P0302 Intermittent Misfire

[Sipafz]

Hello from Wisconsin! New to the site and looking for some guidance on solving this nagging intermittent misfire.

2011 Jeep Liberty 70th Anniversary Edition (dressed up sport) with 150k on the odometer. It’s been good to us as problems and repairs have been minimal. At 50k I pulled the dash out to replace the air distribution housing as one of the blend doors stuck and was damaged by the actuator. At 80k I replaced a coil and again at 110k. That’s it other than regular maintenance.

My current problem started last winter with an intermittent P0302 misfire. I started with the basics:
- Spark Plug, looked ok and gap ok
- Spark Plug Wire, looked ok and resistance in spec
- Swapped Coils, no change
- Swapped Fuel Injector, no change
- Compression Test, all cylinders looked good at 195 / 205psi
- Leak Down Test, all cylinders looked good at 15 / 20%
- Checked for fuel in the coolant (smell and bubbles), negative
- Checked exhaust for excessive white smoke, negative

COVID started up and the Jeep was not used much for the second half of winter. Once it warmed up in the spring, the problem went away. The vehicle passed emissions testing in August and ran good until this month.

I recently repeated all the tests and measures mentioned above. I also performed the following during the last two weeks:
- Check for fuel in the coolant using a chemical test kit, negative result
- Removed Valve Cover, found two loose rockers of which I could compress the lash adjusters with my fingers. One of them on #2 and the other on #4.
- Removed all rocker arms & inspected, they all looked good, normal wear on the pivot points, smooth roller bearings
- Removed all lash adjusters. 3 bad & 3 ok. Replaced all 6 with new. Let them soak overnight in oil prior to reassembly.

Changing the lash adjusters had no positive effect. Problem remains

Looking for input on what to do next.

 

[lfhoward]

Wow you’ve really checked off the boxes on the list of things I would suggest to check.

Maybe peek inside the cylinder with a borescope? Check for carbon deposits in that cylinder that may be messing with your compression ratio, or for damage to the top of the piston?

 

[Sipafz]

Wow you’ve really checked off the boxes on the list of things I would suggest to check.

Maybe peek inside the cylinder with a borescope? Check for carbon deposits in that cylinder that may be messing with your compression ratio, or for damage to the top of the piston?

Wow you’ve really checked off the boxes on the list of things I would suggest to check.

Maybe peek inside the cylinder with a borescope? Check for carbon deposits in that cylinder that may be messing with your compression ratio, or for damage to the top of the piston?

Thanks for the reply. I have a bore scope at work and I plan on checking things out tomorrow. I’m going to peek through the spark plug hole, remove the intake and peek at the intake valve, rotate engine to open the valve and peek beyond. While I have the intake off I’ll replace the gaskets. I’ll post up my findings

 

[David13]

I have been looking for Libertys for sale. A 2011 was for sale cheaper with the P 0305 code intermittent misfire, cyl 5. They insist it's just a bad plug.

However, have searching the internet found about 3 or 4 or 5 threads about this issue and the various and unsuccessful efforts people have gone to to solve it. Unsuccessful except for a few weeks or months, and then it comes back.

I ended up buying one that supposedly doesn't have this problem, or at least not during the test drives. But I'm still curious as to what it turns out to be, or what seemingly solves it.

Mine might start this next week.
dc

One thread mentions vacuum leak. Could be a vacuum leak.

 

[Sipafz]

I have been looking for Libertys for sale. A 2011 was for sale cheaper with the P 0305 code intermittent misfire, cyl 5. They insist it's just a bad plug.

However, have searching the internet found about 3 or 4 or 5 threads about this issue and the various and unsuccessful efforts people have gone to to solve it. Unsuccessful except for a few weeks or months, and then it comes back.

I ended up buying one that supposedly doesn't have this problem, or at least not during the test drives. But I'm still curious as to what it turns out to be, or what seemingly solves it.

Mine might start this next week.
dc

One thread mentions vacuum leak. Could be a vacuum leak.

I didn’t list it, but I did hook up a vacuum gauge at the brake booster connection. The gauge was steady at 20in fluctuating a couple inches when a misfire would happen.

 

[John P.]

From reading many places, if you've rulled out all other potential causes for a misfire you probably need a repair to your TPCM. It's on the Passenger side of the engine bay. Essentially what happens is the coil driver circuit starts to go bad. Vehicle vibration causes a random misfire, eventually the circuit fails and then you will have a constant misfire. Many places do this repair under 300 bucks. I need to do mine but it's due to other issues within my module not the misfire issue.

 

[Sipafz]

From reading many places, if you've rulled out all other potential causes for a misfire you probably need a repair to your TPCM. It's on the Passenger side of the engine bay. Essentially what happens is the coil driver circuit starts to go bad. Vehicle vibration causes a random misfire, eventually the circuit fails and then you will have a constant misfire. Many places do this repair under 300 bucks. I need to do mine but it's due to other issues within my module not the misfire issue.

I think you might be right on the PCM. I spent a good chunk of the day working on this rig today. Removed the intake, plug wires, coils and plugs. Used the bore scope to peek into the cylinders and cylinder heads. With the plugs out I rotated the engine to get a good look at the closed & open intake valve, open exhaust valve, top of the piston and cylinder walls. All looked normal as I compared what I was seeing at cylinder #2 with cylinder #1. I replace the intake gaskets and put everything thing back together. Of course, no improvement on the misfire. Still at idle, but noticed that if I activate the tow/haul mode to keep the rpm’s up, I don’t get many while driving. Turn the tow/haul mode off and misfire returns while driving when the torque converter locks and rpm’s drop below 1,500.

Been reading up on bad O2 sensors and the possibility of them being bad, shutting down the coil driver for a cylinder to give a misfire at idle but no O2 sensor code. I ended up swapping both upstream sensors, but it didn’t change anything. I need to dig into this more.