Another TJ 4.0 Misfire Thread (2005 LJ)

[jw_LJ]

Hello everybody,
I've been visiting the forum for years and have learned so much through this amazing site, and I finally am coming to you guys about an issue I've been having for years now. While I want to provide as much information as possible, I will keep it as brief as possible. I hope you guys like mysteries because this one is a doozy.

*2005 Jeep Wrangler LJ 4.0 6-spd manual 155k miles

Over the last 3 years, it's slowly gotten worse, and a massive amount of time, effort, and money later, the problem is still unsolved.

Some background: First time I remember having a weird misfire was a couple years ago after flooring it on the road and accidently hitting the limiter a couple times, noticed it at idle when I got back home, and it went away. Then, over the months and years, almost every time I got on it hard, it would have a weird rough idle/slight miss at idle. Sometimes it would get especially bad, and the miss would continue through the RPM range, but if I'm easy on it and drive it nice, it almost would always go away. Now more recently (Last year or so) It's gotten worse to the point that if I sit and idle for a while (Drivethru) The roughness gets worse and worse until it throws P0301 and P0300 codes (Cyl 1 misfire and random multiple misfires), and the CEL will often flash. It often has a richness smell to the exhaust. But what's happened in the last few weeks has almost pushed me over the edge. It's been hard starting (4x the average time on the starter before it fires up) and it's started bucking slightly at low RPM cruising leaving the neighborhood, and hesitates slightly sometimes giving it a tap of the gas in neutral.

I've been chasing this issue and here's everything related to it that I have replaced:
-Spark plugs (At least 6 times, multiple different brands, was running NGK ZFR5Ns and recently swapped them out with Champions, but nonetheless, the condition doesn't get worse or better on plugs so I've ruled that out)
-OEM coil rail replaced with Crown coil rail when my OEM one failed, however it didn't fix the issue. Currently running a K Suspension Viper Coil Mod* thinking it was still coil related with no change (Great product, makes messing with anything ignition-wise much easier)
-Cats replaced with flowmaster (Pre-covid when they were $250 for a set, now they're over $1,000) with 2.5in exhaust all the way back.
-OEM Injectors replaced with 12-port injectors, made condition slightly worse and threw rich codes, replaced with remanned 4-hole injectors and rich codes stopped. Underlying issue remained the same throughout.
-OEM OPDA somehow came (slightly) loose on a drive once in 2020, creating a minor oil leak and threw the OPDA codes with limp mode, shop tightened it back up when taken for diag (Likely unrelated, but wanted to include it). Last year I replaced the OEM OPDA with a Crown unit and crown cam sensor under suspicion of it being a worn cam/OPDA* (No monkey noises). This actually seemed to have solved the issue for a bit, I remember it driving great for a week or so before slowly coming back.
-Cam sensor replaced with a NAPA one a couple weeks ago on suspicion of it being the crown one, no change to issue. $50 wasted.

Now here's the kicker:
This is almost exclusively an idle issue, after throwing some new plugs and the cam sensor in it (Which was a hail Mary that fixed nothing) I drove it from NY to FL with no issues aside from hard starts and rough idles. It's still reliable as hell, and it hasn't blown up on me (yet). Once she's off idle, she's absolutely fine. I got 17.5mpg (That .5 really matters to me!) all the way down sitting at 2800rpm going 75mph without a single hiccup. No knocks, ticks, weird noises when cold, warm etc. Once it's driving it's fine, just low rpm stuff. So I think that rules out stuff like fuel pump, lifters, or anything else that would be associated with high RPM power loss.

This leads me to believe it's 1 of 2 things.

1: Cam related: Worn lobes from the OPDA oiling issues or something. I actually spent $700 on cam parts to do the swap, I had it all on the floor in my garage, with the Jeep sitting ready to go under the knife, but I couldn't bring myself to do it. At the time I 95% believed it was the cam, however that 5% kept me from pulling the trigger, I returned everything.
2. Wiring: Engine wiring that has gotten cooked and fragile over the years, while I see some crumbling plastic covers on the wiring, I see no signs of wear or unusual damage to the harness anywhere, especially the main harness behind the firewall. This is what I truly think it is. But would a wiring issue go away with high RPM?

