Yet another misfire

[onsight]

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.

Seconding this. My head was warped, which caused a cyl. 1 misfire. Rebuilt the top end (lifters, rockers, pushrods, etc) and the miss wasn't as bad, but the code would come back intermittently.

One day at work I taped the shop-vac to the tailpipe, set it to blow, and just used my hand to feel around the exhaust manifold. Two big hairline cracks. Replaced it with a $70 manifold from Amazon and a year later, no leaks or cracks, and the CEL hasn't come back.

It's a great test, takes 10 minutes, and is free.

 

[TheDWord]

Oh man, looks like I’ve been neglecting the thread! Thanks for all the input fellas.

At this point I think I’ve ruled out ignition and fueling components as the problem. All the sensors have been checked out, plugs, injectors, intake and exhaust manifolds sealed up (installed a header), etc. Ran some continuity tests on injector and coil circuits and wiggled the crap out of wires. I’m over it.

Found an engine out of an ‘06 with 67k that I intend to snag for long block and plop in. I’m comfortable that the block will work with all my accessory equipment, I just haven’t been able to find anything conclusive on the transmission.

Anyone here with experience in that? Will my 32RH mount up to the block and roll or will I need some parts?

 

[onsight]

Oh man, looks like I’ve been neglecting the thread! Thanks for all the input fellas.

At this point I think I’ve ruled out ignition and fueling components as the problem. All the sensors have been checked out, plugs, injectors, intake and exhaust manifolds sealed up (installed a header), etc. Ran some continuity tests on injector and coil circuits and wiggled the crap out of wires. I’m over it.

Found an engine out of an ‘06 with 67k that I intend to snag for long block and plop in. I’m comfortable that the block will work with all my accessory equipment, I just haven’t been able to find anything conclusive on the transmission.

Anyone here with experience in that? Will my 32RH mount up to the block and roll or will I need some parts?

Do you actually 'feel' the misfire or is it just the CEL?

Personally, if it's just the light and you can't feel an actual misfire, I'd just send it and drive with the CEL on. I totally get the 'over it' feeling.

 

[TheDWord]

Do you actually 'feel' the misfire or is it just the CEL?

Personally, if it's just the light and you can't feel an actual misfire, I'd just send it and drive with the CEL on. I totally get the 'over it' feeling.

Oh absolutely, if it weren't noticeable, I'd move on. I'll try to describe it as accurately as I can:

When first starting up, at idle, it feels pretty good, maybe like theres a slight hiccup in a cylinder occasionally. Until it really warms up driving around on city it feels great, plenty of power and doesn't really run too rough.

If I take it on the freeway is when things really start to get bad. Once the RPMs get over about 2500ish (I have 33s on 4.88 gears and 3 speed auto so that does take too long) I'll start feeling a vibration and that will stick around unless I push close to redline. At higher RPMs the PCM is unhappy and starts flashing CEL.

Once things really get warmed up, after driving mildly for a while or hitting the freeway, city driving isn't great. Idle is OK, but power is noticeably decreased and mid range is super rough, starting around 1800-2000 RPM the engine has an obvious miss on at least 1 cylinder.

 

[kincaid.0678]

Have you checked/ replaced your timing chain?

 

[TheDWord]

Have you checked/ replaced your timing chain?

No, I have not. Short of getting into disassembly and physically checking the chain, I haven’t hear of a reliable way to check if it’s good or not. After everything I’ve done, I’m hesitant to get into pulling apart and replacing internals.

Plus, the long block was replaced around 136k miles, I’m now at 159k. I didn’t own the Jeep during that repair but I have records of it. Gotta assume the chain was replaced along with that.

 

[pagrey]

No, I have not. Short of getting into disassembly and physically checking the chain, I haven’t hear of a reliable way to check if it’s good or not.

It is very easy, you put a breaker bar on the crankshaft and push it back and forth. It is extremely easy to feel the play in the chain. Often you can even hear it through the timing cover when you do this if it has slack. If you are really not mechanically inclined you can pull the OPDA cover or distributor cap and watch the rotor but it's hardly necessary. A loose chain is very easy to feel.

