08 JK engine miss ,baffling.

Gjac wrote:
Glad you found the problem, as you can see from my original post I had a cracked spark plug also and it was very hard to see.

#5 is the toughest plug to replace and it was the last one I removed. I should start with the most difficult ones first then breeze through the easy ones but I seldom do. Sometimes it's all about going back to basics.

Glad you found the problem, as you can see from my original post I had a cracked spark plug also and it was very hard to see.

Gjac wrote:
Did you ever find the problem?

I did and I forgot to post the results. I found a hairline crack in spark plug #5. I changed the wires and plugs as part of a regular maintenance issue and #5 was the last plug to remove. I looked at it and it seemed ok, with dirty gloves I wiped around the plug and the dirt/grease went into the crack and exposed the problem. The Jeep runs fine now. I do have a new scanner now that I was hesitating to buy for whatever reason, I carry one in the coach when I travel.

Did you ever find the problem?

j-d wrote:

klutchdust wrote:
I owned a 1985 "birthday bird" Thunderbird that ran when it felt like it and stopped when it felt like it regardless of where you happened to be. Many years later Ford fessed up and said a module in the ignition system would shut off the fuel pump once it got hot, not always but sometimes.

We had an '84 and an '86 TBird. Both V6 engines with EFI. The '86 was a deluxe one with six way power seats both sides, electronic dashboard and AOD electronic overdrive. I really liked those cars. '84 was plainer with 3-speed auto and one 4-way power seat. It seemed to have more "heart" though. I think the distributor module was "TFI" Thick Film Integrated (Circuit) and some of those modules were troublesome.

Very troublesome.
So we just took the jeep to the store ,about 5 miles each way,and it didn't miss at idle, miss during heavy acceleration (or as the wife said, man ,you really punched it) and just ran like normal. Tomorrow I am going 60 mile roundtrip on the freeway. That seems to be the time it may stumble and even backfire. Once driving it hard and decelerating for an off ramp it burped through the intake. Heat related...hhmm.....

klutchdust wrote:
I owned a 1985 "birthday bird" Thunderbird that ran when it felt like it and stopped when it felt like it regardless of where you happened to be. Many years later Ford fessed up and said a module in the ignition system would shut off the fuel pump once it got hot, not always but sometimes.

We had an '84 and an '86 TBird. Both V6 engines with EFI. The '86 was a deluxe one with six way power seats both sides, electronic dashboard and AOD electronic overdrive. I really liked those cars. '84 was plainer with 3-speed auto and one 4-way power seat. It seemed to have more "heart" though. I think the distributor module was "TFI" Thick Film Integrated (Circuit) and some of those modules were troublesome.

j-d wrote:
Does it sit there and miss at idle, when you can dink with it, like unplugging the injector connector and seeing if that makes a change? I wish we could narrow between Ignition and Fuel before going all out on Ignition.
I don't have any of these, but you can get a "Noid Light" that plugs where an Injector connects. Doesn't tell you injector efficiency, but at least confirms the PCM is pulsing the injector.
Comfort amid Frustration: Coil is probably less expensive than a diagnostic hour at a shop.
It'll be a little while but I'll start a thread in automotive to see if anybody has a tip.

Great ideas. I registered on that site you mentioned in the PM. I am going to tinker around with it again and start from basics to be sure I am not missing something obvious. That coil though, could it break down or fail after getting hot. What is it that is getting hot then failing.
I owned a 1985 "birthday bird" Thunderbird that ran when it felt like it and stopped when it felt like it regardless of where you happened to be. Many years later Ford fessed up and said a module in the ignition system would shut off the fuel pump once it got hot, not always but sometimes. You never know.

Does it sit there and miss at idle, when you can dink with it, like unplugging the injector connector and seeing if that makes a change? I wish we could narrow between Ignition and Fuel before going all out on Ignition.
I don't have any of these, but you can get a "Noid Light" that plugs where an Injector connects. Doesn't tell you injector efficiency, but at least confirms the PCM is pulsing the injector.
Comfort amid Frustration: Coil is probably less expensive than a diagnostic hour at a shop.
It'll be a little while but I'll start a thread in automotive to see if anybody has a tip.

j-d wrote:
It has a six-hole pack and missing on #5 only...? Is this one of the "waste spark" systems where a Six would have a Three-Coil Six-Hole pack? Parts lookup shows that. Usually when one cylinder misses with a pack like that, so does the opposite one in the firing order, the one connected to the opposite side of that one coil out of the three in the pack.

Is this the setup you have? Is the engine a V6 3.6L PentaStar? I'm wondering if the sparkplug cables reach, could you switch them, and see if the problem migrates from #5 to its opposite #6.

We nearly scrapped a Subaru where a NEW coil was dropping two cylinders. Another NEW coil and it was back to its old self.

Please send the Engine's Name, Model Code, and Displacement. I want to try this on another forum.

The 08 were pre pentastar engines, I have the 3.8 . When checking the coil The resistance values on the primary and secondary were consistent with one i checked at the local auto parts store. Something is failing after the jeep warms up or is driven long distance like freeway driving.
Counting the original coil, this jeep is on number 3. The first two that failed showed a low resistance on the coil pack. Setting the meter to 200 ohms I checked the primary coils and they read 1.9/1.0/1.9. Ahaa I said, one is reading lower however the new one reads the same thing.
I may just buy a new coil as the history of this Jeep has shown it eats coils. I am missing something, I need to start at square one again. The spark plug wires are so short I can't swap over to check.
I have read a lot of posts on different forums and the results have been all over the place, bad plug, wire, connector. I'll keep trying, thanks to all that have offered opinions, all good suggestions.

I had this happen on my tow car several years ago and it drove me crazy until it finally started to throw a code, miss fire #3 cylinder. I switched #3 coil with #4 still same code, so I knew coil pack was good. Switched #3 and number #4 plug and code went to misfire #4 cylinder so I new it was the spark plug. After careful inspection I noticed a tiny black streak in the white insulator indicating a very small crack. I never noticed it by just looking at it before I ran this test. Good luck and hope you find the problem.