A ghost of a misfire

[Mercury]

Alright, I have a misfire. No one has been able to find it, it's a ghost, and it's so slight it's almost a ghost as well.

So, tiny misfire, but consistent. You can hear it every second, or every couple of seconds, or several times a second. There's no predicting. It happens if it's under load, it happens at idle. The only, only time it doesn't miss is during the first 10 to 20 seconds after starting. It'll sound just fine until it's slightly warm and the miss comes right back. The only thing that impacts it is a vacuum leak which makes it louder.

It's never thrown a misfire code of any kind.

I've had three Jeep mechanics look at it, one of which rebuilds 4.0s regularly, and no one has an idea. The rebuilder said from the sound of it he thinks it's electronic, but doesn't know where.

So far I've replaced every sensor of which I can think. O2 sensors, TPS, CPS, MAP (I switched back to the Mopar as it had no impact), all Mopar. I've replaced the throttle body as the old one leaked. I've smoked every location on this Jeep and plugged every leak. Distributor replaced, spark plugs, wires, cap, rotor. I've replaced the computer. No exhaust leaks before the muffler. Cooling system flushed with Prestone and Thermocure, new radiator, water pump, holds pressure. No exhaust leaks in the cooling system. Compression test showed all within 10% except one which was slightly lower. Gas mileage is good, around 15.5 mpg if there are no vacuum issues.

When I plugged the vacuum canister line today the miss got a little quieter, so I'm ordering a universal vacuum canister.

Does anyone have any ideas?

Yes, I've posted in the past, but I've performed more repairs/upgrades in the past several months and wanted a clean-slate start.

 

[MikeE024]

Alright, I have a misfire. No one has been able to find it, it's a ghost, and it's so slight it's almost a ghost as well.

So, tiny misfire, but consistent. You can hear it every second, or every couple of seconds, or several times a second. There's no predicting. It happens if it's under load, it happens at idle. The only, only time it doesn't miss is during the first 10 to 20 seconds after starting. It'll sound just fine until it's slightly warm and the miss comes right back. The only thing that impacts it is a vacuum leak which makes it louder.

It's never thrown a misfire code of any kind.

I've had three Jeep mechanics look at it, one of which rebuilds 4.0s regularly, and no one has an idea. The rebuilder said from the sound of it he thinks it's electronic, but doesn't know where.

So far I've replaced every sensor of which I can think. O2 sensors, TPS, CPS, MAP (I switched back to the Mopar as it had no impact), all Mopar. I've replaced the throttle body as the old one leaked. I've smoked every location on this Jeep and plugged every leak. Distributor replaced, spark plugs, wires, cap, rotor. I've replaced the computer. No exhaust leaks before the muffler. Cooling system flushed with Prestone and Thermocure, new radiator, water pump, holds pressure. No exhaust leaks in the cooling system. Compression test showed all within 10% except one which was slightly lower. Gas mileage is good, around 15.5 mpg if there are no vacuum issues.

When I plugged the vacuum canister line today the miss got a little quieter, so I'm ordering a universal vacuum canister.

Does anyone have any ideas?

Yes, I've posted in the past, but I've performed more repairs/upgrades in the past several months and wanted a clean-slate start.

Some folks may not know it, but you can have misfires without popping an engine code. It appears that the misfires don’t pop a CEL unless they happen enough in X amount of time. My PCM didn’t pop a code when I had many misfires reported on my gauge in real-time...eventually I’d get a code when the number was high enough in the cycle time before it reverted back to zero for the next test cycle.



All we can do at this point is throw ideas at it. Also, we don’t know what we’ve already suggested so we are bound to repeat ourselves and you’ll tell us you already checked it.

That said, was your computer replaced with a Wranglerfix unit? I ask because that’s what solved my random misfires after the hood louver and heat wrapping the injectors and fuel rail didn’t help.

You’ve stated that one of your cylinders was outside of 10% compared to others, so maybe that’s part of it. Also, were the compression numbers good or low?

