TJ misfiring cylinders 2&6, can't identify problem

[2005 Wrangler Rubicon]

Help!
I'm out of ideas and looking for some advice. My check engine /malfunction indicator light started flashing the other day and I’ve been unable to diagnose the problem.
I had been highway driving for about an hour, and stopped for 25 mins or so, and upon starting my jeep back up the check engine light came on, staying on for about 10 seconds, and then started flashing. I also noticed a slight burning smell in the exhaust and a rough idling feel to it. I turned the car off, took a look under the hood for anything noticeable, and made the mistake (as I’ve been told since) of driving it home (another hour). All gauges are normal, but the acceleration feels somewhat restricted.


Since then, I’ve determined that I’ve got 2 cylinders misfiring (2 & 6). I checked spark plugs & wires (all good) and my mechanic checked the computer, the fuel injectors, and did an engine compression test, but he is unable to determine the cause. As we speak, he's checking the valves and valve springs, but is running out of ideas if that fails to diagnose the problem.


Does anyone have any other ideas??


As per usual, thanks in advance for any help.

 

[Jester32]

Plugs. Wires. Moisture in the cap. Plug wire arcing due to an open plug. What year. Does it have a distributor or is it a distributorless ign.

 

[2005 Wrangler Rubicon]

Plugs & wires are good. Mechanic double checked them too. It's a 2005 wrangler Rubicon, and I'm fairly certain is distributorless.

 

[JPNinPA]

Plugs. Wires. Moisture in the cap. Plug wire arcing due to an open plug. What year. Does it have a distributor or is it a distributorless ign.

Uh Uh maybe a 2005 Rubicon?? The 2005 does not have wires and a distributor. It has the coil pack or rail.


The misfiring could be the coil pack or rail, Plugs (what did they look like?) stuck injector, and could be other things like the CPS. Also 2005 have a TSB for the Cam and CPS wearing out Check the link.

Chrysler is paying to install a new cam (2005/2006 owners should read this) - Page 11 - JeepForum.com

What codes?? USe the on/off key trick to read the codes.

Also the current lack of power is when the Jeep goes into LIMP mode. This was after the CEL blinked a number of times.

 

[Jester32]

Uh Uh maybe a 2005 Rubicon?? The 2005 does not have wires and a distributor. It has the coil pack or rail.

The misfiring could be the coil pack or rail, Plugs (what did they look like?) stuck injector, and could be other things like the CPS. Also 2005 have a TSB for the Cam and CPS wearing out Check the link.

Chrysler is paying to install a new cam (2005/2006 owners should read this) - Page 11 - JeepForum.com

What codes?? USe the on/off key trick to read the codes.

Also the current lack of power is when the Jeep goes into LIMP mode. This was after the CEL blinked a number of times.

That's why I asked the year. Check tge spark plug boot for carbon tracing as well as the ceramic on the plug. See lots of problems with jobber plugs. We had alot of problems with the crank and cam sensors. Also cams wearing with the removal of additive packs from oils. Seen lots of Mis fires from that but it's usually accompanied by a noise complaint.

 

[Jester32]

Hope it's an easy fix. Cheers. .

 

[Utah_jeepster]

Hopfully this thread will give you some ideas
http://www.jeepz.com/forum/cj-yj-tj-jk/29788-4-0l-multiple-cylinder-misfire.html

 

[SteelHeadz]

Have you had the catalytic converter checked?. The misfire could be caused by a plugged catalyst since there is an upstream and downstream O2 sensors along with the symptoms you described with the lack of power and burning smell. The link that Steven (Utah Jeepster) had posted will help you diagnose a lot of misfire issues as well.

 

[2005 Wrangler Rubicon]

Thanks for the tips, but still nothing yet.

My mechanic was in the process of checking the valves and valve springs, but has since come down with a bug...figures!
Anyway, the codes I'm getting are P0302 & P0306, and I'm not 100% convinced my Jeep is going into Limp Mode, as the acceleration reduction is only slight/minor (I could still hit 3000 RPMS). What I did notice is that when idling, or driving at low speeds, the entire Jeep shakes. As well, the exhaust appears to be puttering, as in, not a constituent output of the fumes.

So far, the plugs & coil appear to be in good shape, the fuel injectors are fine, computer appears to be good, and it can't be bad gas, as I hadn't topped up in between.
What I've taken from the responses, is that if nothing is wrong with the valves, the next best bet would be to check the catalytic converter, the CPS & catalyst, and for a carbon build up on the valves. That about right?

Thanks again for all the responses.

 

[Utah_jeepster]

Random or cylinder mis-fire is the hardest code to nail down since combustion is a three legged stool.

You need air, spark and ignition to get it, with either of those three not working the stool falls over.
Start with the cheapest then proceed to to the most expensive.
First has your mechanic smoked the engine? This simple test shows if you have a vacuum or air leak.
Next I would change plugs and see if that helps, then check and or replace the crank and cam position sensors, with careful inspection of the crank angle sensor gears (Cam position sensor housing)
Next is changing the two injectors around to see if the problem changes cylinders. Then change the coil rail. Then try a decarbonizing of the valve seats as outlined in the first part of my thread.
Then go for the next phase as I am by trying to replace the fuel pump and strainer. Making sure you clean and or replace the fuel pulse valve on the fuel rail. As I said I have good fuel pressure but what I am thinking might be the problem is the fuel volume.
Ask you mechanic if he has access to Mitchelle's online service, it should list the technical service bulletins or your current year. It should follow the one in my posting as a possable cause to your mis-fire.

All in all this is one of the most stubborn and hardest problems for to try and nail down the cause. With my part time work at a local parts store this particular code is the hardest to explain and help our customers to diagnose. P0300-P0308. So dont feel your alone here we are trying to help, but like I said, this problem is a three legged stool and trying to nail down which leg is broken is the hardest problem out there.

 

[2005 Wrangler Rubicon]

Ok, so the mechanic thinks he has figured this out, but I still have a couple of questions.


My mechanic re-checked the fuel injectors today, when he noticed that one did not have a pulse. He traced it back to the PCM, which he says is shot and must be replaced. He says he’s going to ‘comp’ me the PCM because of the trouble I went through, but I feel like he knows something that I don’t.
What I don’t understand is how they missed this on the first time around while check the fuel injectors, and why the PCM error didn’t get caught when they check it. Any insight?


As well, does this sound reasonable that a faulty PCM would cause the problems I described?
Anyway, thanks again for all the responses and hopefully this resolves this issue!

 

[JPNinPA]

How did he check the injector?
It's nice he is comping the PCM. That's a $600 item. I would call a dealer on this one as there are reflashed programming for the PCM. But I don't know all the reasons for the various flashes.

 

[volaredon]

coil pack. had something like that on my 01 XJ;
I don't think it was those 2 cyls but it was 2 adjacent firing ones which 2 and 6 are (firing ord; 1-5-3-6-2-4) see; 6 & 2 fire one right after the other; if they were next to each other in the block I'd say head crack, gasket etc; but being as they are next to ea other in the FIRING order, what else "connects" them but that coil pack>? If it had a distributor I'd be looking for bad wires or cracked cap.