to be at its roughest when the AC is off and it smooth's out some when the AC is on. Then last night I was out and about and while I was sitting at idle for a little bit the check engine light started to flash. I shut the engine off and restarted it and it was gone. Overall it runs fine otherwise. It has good acceleration. If anyone has any ideas please tell me.

Also, I'm not sure if this is pertinent, but it idles at 600 rpm. I'm not sure if this is the normal idle speed for these engines or if this is another symptom.

 

[ltd02]

600 sounds about right to me. The 3.7 isn't the smoothest engine out there but if you're getting CELs then that's a problem. Was the latest code a #6 misfire too? I'd swap the #6 coil to another cylinder and see if it follows. Maybe have a look at that plug too. Make sure nothing is "contaminating" the #6 coil. It sits beneath the coolant reservoir near the firewall on the passenger side. If you get sloppy when filling or there is a leak from the hose at the bottom it can get down into the coil.

I had a flashing CEL about two years ago and shut it down immediately. Was #6 misfire. Started it back up and it never came back (two years later).

 

[uss2defiant]

agree w/ ltd02.

1. move coil #6 and see whether the problem follows.
2. check the condition of the spark plugs. should be NGK 6987 spark plugs.

otherwise, bring it back in to get it checked out. I assume it is under some sort of warranty?

 

[jthoman]

I did replace the plugs the day after I bought it with ngk plugs. Sorry, I forgot to mention that. But I will try moving the coil and see if that makes a difference. I wouldn't mind a slightly rough idle but is it supposed to make the whole car vibrate?

 

[HoosierJeeper]

No...my KJ and WK with the 3.7 idle smooth, smooth as my LR3 with the 4.4 Jag engine. RPMs should stay dead steady nearly.

 

[jthoman]

According to the tac it holds steady. Is it possible the IAC is gummed up with carbon and causing it to idle rough? I am going to try moving the #6 coil pack to a different position and see if the code follows.

 

[HoosierJeeper]

For sure that's possible...yeah try cleaning the IAC and the TB.

 

[uss2defiant]

The TB might be cleaned if they used the can of Mopar combustion chamber cleaner.
You could look at some live data from the obd2 and see where the issue is.

 

[jthoman]

Well I cleaned the TB today and it wasn't horrible. I'm not sure what they did exactly just that they said they were going to do something to clean the carbon build up off of the valves. The manager at the dealership said these engines are notorious for carbon build up. I also cleaned out the PCV as well. I did replace the coil pack on the #6 cylinder to see if that works and its still running rough and the CEL did start to flash after idling for a while again. The guy at the parts store said he can feel a misfire but the vibration in the car. But again it seams to only flash when the AC is off. Can a bad valve cause a misfire?

 

[Hockeygoon]

Well I cleaned the TB today and it wasn't horrible. I'm not sure what they did exactly just that they said they were going to do something to clean the carbon build up off of the valves. The manager at the dealership said these engines are notorious for carbon build up. I also cleaned out the PCV as well. I did replace the coil pack on the #6 cylinder to see if that works and its still running rough and the CEL did start to flash after idling for a while again. The guy at the parts store said he can feel a misfire but the vibration in the car. But again it seams to only flash when the AC is off. Can a bad valve cause a misfire?

A bad valve can cause a miss but I doubt it would go away when you turn on the AC and would most likely get worse.

Did they do any diagnostics on the injectors? I say this because the engine throttles up when the AC compressor kicks on - and your miss goes away. A partially clogged injector that leaves that piston fuel starved at idle could be the issue.

Troubleshooting and Cleaning Dirty Injectors


Troubleshooting can lead you to the fuel injectors needing to be cleaned, if an engine is experiencing any of the classic symptoms of dirty injectors, such as lean misfire, rough idle, hesitation and stumbling on light acceleration, a loss of power, and higher hydrocarbon (HC) and carbon monoxide (CO) emissions. Lean misfires can cause newer systems (OBDII) to read a misfire code or multiple codes depending on the effected injector(s).