I don’t know how your mechanic can claim it sounds electrical. That statement sounds irresponsible.

Was your vacuum leak tests done with smoke and pressure? You plugged up the butterfly valve area with new seals or o-rings?

Have you confirmed that your fuel pressure is good?

Your exhaust manifold bolts and nuts are tight…and your manifold isn't leaking at all?

Spark plugs? injectors? Distributor and wires?

Good grounds? Wires to the harness all look healthy, especially near the back corner of the valve cover?

I’ll let others throw more spaghetti at the wall. Best of luck with this.

 

[macleanflood]

Mechanic sounds exasperated and +2 on checking fuel pressure.

Driving around reading/logging live ODB data is how I solve these.

We just had a forum poster with a collapsed lifter causing misfires.

Chase that leaking cylinder.

-Mac

 

[Mercury]

Some folks may not know it, but you can have misfires without popping an engine code. It appears that the misfires don’t pop a CEL unless they happen enough in X amount of time. My PCM didn’t pop a code when I had many misfires reported on my gauge in real-time...eventually I’d get a code when the number was high enough in the cycle time before it reverted back to zero for the next test cycle.

View attachment 601819View attachment 601820

All we can do at this point is throw ideas at it. Also, we don’t know what we’ve already suggested so we are bound to repeat ourselves and you’ll tell us you already checked it.

That said, was your computer replaced with a Wranglerfix unit? I ask because that’s what solved my random misfires after the hood louver and heat wrapping the a ac injectors and fuel rail didn’t help.

You’ve stated that one of your cylinders was outside of 10% compared to others, so maybe that’s part of it. Also, were the compression numbers good or low?

I don’t know how your mechanic can claim it sounds electrical. That statement sounds irresponsible.

Was your vacuum leak tests done with smoke and pressure? You plugged up the butterfly valve area with new seals or o-rings?

Have you confirmed that your fuel pressure is good?

Your exhaust manifold bolts and nuts are tight…and your manifold isn't leaking at all?

Spark plugs? injectors? Distributor and wires?

Good grounds? Wires to the harness all look healthy, especially near the back corner of the valve cover?

I’ll let others throw more spaghetti at the wall. Best of luck with this.

I'm happy to hear any options, at this point I just don't know what to check.

I replaced it with a PCM I bought off eBay. No change at all in anything after replacement. I switched it back not long ago to the original, no change.

I'm not sure how he thought that was the case. He didn't charge me, just offered to give it a listen and see what he thought, so I'm not mad.

They were all within spec, even the low one, just lower than the others. I don't recall the exact values as it was late last year, but they weren't alarming.

Vacuum tests done with a hand vacuum pump at various points in the system. I basically pulled each hose and created vacuum and plugged every loss point. I replaced one of the one-way valves. Everything holds vacuum up to 20.

The smoke tests were just using the pressure of the smoke tester. Should I try smoking, pulling the tester and then using my pump to add pressure?

Fuel pressure is good. It was 1 psi below the normal value I found on the forums, but no drops when revving or at idle.

I replaced the exhaust manifold late last year and tightened them as much as I could without rounding the bolts. I smoked it as well and no leaks (but it's just the pressure of the smoke tester).

Plugs, wires, and distributor are all new. Injectors were removed and cleaned by a guy with a machine, and they all were in good shape. New o-rings when I installed them.

Grounds should all be solid. I upgraded all of the grounds to 1/0 OFC wire, and added a ground to the alternator bracket and another to the frame.

The harness at the back of the valve cover was rubbing through, bad enough that it started to blow the fuse for the fuel injectors. We replaced the damaged sections and the injectors are working normally now.

 

[Mercury]

Mechanic sounds exasperated and +2 on checking fuel pressure.

Driving around reading/logging live ODB data is how I solve these.

We just had a forum poster with a collapsed lifter causing misfires.

Chase that leaking cylinder.

-Mac

Oh, I should mention my long-term fuel trim is always positive. The short-term goes positive to negative, but not the LTFT. I've seen it spike as high as 8.

