Yet another misfire

Today while I was poking and prodding the Jeep in hopes it would give me a sign, I found a problem. One that from all I can tell would cause the PCM to report misfires that it would have trouble pinpointing.

There is a crack in the intake plenum just forward of the injector for cylinder 1. I found this because the engine started whistling at me. I've heard this sound before while diving and infrequently while parked and idling, any time I've tried to pop the hood and find it, it stops the moment the hood opens. Today, it was happy to let me pinpoint.

I dont have any carb cleaner, but I do have some electronics parts cleaner. Sprayed some of that at the crack and the engine wasn't too happy about it.

I'll be getting a new intake plenum and swapping that out when I have time. I'll report back when that gets done!

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I think the OPDA sensor sold by NAPA has worked, this is from another thread.

“Also, if your camshaft position sensor is going bad you can use the Napa Echelon series sensor to replace it without any issues (based on others' opinions here and my own experience). I pulled my OEM CPS when changing the OPDA just to see if the Napa part would work okay. It worked great, so I left it installed and keep my stock CPS as my spare (since they are a unicorn nowadays). The Napa part has been running fine for 7 months.”
 
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I had a couple friends used JB Weld to fix a hole in the oil pan of an old commercial fishing boat they had harbored in Half Moon Bay, just south of San Francisco. This was 40years ago at least and for whatever reason this seemed the best solution to them anyway.

They fished that boat for years until one of their other Shabby repairs an electrical one this time ended their season burning it to the waterline, the JB Weld on the oil pan never failed.

Here’s another epoxy tip, I was talking to a tile guy I know about some tiles that came off the negative edge on my pool, he recommended auto body filler said it’s an old trick in the tile business. Works amazing to replace any kind of tile, brick or pavers too.
 
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1515art said:
Sounds like you found your problem, congratulations!
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We'll see! It's certainly A problem, don't know yet if it's THE problem. I took that JB Weld suggestion and ran with it; I did notice an improvement, but the misfire is still present. I went ahead and ordered a replacement manifold and gasket, they should both be in next week and hopefully I'll have the opportunity to install next weekend. Fingers remain crossed until then!
 
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williambmac said:
So why would a crack by the #1 injector be causing a P0306?
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I'm no master of technical aspects of engines but I can take an educated guess.

These motors don't have sensors on each cylinder to analyze performance individually, the PCM detects change in cylinder RPM through the crankshaft position sensor and uses that determine which cylinder is missing. Through each cycle of the engine 2 pistons are at TDC together (1&6, 2&5, 3&4).

Because no system is perfect, I could imagine the PCM might notice the variation in engine RPM during the cycle where cylinder 1 is firing incorrectly as a miss at 6. Again, that a total guess and I could be way off, but either way a crack in the intake is bad and needs to be fixed.
 
TheDWord said:
I'm no master of technical aspects of engines but I can take an educated guess.

These motors don't have sensors on each cylinder to analyze performance individually, the PCM detects change in cylinder RPM through the crankshaft position sensor and uses that determine which cylinder is missing. Through each cycle of the engine 2 pistons are at TDC together (1&6, 2&5, 3&4).

Because no system is perfect, I could imagine the PCM might notice the variation in engine RPM during the cycle where cylinder 1 is firing incorrectly as a miss at 6. Again, that a total guess and I could be way off, but either way a crack in the intake is bad and needs to be fixed.
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So, you're thinking a ghost misfire? Possible, but usually when you have something like that going on, you will have 2 cylinders showing a misfire. One with a high misfire count, and one with a much lower misfire count.
 
williambmac said:
So, you're thinking a ghost misfire? Possible, but usually when you have something like that going on, you will have 2 cylinders showing a misfire. One with a high misfire count, and one with a much lower misfire count.
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Probably convincing myself it’s the issue more than anything. I just want this misfire to be gone!

6 is not the only cylinder showing misfires btw, it’s just consistently the first. Others do pop up depending how long I keep RPMs up. Like on my 5ish mile test drives after part swaps, 6 will be confirmed and others will be pending. I’m turning about 3.1k for a few minutes on those drives; I’m not exactly taking it easy but also not taking it far.
 
If the manifold doesn't fix it there's some information about carbon keeping the valves from rotating and a factory fix using water or something to clean out the carbon. It's worth some research as a next step.
 
Ok so I’m working on replacing the intake right now and while inspecting inside the ports to check the valves I found a couple pieces of plastic inside the port for the intake valve of cylinder 5. Anyone have thoughts on what it might be?

I can tell it’s been there a while, it’s worn like it’s been bouncing around a while so I don’t think it was introduced while I was disassembling things.

EDIT: I maybe could have added something for size reference. The larger bit is about the size of a quarter.

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Update: Got a replacement intake manifold (and a Banks header while I was at it) install finished up and... no difference. 😭 First drive out after getting everything buttoned up popped the same codes; P0306, P0305 and P0300.

One thing that I can note through all of this is that the misfire is more dramatic after warming up fully. Like when I first pull it out of the garage and start driving it seems OK, then I take it on the freeway. Typically, once I get to highway speed, the codes show up and I'll exit. After that, the roughness starts.

