2008 Jeep Wrangler cylinder 5 misfire

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#1
Make
Jeep
Model
JK
Year
2008
Miles
123K
Engine
v-6 pre-pentastar
So my JK, original owner well maintained, has an engine misfire on #5, not around town (Southern Cal.) but after driving it down the freeway for 30 miles or so. It burps, hesitates, backfires occasionally , once through the intake during deceleration. 123K miles, have replaced the coil twice already in the past, each time verifying by doing a resistance test on the coil, one bank was weak coinciding with that misfire. This coil that is 30K miles old checks with the same resistance values as a new one at the store, I checked them both. Plugs and wires 30K miles old. Pulled plugs on drivers side of engine, visual and ohm and the plugs look good, swapped wires around the cylinder and no misfire. Suggested I allow an air gap where the coil mounts to it's plate, installed new crankshaft positioning sensor. Around town it's fine, get her toasty and she barks. Oil is clean, coolant is clean, trans was serviced at 100K. Garage kept, non smoker non abuser of my jeep out in the desert. Heat seems to be breaking something down, what though.
 

billr

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#2
I gotta ask... when the plugs were out, did you take compression readings? Is this port injection? If so, have you tried swapping injectors around?
 
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#3
If your "new" plugs are copper/nickel or Bosch, it's time for a fresh set at 30 k miles. And remember...for cyl #5 to fire, cyl #2 plug and wire need to be in good shape. That coil pack is essentially three coils, with each coil lighting two spark plugs every time it fires. 1521943883910.png
 
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#4
I found some cracks in #5 plug, new wires and plugs and it is running fine. Of course it did that before until I hit the open road. It took a lot of heat for the plug to fail and start misfiring. When cold it would not misfire. Even with the original miss occasionally it never threw a code.
 
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#5
You stated "It burps, hesitates, backfires occasionally "
Have the jeep SCANNED? - post all codes. Even if CEL is off.
Any chance yu have a scanner - that shows live data? Make this easier.
If no scanner check fuel pressure WITH a gauge. Sometimes easier said than done*. let us know
What plugs are you using? yeah it matters.

Jeep is STOCK no aftermarket BS?

Jeeps are pretty easy to work.

* might not be a schrader valve on this model.
 

billr

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#6
OP, are you saying this is fixed now, that you simply didn't see the cracked plug when you checked them previously?
 
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#7
I did not see the cracked plug when they were checked previously . What was interesting is that the misfire occurred only when the Jeep was driven for a longer distance, let's say on the return of a 30 mile one way trip, half way home bing,bang,boom. Around town it was fine. Jeep is bone stock drivetrain wise.

UPDATE: 30 miles each way and Jeep ran fine. Scanner not picking up any codes. Thank you to those that offered suggestions.
 
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