What else should I look for?

 

[Mercury]

Some folks may not know it, but you can have misfires without popping an engine code. It appears that the misfires don’t pop a CEL unless they happen enough in X amount of time. My PCM didn’t pop a code when I had many misfires reported on my gauge in real-time...eventually I’d get a code when the number was high enough in the cycle time before it reverted back to zero for the next test cycle.

View attachment 601819View attachment 601820

What is that computer/logger you're using in that picture?

 

[macleanflood]

Oh, I should mention my long-term fuel trim is always positive. The short-term goes positive to negative, but not the LTFT. I've seen it spike as high as 8.

What else should I look for?

O2 voltages, engine temperature, vacuum and timing.

-Mac

 

[MikeE024]

What is that computer/logger you're using in that picture?

Sadly, the gauge only works with the 05-06 models. I shared so you know that misfires do indeed happen without necessarily popping a CEL.

 

[Mercury]

Sadly, the gauge only works with the 05-06 models. I shared so you know that misfires do indeed happen without necessarily popping a CEL.

Oh, that's a shame. I love seeing data from my OBD2. I don't understand half of it, but what I do understand is pretty neat to know.

O2 voltages, engine temperature, vacuum and timing.

-Mac

That's what I have up usually, along with STFT, LTFT. Not sure if I have an option for timing, need to check that. I think it does.

 

[Mercury]

I'm just at a loss for what to test next. I could throw a new fuel pump at it, see if that does anything. It just doesn't act starved in any other way; I press the gas, it moves. I ran a vacuum test on it yesterday and the only loss was the new AC switch panel and the canister. The switch panel is now replaced and functioning. When I replace the vacuum canister I think that will resolve all of the vacuum issues. Unless there's some weird leak in the brake booster I just don't know what else could be leaking.

It seems like with the LTFT being constantly in the positive that I'm seeing a vacuum leak, just not sure where it could be. When I blocked off vacuum to the canister I noticed the misfire was quieter. Not gone, but quieter. I don't know if that means that the miss itself is vacuum related or if the vacuum leak was just exacerbating the misfire. If it's electrical, where could that be? I could pull the insulation from the bundle that passes behind the valve cover and see if we missed any of the shorts. Other than that I don't know what else to check.

Anyone have any ideas? Random, spitball type of thoughts? I'm up for just about anything at this point, I've had the misfire for two years now.

 

[macleanflood]

Need to fix compression issue. I suspect your TJ is running under temp compounding that issue.

-Mac

 

[LostintheForest]

Was just about to ask, have we checked compression/ leak down more recently?

I guess we can just refresh ourselves of the basics needed to make an engine run:
Suck, squeeze, bang, blow.
Verify which of these 4 things is not happening correctly, and narrow down what is need to make it happen by process of elimination.

Suck: you have verified you have good vacuum, and confirmed it with multiple tests.

Squeeze: compression was “within spec” a few years ago. I’d recheck, and do a leak down as well.

Bang: literally everything has been replaced it seems,primary and secondary ignition wise. Probably not ignition in that case, verify fuel pressure and volume.

Blow: it sounds like your exhaust is fine, but verify you have good flow. All metering sensors have been replaced, too it looks like, so that’s probably not it.
If it was a a restriction, it’d been sluggish and have crap mileage.

Best of luck

 

[Mercury]

Need to fix compression issue. I suspect your TJ is running under temp compounding that issue.

-Mac

Funny enough I've had overheating problems off and on for years. Last year if the ambient temp was over 88 it would overheat at idle. Spike up over 220. I ran two coolant flushes through it, and while they pulled out some stuff it was overall pretty clean before. Squeaky clean now. Then last weekend it was 80, I was waiting for my daughter, and the temp was over 220 within a few minutes.

We replaced the water pump, upper and lower radiator hose, and the radiator. I had the pump and the radiator on hand for a bit, glad I did. It hasn't reached that outside temp again yet so I'm not sure if it's helped the issue or not.

Was just about to ask, have we checked compression/ leak down more recently?