I know theres something about the engine going into a "closed loop" operation mode after warming up. Considering the symptoms, I guess it could be something that changes after that happens, but I have no idea what this closed loop operation is. What might I check out related to that?
 
TheDWord said:
2001 TJ 4.0 Auto

I’ve been dealing with a misfire for a while now and it’s time to resort to some direct help from the community. I’ve been following forum advice as much as possible through this process but I’m still stuck. Through everything I’ve done the misfire has remained on 6, P0306 code. Slightly rough at idle when cold smooths after warming. Generally don’t feel the miss when driving until 1500ish RPM, then it’s rough all the way through. If I put distance on it more cylinders will pick up a miss, usually 5 come second. Seems to be worse after warming up.

So. Many. Fixes. Attempted before coming here to post about my woes! I’ve probably gone through some parts that were more than necessary but I just want this misfire to go away. I’ll list everything I’ve tried in the order I tried:

  • Spark plugs - Autolite iridium and champion coppers, just to be sure.
  • Coil rail - Autozone as I didn’t want to wait 3 months for the Mopar part.
  • Crank position sensor - Mopar part
  • Fuel pump - Delphi part
  • Injectors - https://www.ksuspensionfab.com/store/p71/Remanufactured:_4_Port_Upgrade_Injectors_YJ/LJ/TJ.html#/
  • Battery - Big ass Odyssey. Needed this anyway for some electronics upgrades
  • Compression checked out - don’t have values, took it to my local trusted mechanic for this.
  • PCM - wranglerfix.com
  • Cleaned and checked grounds
  • 2/1 Downstream O2 sensor - NTK part - replaced exhaust and sensors last year. Tried a new one recently just in case
I realize I could have probably gone about this in a better order. This thing is just making me insane. I put a sweet new axle in before this issue reared it’s ugly head and I would like to enjoy it eventually. Help!

Time for an LS swap?
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I chased a misfire in my 2.5 for longer than I’d like to admit. Changed plugs, wires twice, the coil, injectors, CPS, distributor and rotor. Pretty much you name it I replaced it or at least tested it. Wasn’t until I replaced the gears and timing chain ( which didn’t look that bad) did I get the result I was looking for. My Jeep only has 137k original miles so it could be worth looking into.
 
TheDWord said:
I know theres something about the engine going into a "closed loop" operation mode after warming up.
Click to expand...

The mystery object looks like regular automotive hose. I think the white at the ends are the worn fibers. You can cut it and it should look like standard hose. Closed loop is normally reached after about 60 seconds once the 02 sensor heaters have warmed the sensors enough to operate. Way too short for what you describe, you are describing regular operating temp. I know, not too helpful.
 
When you did the leak down test did you pull the radiator cap to see if you could hear any air coming from the cooling system?
 
TheDWord said:
2001 TJ 4.0 Auto

I’ve been dealing with a misfire for a while now and it’s time to resort to some direct help from the community. I’ve been following forum advice as much as possible through this process but I’m still stuck. Through everything I’ve done the misfire has remained on 6, P0306 code. Slightly rough at idle when cold smooths after warming. Generally don’t feel the miss when driving until 1500ish RPM, then it’s rough all the way through. If I put distance on it more cylinders will pick up a miss, usually 5 come second. Seems to be worse after warming up.

So. Many. Fixes. Attempted before coming here to post about my woes! I’ve probably gone through some parts that were more than necessary but I just want this misfire to go away. I’ll list everything I’ve tried in the order I tried:

  • Spark plugs - Autolite iridium and champion coppers, just to be sure.
  • Coil rail - Autozone as I didn’t want to wait 3 months for the Mopar part.
  • Crank position sensor - Mopar part
  • Fuel pump - Delphi part
  • Injectors - https://www.ksuspensionfab.com/store/p71/Remanufactured:_4_Port_Upgrade_Injectors_YJ/LJ/TJ.html#/
  • Battery - Big ass Odyssey. Needed this anyway for some electronics upgrades
  • Compression checked out - don’t have values, took it to my local trusted mechanic for this.
  • PCM - wranglerfix.com
  • Cleaned and checked grounds
  • 2/1 Downstream O2 sensor - NTK part - replaced exhaust and sensors last year. Tried a new one recently just in case
I realize I could have probably gone about this in a better order. This thing is just making me insane. I put a sweet new axle in before this issue reared it’s ugly head and I would like to enjoy it eventually. Help!

Time for an LS swap?
Click to expand...
I had a cylinder 6 misfire and ended replacing the #6 plug wire. Fixed! Worth a try?
 
Maybe check/inspect if you have any wires (including the 02 sensors) near the manifold or headers that might get hot and cause you fits as things heat up. Could also wiggle the wire bundle behind the valve cover (while idling) to simulate road conditions and see if you can influence the outcome.

My 99 died leaving the shop (once warmed up) because the shop improperly ran the crank position sensor wiring so the insulation melted and wire heated up. Once cool, it would start again (likely due to the resistance dropping). Just throwing this out there in case it helps.

On a gas getter car, I’ve used a shop vac to push air into the tail pipe (using lots of tape to seal it off) then sprayed soapy water on the manifold and all couplers. This exposed way more leaks than I expected to find. I used exhaust putty to seal it all up because I needed the car to drive the next morning.
 
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