What do you guys think? This has driven me absolutely nuts and as I inch closer to an LS swap I just wanted to see what you guys think, or if anyone's had this absolutely unusual nightmare of an issue on their OPDA equipped 4.0. Also take a look at the spark plugs* after I pulled them, notice plugs 1 and 6 have slightly more carbon buildup than the rest? But if it was the wiring for cyl 1&6 which share the same wire, wouldn't I have a cyl 6 misfire as much as cyl 1? I think it's possibly wiring nonetheless. Apologies for the long post, I'm just stumped. If you guys want I can add more pics of engine bay, video of it hard starting and idling if you want.

*Attached are some pictures: 1: My Jeep after she made it to Florida 2: The K Suspension Coil Mod for those who aren't familiar. 3&4: Best pics of cam and OPDA wear I have. 5: My latest spark plugs pulled (cyl 1 is on the right, cyl 6 is on the left) (I know there's only 5, one of the middle ones is missing but it's identical to the others)

 

[Crom]

The OPDA gear looks terrible and needs to be replaced. However, before you go any further take the jeep to a garage and have them perform a compression and leak-down test. It would not surprise me to learn your #1 cylinder piston rings are worn. If that is the diagnosis you will need a new engine.

 

[jw_LJ]

The OPDA gear looks terrible and needs to be replaced. However, before you go any further take the jeep to a garage and have them perform a compression and leak-down test. It would not surprise me to learn your #1 cylinder piston rings are worn. If that is the diagnosis you will need a new engine.

Hey Crom, I appreciate the insight. The pic is of the old OPDA and was already replaced. Also, I forgot to add that I had taken it to a shop in 2021 and they performed a compression/leakdown and came back with nothing out of the ordinary. I might get another test done to get a second opinion.

 

[sab]

When a driven gear (on the OPDA) is worn like that, the drive gear (on the cam) usually is damaged, too. That kind of damage will screw up the cam/crank sync. How does the cam gear look?

 

[jw_LJ]

When a driven gear (on the OPDA) is worn like that, the drive gear (on the cam) usually is damaged, too. That kind of damage will screw up the cam/crank sync. How does the cam gear look?

IIRC it was definitely worn, not quite as bad as the OPDA gear but had some noticable wear. I wasn't able to get a good pic of it but the pic I attached you can see the discoloration from the wear even though it's out of focus.

 

[TimV]

Checked fuel pressure?
Vacuum leak at throttle body and/or IAC?
Cam crank sync and relearn? Good threads around about that topic, I am insufficiently learned to speak further.

 

[gatertamer]

I know after all the money you’ve sunk into this it’s a silly question but have you checked resistance on the plug wires?….that’s what happened to me with two different rigs

 

[sab]

IIRC it was definitely worn, not quite as bad as the OPDA gear but had some noticable wear. I wasn't able to get a good pic of it but the pic I attached you can see the discoloration from the wear even though it's out of focus.

I think that's your problem (worn cam drive gear). Based on this:

-OEM OPDA somehow came (slightly) loose on a drive once in 2020, creating a minor oil leak and threw the OPDA codes with limp mode, shop tightened it back up when taken for diag (Likely unrelated, but wanted to include it). Last year I replaced the OEM OPDA with a Crown unit and crown cam sensor under suspicion of it being a worn cam/OPDA* (No monkey noises). This actually seemed to have solved the issue for a bit, I remember it driving great for a week or so before slowly coming back.

Here's my thinking. The gear on the old OPDA and the cam gear got worn when the OPDA "loosened." That's in quotes because I think that you suffered the well known OPDA failure. Once that happened, the two gears destroyed themselves. You didn't hear laughing monkeys because the OPDA screw loosened up from the vibration and limited the amount of damage, preventing the noise. You then replaced the OPDA. It ran okay for a short time because one of the gears was new, and the slop between the gears was improved enough to not have cam/crank sync problems. However, the cam gear was still bad, and over a short period of time, it destroyed the new OPDA gear, and you're now back to having sync issues. The misfire codes are tripped when the PCM sees unusual (unexpected) time pulse widths between cam pulses. I think you're seeing this at idle because when the engine has no load on it, the cam/crank sync has more variation in those pulse widths than it does under load. There is a time factor to the misfire codes - if the errors happen frequent enough, it triggers a code, but if they are very infrequent, it doesn't. At idle, they'd happen more often because the engine has no load on it.

I think that a solution will be to replace the cam and the OPDA to get new gears in place, but before doing that, I'd look real good at the cam and OPDA gears. If there's visible damage on both, I'd then proceed with replacement. If not, this is probably just a bad theory.