 

[TheDWord]

Thanks @pagrey for the input there! Thats going on the list of things to check. However, I found some new and interesting info about my bastard Jeep today.

I took it to the local Jeep dealer to have the run their diagnostic tool and see if there might be anything more they could tell me. All they were able to tell me is that the PCM has no VIN coded. I have a couple new PCMs from Wranglerfix that didn't work properly, and now I'm thinking we have more to investigate.

More fun decoding this mystery Jeep!

 

[TheDWord]

Seems I've located a problem.

 

[pagrey]

Is that a busted piston, how'd you find that?

 

[macleanflood]

Is that a busted piston, how'd you find that?

He pulled the head... =)

And yet another good reason to have a cheap borescope...

Another place to check slack on the timing chain is when you have the oil pan off...

I'm getting very occasional misfires on my 97...very...no particular cylinder... usually 4 to 6 of them at a time...done most of what you've done...sans head/engine.

I had my pan off and it felt like there was excessive play in the timing chain...I could deflect it about a 1/4" with my finger...maybe more...so watching this thread...and maybe I need to borescope my piston heads.

-Mac

P.S. with a fully rebuilt engine costing around $2k...check out sandjengines.com...why gamble on the mystery of an unknown engine?

 

[TheDWord]

He pulled the head... =)

And yet another good reason to have a cheap borescope...

Another place to check slack on the timing chain is when you have the oil pan off...

I'm getting very occasional misfires on my 97...very...no particular cylinder... usually 4 to 6 of them at a time...done most of what you've done...sans head/engine.

I had my pan off and it felt like there was excessive play in the timing chain...I could deflect it about a 1/4" with my finger...maybe more...so watching this thread...and maybe I need to borescope my piston heads.

-Mac

P.S. with a fully rebuilt engine costing around $2k...check out sandjengines.com...why gamble on the mystery of an unknown engine?

Agreed on the borescope! If I had one when I had the intake and exhaust manifolds off I may have noticed this issue sooner.

I decided to go ahead and pull the head. Considering everything I’ve been through at this point, I figured it couldn’t hurt. Honestly, glad I did since I can clearly see an issue. The surface of the cylinder 6 piston is badly pitted and has a chunk of metal lodged into it. That piece appears to be the same size as piece missing from it. Can’t tell what happened to it, but it wasn’t good.

Thanks for the tip on sandjengines.com, wish I’d seen it earlier. I ordered a junkyard engine with 62k. Though there’s always risk there, I feel pretty good about it.

 

[TheDWord]

Quick update:
Received the engine, it look clean and health, and proceeded with the ol' swaperoo.

After finishing the engine swap, which was not nearly as bad as I thought it would be in my tiny garage, the godawful misfire is still present . I have to assume at this point that the issue is the flex plate, it's literally the single part that remains from the old engine. Seems I'll be pulling the transmission and getting at that next, which is a bummer because I thought to myself "self, you should just change the flex plate while the engine is out." I didn't because it was late and I wanted the engine in before I went to bed; not running, just mounted.

Lesson learned: slow down, take it easy.

This has been exhausting! Hopefully my documented pain will help someone else in the future.

 

[macleanflood]

If you still have your original sensors you might want to switch back and try things out.

How extensively have you checked the wiring harness? I think if I pulled my engine I would have untaped and unwrapped the entire engine harness.

I went through a couple of bad oil pressure sensors before I found a working one.

-Mac

 

[TheDWord]

If you still have your original sensors you might want to switch back and try things out.

How extensively have you checked the wiring harness? I think if I pulled my engine I would have untaped and unwrapped the entire engine harness.

I went through a couple of bad oil pressure sensors before I found a working one.

-Mac

Oh yeah I’ve kept and retried many sensors. Tried 3 different crank position sensors and 2 cam sensors just during this swap. I’ve swapped the TPS, MAP, and various other sensors multiple times.

