Wrangling an intermittent misfire

Jeff Diagnostician Cathedral City, California Posted   Latest   Edited  
Case Study
Driveability
2014 Jeep Wrangler Unlimited Rubicon 3.6L (G ERB) 5-spd (W5A580)—1C4BJWFGXEL136753
P0304 - Cylinder 4 Misfire Detected

This Jeep comes to me this week with an intermittent misfire but runs well otherwise, someone has already put 6 plugs and 3 coils on cylinders 2,4 and 6 under the intake. Scan the pcm found P0304 , theres also a random tapping under the bank 2 valve cover that happens as well but doesn't occur at the same time as the misfire. A quick look at the data stream but nothing sticks out to me, so I throw the wps in the tail pipe to verify the misfire is on cyl 4 ​ ,I find that it is in fact cyl 4 . Cranking compression test with the amp probe is next up BUT I learned in class a few weeks back the Pico Diagnostic software has an option in the compression test to use the wps in a cylinder and it calculates the compression in the others, so out of curiosity I do that and it comes up with cylinder #4 10 lbs lower than the rest ( I didnt save those results wish I did).

I take a cranking vacuum waveform with the First look sensor, all the pulls look even, next I get authorization to pull the intake so I can get a cranking compression waveform from cylinder 4 . What I found was a random compression peak that was 8 lbs lower than the rest , due dilligence has me do a cranking compression on cyls 2 and 6, those looked good. I gotta see whats going on inside cylinder 4 while its running so I bolt the intake back on a get a running compression waveform of cylinder 4, I find random uneven compression peaks, A closer look at the waveform and I see the decompression pocket start to round out when it drops compression, HMMMM Now I gotta see whats goin on underneath that valve cover, Im already suspecting bad rockers the intermittent tapping was a clue plus these engines are notorious for that. I get the valve cover off, nothing jumps out at me, I roll the engine around and get the #4 rockers off the cam lobes and give em a quick wiggle, seems tight, there are no tell tale signs on the cam lobes either. I get cyl # 4 exhaust rockers out and I find just 1 of the 2 rockers has a bad roller bearing, I posted a video of the bearing play, if i push on the bearing and try to roll it there is a flat spot where it locks up as well. Now I gotta admit Im skeptical here, I get 4 rockers from the dealer for the shocking price of 8 dollars each, install them reassemble everything else and all is well and runs perfect.

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Geoff Diagnostician
Lahaina, Hawaii
Geoff Default
 

Nice write up. How many miles? With parts that cheap on these engines, it's no wonder they (all) break, right? It's hard to keep track of which heads are under permanent warranty anymore. So many stories of cams, valves, and now rockers. We had a 5.7L Jeep bend a valve while it was here for something else. That went over really well. Thanks!

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Jeff Diagnostician
Cathedral City, California
Jeff Default
 

It had 60k on the clock , we called the dealership for any warranty unfortunately there wasnt any

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Brian Owner
Parma, Ohio
Brian Default
 

Nice work! Thanks for sharing.

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