After you replace the OPDA, you'll then need to find someone with a DRBIII or aftermarket computer to run the "ReLearn Cam Crank" function. Also, I have a working theory, with some anecdotal evidence to support it, that you can rotate the OPDA with the engine running prior to the Relearn, to set the Cam Difference variable as close to zero as possible so that you can use a non-Mopar cam sensor successfully. I haven't written that up because it's only been looked at on one TJ that I know of.

Here's a thread that covers how to do the ReLearn with a DRBIII:

https://wranglertjforum.com/threads...am-sensor-in-the-4-0l-engine-2000-2006.69520/

 

[jw_LJ]

I think that's your problem (worn cam drive gear). Based on this:



Here's my thinking. The gear on the old OPDA and the cam gear got worn when the OPDA "loosened." That's in quotes because I think that you suffered the well known OPDA failure. Once that happened, the two gears destroyed themselves. You didn't hear laughing monkeys because the OPDA screw loosened up from the vibration and limited the amount of damage, preventing the noise. You then replaced the OPDA. It ran okay for a short time because one of the gears was new, and the slop between the gears was improved enough to not have cam/crank sync problems. However, the cam gear was still bad, and over a short period of time, it destroyed the new OPDA gear, and you're now back to having sync issues. The misfire codes are tripped when the PCM sees unusual (unexpected) time pulse widths between cam pulses. I think you're seeing this at idle because when the engine has no load on it, the cam/crank sync has more variation in those pulse widths than it does under load. There is a time factor to the misfire codes - if the errors happen frequent enough, it triggers a code, but if they are very infrequent, it doesn't. At idle, they'd happen more often because the engine has no load on it.

I think that a solution will be to replace the cam and the OPDA to get new gears in place, but before doing that, I'd look real good at the cam and OPDA gears. If there's visible damage on both, I'd then proceed with replacement. If not, this is probably just a bad theory.

After you replace the OPDA, you'll then need to find someone with a DRBIII or aftermarket computer to run the "ReLearn Cam Crank" function. Also, I have a working theory, with some anecdotal evidence to support it, that you can rotate the OPDA with the engine running prior to the Relearn, to set the Cam Difference variable as close to zero as possible so that you can use a non-Mopar cam sensor successfully. I haven't written that up because it's only been looked at on one TJ that I know of.

Here's a thread that covers how to do the ReLearn with a DRBIII:

[URL]https://wranglertjforum.com/th...am-sensor-in-the-4-0l-engine-2000-2006.69520/[/URL]

I think you just about hit the nail on the head with what I was thinking and why I was going to replace the cam. When I replaced the OPDA I didn't do any sort of relearn, and I believe you're correct in saying that it ran well because of the new OPDA but quickly got worn down. You have given good information leading me to believe it could be the worn cam causing all of these weird issues. Cam/crank sync causing the misfires and hard starts. I appreciate the insight Sab. Now the question is: Put $700 and hours of labor into an underpowered 4.0, or just run it till I have enough set aside for an LS swap

 

[sab]

Now the question is: Put $700 and hours of labor into an underpowered 4.0, or just run it till I have enough set aside for an LS swap

Since you did pose the question, I'm going to opine. To swap or not is a personal decision, so you'll hear various opinions. Here's mine. With any given production vehicle, the more mods you make, the more likely you are to have unexpected failures because no aftermarket company tests components more than the OEM tests their components. Some (most?) don't test them at all. They just design, build, sell, and then maybe make improvement when the customers test the stuff and have issues. Many times, those issues are ignored. Granted, sometimes the OEM's don't test in actual use conditions, but from my experience in the automotive industry, that's the exception these days. Most of the manufacturers work very hard with their customers to understand what they are doing with their vehicles because that's the only way to keep warranty costs in line.

With TJs, we typically perform a bunch of mods to make them much more off-road capable and to meet our personal needs and desires. Every one of those mods increases the possibility of a failure because you're using parts that aren't tested as well. If you're in the backcountry when you experience a failure, the best case scenario is a huge inconvenience. The worst case scenario is a life threatening situation (I know this from personal experience on snowmobiles 50 miles from civilization when the high temp for the day is -25F.) My personal philosophy with my build is to try to limit, as much as possible, the potential for failure.

