Hey everyone,

First off this is going to be a bit long but I want to make sure I mention as much info as I can!!

My 2001 Jeep Cherokee Classic (174k miles, I6, 4.0) has been giving me a random misfire on Cylinder 5 and a rough idle for the last few months or so. I am a student and it's my daily driver so I haven't really had a choice in whether or not I could send it to the shop for a big fix until recently. I just got it back after it was in the shop for a week, and had the head replaced. The old head was a TUPY put on by the previous owner and the shop told me that it was machined terribly and had warped since it wasn't torqued down right. The new head is a clearwater with new valves and springs too. Work was done at the shop. That was about three weeks ago. The rough idle is still there and now I am getting concerned considering the new head/valves/springs didn't fix it. I was sitting today in the parking lot waiting for the car to warm up since it's starting to get cold here in Michigan. The engine warmed up, ran rough, and then stalled out. All while in park and it died after I feathered the engine a bit (brought it up to just over 1000 RPM and then let it go, did this about three times and then it died). I started it back up and it continued to run a bit rough but didn't throw a code. I put it in drive and pulled out of the parking lot. It ran/runs well when I'm driving down the road, it just runs rough when I am sitting at a stoplight (RPMs bounce a bit and hover right in between the two small dashes under 1000 RPMs on the tach) or in park and is only rough when the engine is warmed up. Later I was driving down the road and the CEL comes on. No changes in RPM, no bucking or jerking, it doesn't stall or hesitate, feels like nothing changed but the light came up. Got to my class and checked the codes before running in and this is what I can up with:

P0300
P0304
P0305
P0306

Things changed leading up to this:
- New clearwater head
- New valves and springs, from clearwater
- New gaskets (head, exhaust, valve, etc)
- New NGK copper plugs (NGK 3459 ZFR5N V-Power Plug)
- Changed the coil pack about 30k miles ago (from O'Reilly's not sure of the brand)
- new 4 hole injectors 30k miles ago (Bosch injectors from eBay with plenty of good reviews)
- Did "Big 3" cable upgrade through JeepCables.com
- new "high output" (160A) alternator
- Seafoamed intake and gas tank recently thinking maybe an injector is clogged

My thoughts moving forward:
- Swap out coil pack for a new coil pack from O'Reilly's (I think I have a lifetime warranty so hopefully it won't cost anything)
- If that doesn't catch it, swap injectors on 1, 2 and 3 with 4 5 and 6 injectors and see if it follows, if so replace the injectors
- Replace TPS?
- Replace the crankshaft position sensor? (Wouldn't it not start or die while driving?)
- Replace the camshaft position sensor? (Wouldn't it not start or die while driving?)
- O2 Sensors? (Wouldn't I get a code for this?)
- Possible exhaust issue causing rough idle and misfire?
- Could the alternator be bad? It has a bit of a whine when running, but it goes away when I rev the engine and the volts are strong at 14 when driving down the road and only dip a bit below 14 at idle under load (i.e. with headlights on high beams, aux lights, full blower on, etc.)

I have 6-months to bring it back to the shop if there was an issue with the head for the repair to be covered, but I really want to avoid doing that since 1. they may charge me considering that they will have to figure out if the misfires have to do with the work they did, and 2. I don't think this issue links back to the head job considering its intermittent misfire and is a continuation of the issues I was having BEFORE the head was changed.

Sorry for the novel. Hoping someone can help me out with this. Any help would be greatly appreciated!! I am not super mechanically inclined, I only started doing research and working on cars once I bought this jeep two years ago, but I am a fast learner and I love working on this thing. I want to fix it so it isn't suffering anymore! If you have any questions or need any additional information that would help please let me know and I'll answer whatever I can to get closer to a solution!

THANK YOU!

 

After warm up does it idle normally?

I had a similar idle issue (98 XJ) and had to keep my foot in it until a certain temp (120°F or abouts) and then it would settle down to a rough idle on its own. It turned out to be the idle air controller (IAC) in conjunction with the #1 O2 sensor. I think the combo failure of the two devices fooled the computer because with both faults going simultaneously no code could be distinguished and thrown. Anyway, I changed the IAC and that fixed the idle to the extent that it would run (cold engine) without my feathering the pedal, but the rough idle problem remained. A new O2 sensor returned the engine's smooth idle. I'm actually pretty confused that the shop performing the work did not zero in on this for you.

 

Hey!

Thank you for answering! I forgot to mention in the list above that I just changed the IAC a little bit ago as well. It was a 'Standard parts' company that had some good reviews, but I could order a mopar one and see if that helps! Sounds like swapping out the 4 O2s on it wouldn't hurt either!

And yeah, truth be told I feel like I got taken for a ride on this and the shop wanted to order some junky head and throw it on there with barely any warranty, I hopped on here and found clearwater and had them order that instead. I'm looking at it as at least I know I havea new head that is high quality with new values springs and gaskets! hahah that is one expensive lesson though

 

With all that work done, two things come to mind. These coil packs are fidgety and if the little springs get bent inside you get a miss fire. With all that work done, someone could have dropped something on it and tweaked something. I would start with that. But also first check all wire connections because a misfire is where the spark and gas aren't working. If you take a straight metal ratchet bar and put the handle side to your ear and the other side on the base of the plug you should be able to hear the tick of the spark plug firing. So... check all wires, check coil pack, check plugs (its not uncommon to have a bad plug from factory), then check injectors. In my opinion Seafoam is a bunch of crap. I wouldn't use it and I am a fan of stock injectors. Good luck.

Also found this in forum: If you know which 2 cylinders are misfiring, it's easy enough to find out if it's the coilpack. Coil #1 fires cylinders #1 and #6, Coil #2 fires cylinders #3 and #4, Coil #3 fires cylinders #5 and #2. I've been driving a 4.0 with a coil rail for 6 years and have already had to replace the SOB! It's a known failure point on the newer 4.0 engines. A scanner should be able to tell you which 2 are misfiring. If they are a matched pair, the answer is obvious. Pull the plugs and look at them!

 

It's not uncommon for the coil rail to short internally and cause random misfires. I'm not sure how they are laid out internally, but think about this. Coil #1 fires #1 and #6 at the same time and they are on opposite ends of the rail. I put a Standard Motor Products rail in my 2000 WJ around 50K miles ago.

 

Hey everyone!

Wanted to follow up on this. Haven't had very many issues with the misfires since I replaced my battery, radiator, thermostat and housing, and water pump. Not sure if any of that helps but that is what ended the problem. The shop also said that after I installed brown dog motor mounts the knock sensor could be getting confused. They said there are no issues and just drive it and watch out for anything that pops up. I replaced the coil pack also as folks mentioned above so that could also have helped. Honestly, I don't have a garage where I live so I can only work on this when I come to my parent's place (8hrs away) so when I get here I try to load as much into the parts cannon as I can.

I hope that helps if someone is looking for an answer to this same problem I had.

 

Glad it finally worked out...to the best of my knowledge, the 4.0 has NO knock sensor