The harness has been replaced. The new one was inspected closely before going in.

At this point, the one thing I can fathom would cause such a consistent misfire is the flex plate; more specifically, the tone ring.

 

[TheDWord]

My pain is not over... After engine swap and many sensor replacements, I'm still experiencing the same symptoms as always. I've gone down a road of diagnosing on data; the charts in the image are from a recent drive where I noted misfire timing against the live data. During the periods where the fuel trim is flat the reading is 0, the engine light was flashing, indicating active misfire. Makes sense if fuel trim is non existent in either direction.

I'm not sure where to go from here. I want to consider O2 sensors possibly being defective but voltage on both look good to me throughout the drive. Theres a clear problem with fuel trim, PCM is flattening out the trim and causing the misfire despite what appears to me to be normal O2 sensor readings and otherwise normal trim. Although, during the first flatline, the valleys in the O2 voltage curves do read 0.0. I don't know if thats an indicator or a byproduct.

I guess the question is, what could cause the fuel trim to go to 0 briefly?

Yes, I've tried replacing the PCM. I have 2 on hand from WranglerFix, I experience the same problem as I do both as I do with the factory one.

 

[primetime4]

Just wanted to link to another thread with very similar issue here: https://wranglertjforum.com/threads/check-engine-light-flashing.1219/

I am also currently battling engine misfires with my 2001 TJ with 32RH. Not sure if there is a trend here but 3 shops have tried just about everything. I might need to have them check the timing chain mentioned. Haven't heard of the flex plate potentially being an issue but any suggestions on what to check next helps.

 

[TheDWord]

Just wanted to link to another thread with very similar issue here: https://wranglertjforum.com/threads/check-engine-light-flashing.1219/

I am also currently battling engine misfires with my 2001 TJ with 32RH. Not sure if there is a trend here but 3 shops have tried just about everything. I might need to have them check the timing chain mentioned. Haven't heard of the flex plate potentially being an issue but any suggestions on what to check next helps.

That's an interesting thread for sure. Sounds like you have a lot of the same issues a mine, I'll certainly be following your story.

I think in both of our cases the flex plate was/is low on the list of likely culprits. The crank sensor sends a pulse to the PCM by reading gaps in a tone ring attached to the flex plate. If damaged somehow (bent/cracked), it could throw off ignition timing by sending a bad signal to the PCM. It's not really a part that will typically go bad over time.

I'd be curious to know if, when your CEL is flashing, your short term fuel trim is flat like mine is. In the graphs that I posted, the flat 0 trim correlates with the times my CEL is flashing. While the condition exists, the engine feels rough, and the longer it goes the more rough it seems to get.

 

[primetime4]

I noticed that data point too but interpreting that is way over my head. What the dealer indicated to me is that the engine appears to be running lean when the light if flashing. Not sure if that is the same as a flat 0 trim. However, they tested the fuel pressure and it was fine so did not think it was a failing fuel pump causing the issue.

 

[TheDWord]

I noticed that data point too but interpreting that is way over my head. What the dealer indicated to me is that the engine appears to be running lean when the light if flashing. Not sure if that is the same as a flat 0 trim. However, they tested the fuel pressure and it was fine so did not think it was a failing fuel pump causing the issue.

I'm really just learning about fuel trims and how the PCM adjusts them myself, but here's what I know so far. The PCM uses readings from the upstream O2 sensors to determine lean or rich conditions, it uses that information to calculate required injector pulse width to compensate. That is fuel trim; a negative fuel trim means the injector pulse is shortened (less fuel), positive trim means the injector pulse is widened (more fuel). STFT should always be within +/- 3%, optimally.

Hypothetically, if the PCM were not adjusting the injector pulse (0 trim) and conditions required positive trim (more fuel), you would have a lean condition.

In my case, the O2 sensors appear to be sending valid info, but fuel trim is cutting out. I'm going to run some more road tests with live data to find if that was a fluke, but I don't think it was. I just don't know at this point what would cause the PCM to briefly quit adjusting fuel trim.