The 4L is a proven, well-tested production engine. It's not perfect, and it doesn't make 400hp, but it does the job, and it's not prone to major-melt-down-type failures. The LS is very reliable, too. However, all of the support parts for an LS in a TJ after a swap are not. Those are basically custom-built parts. Since the engine, in my way of thinking, is not a limiting factor with 4L TJs, I'm inclined to keep mine as stock as possible, including those support parts. I'm modifying enough of the other parts that I don't want to increase my chances of backcountry failures whenever I'm out.

That's just my opinion. YMM(and probably will)V. You'll hear others chime in with their opinions, I'm sure. You need to do what works for you. My philosophy is just one crusty, old dude's opinion in a sea of opinions. Good luck with your TJ, and may the road rise up to meet you!

 

[jw_LJ]

Since you did pose the question, I'm going to opine. To swap or not is a personal decision, so you'll hear various opinions. Here's mine. With any given production vehicle, the more mods you make, the more likely you are to have unexpected failures because no aftermarket company tests components more than the OEM tests their components. Some (most?) don't test them at all. They just design, build, sell, and then maybe make improvement when the customers test the stuff and have issues. Many times, those issues are ignored. Granted, sometimes the OEM's don't test in actual use conditions, but from my experience in the automotive industry, that's the exception these days. Most of the manufacturers work very hard with their customers to understand what they are doing with their vehicles because that's the only way to keep warranty costs in line.

With TJs, we typically perform a bunch of mods to make them much more off-road capable and to meet our personal needs and desires. Every one of those mods increases the possibility of a failure because you're using parts that aren't tested as well. If you're in the backcountry when you experience a failure, the best case scenario is a huge inconvenience. The worst case scenario is a life threatening situation (I know this from personal experience on snowmobiles 50 miles from civilization when the high temp for the day is -25F.) My personal philosophy with my build is to try to limit, as much as possible, the potential for failure.

The 4L is a proven, well-tested production engine. It's not perfect, and it doesn't make 400hp, but it does the job, and it's not prone to major-melt-down-type failures. The LS is very reliable, too. However, all of the support parts for an LS in a TJ after a swap are not. Those are basically custom-built parts. Since the engine, in my way of thinking, is not a limiting factor with 4L TJs, I'm inclined to keep mine as stock as possible, including those support parts. I'm modifying enough of the other parts that I don't want to increase my chances of backcountry failures whenever I'm out.

That's just my opinion. YMM(and probably will)V. You'll hear others chime in with their opinions, I'm sure. You need to do what works for you. My philosophy is just one crusty, old dude's opinion in a sea of opinions. Good luck with your TJ, and may the road rise up to meet you!

You absolutely provide some great points. I do like the 4.0, I've had quite a few TJs and it's adequate power to get everything done. It's a shame this one's been a headache for the last couple years that's turned me off to them. I agree with you on the OEM is best for reliability and simplicity. I've tried my best to keep the technical stuff stock and just modify the easy stuff for my needs. I know every nut and bolt on these jeeps and that's why I'm liking the idea of going LS, I know this jeep inside and out and I'll be able to diagnose any issues I have pretty well since I'd be doing the work myself. So it's a toss up..do I need to put $5k+ into my jeep right now? Absolutely not. Do I want to? Yes.

Sometimes I do wish I'd just kept the thing stock and lightly wheeled it on 33s and stock axles, it can do 95% of what all Jeeps do in that config. But, I'm in too deep at this point to stop so what the hell, it's not rusting apart and I plan on keeping it for a long time.

But I digress, I appreciate the help Sab, I'm probably going to rethink a cam kit for now.

 

[jw_LJ]

Welp, I can finally say I fixed my misfire problem. With the unknown fear of what was causing it (most likely wiped cam, but not 100% sure) I bought a 90k mile 4.0 and swapped it out over a couple days. Put a set of headers on it, new clutch, ignition system, some sensors, checked and rewrapped the entire wiring harness and she fired right up.

Runs perfect, no more crappy idle, no more CELs, no more hard starting, no more engine roughness. I've been dealing with it since 2019 and for 40k miles and it's so nice to not need to anymore. I just enjoyed sitting and quietly idling in a parking lot for a few minutes, it's the little things lol. Drives and sounds great, it's a blast to drive again. Thanks everyone for the advice, while I was about to order the cam, I found this engine for sale for about the same price and said screw it. Just wanted to post a follow up to wrap up this thread.

By the way, swapping these 4.0s is super easy, for anyone wondering. I did it totally solo over 4ish days with a cherry picker and a socket set.