I guess we can just refresh ourselves of the basics needed to make an engine run:
Suck, squeeze, bang, blow.
Verify which of these 4 things is not happening correctly, and narrow down what is need to make it happen by process of elimination.

Suck: you have verified you have good vacuum, and confirmed it with multiple tests.

Squeeze: compression was “within spec” a few years ago. I’d recheck, and do a leak down as well.

Bang: literally everything has been replaced it seems,primary and secondary ignition wise. Probably not ignition in that case, verify fuel pressure and volume.

Blow: it sounds like your exhaust is fine, but verify you have good flow. All metering sensors have been replaced, too it looks like, so that’s probably not it.
If it was a a restriction, it’d been sluggish and have crap mileage.

Best of luck

It was checked late last year, but it's been six months.

I've checked fuel pressure, but how would I check flow? None of the vids/write-ups I've gone through have mentioned this and now I'm interested.





Would 10 - 12% difference in compression lead to an intermittent misfire like this? To me it seems like with the number of misses that happen (it can be several per second), if was localized to one cylinder wouldn't that be enough to trip the CEL? Is there a problem that can exist on one cylinder that can cause misfires in other cylinders? I'm just trying to wrap my head around the logic.

At this point I wonder if I should just pull the head. Growing tired of messing with it at this point. If there is a compression issue it's going to require this anyways from what I understand.

 

[NskLJ]

Alright, I have a misfire. No one has been able to find it, it's a ghost, and it's so slight it's almost a ghost as well.

So, tiny misfire, but consistent. You can hear it every second, or every couple of seconds, or several times a second. There's no predicting. It happens if it's under load, it happens at idle. The only, only time it doesn't miss is during the first 10 to 20 seconds after starting. It'll sound just fine until it's slightly warm and the miss comes right back. The only thing that impacts it is a vacuum leak which makes it louder.

It's never thrown a misfire code of any kind.

I've had three Jeep mechanics look at it, one of which rebuilds 4.0s regularly, and no one has an idea. The rebuilder said from the sound of it he thinks it's electronic, but doesn't know where.

So far I've replaced every sensor of which I can think. O2 sensors, TPS, CPS, MAP (I switched back to the Mopar as it had no impact), all Mopar. I've replaced the throttle body as the old one leaked. I've smoked every location on this Jeep and plugged every leak. Distributor replaced, spark plugs, wires, cap, rotor. I've replaced the computer. No exhaust leaks before the muffler. Cooling system flushed with Prestone and Thermocure, new radiator, water pump, holds pressure. No exhaust leaks in the cooling system. Compression test showed all within 10% except one which was slightly lower. Gas mileage is good, around 15.5 mpg if there are no vacuum issues.

When I plugged the vacuum canister line today the miss got a little quieter, so I'm ordering a universal vacuum canister.

Does anyone have any ideas?

Yes, I've posted in the past, but I've performed more repairs/upgrades in the past several months and wanted a clean-slate start.

Here is the way I have diagnosed random misfires. If you can reproduce it while driving hold it in misfire until the check engine light turns on. I found slower speed lugging the throttle will usually recreate the misfire. I will hold the throttle there even if I have to use the brakes to slow the vehicle. Once you find the cylinder I would pull the plugs and inspect looking for cracked porcelain around the electrode. From this point I would swap start swapping parts (plugs, injectors) one at a time from misfire cylinder to another cylinder to try to get the misfire to move. This can be a long process, doubt you will find a mechanic to spend the time to do this. You may also have some carbon on a valve causing issues a seafoam top end cleaner may help if that is the issue. If you have an intake valve with excessive carbon the fuel can soak into it causing a lean misfire on that cylinder.

 

[LostintheForest]

Funny enough I've had overheating problems off and on for years. Last year if the ambient temp was over 88 it would overheat at idle. Spike up over 220. I ran two coolant flushes through it, and while they pulled out some stuff it was overall pretty clean before. Squeaky clean now. Then last weekend it was 80, I was waiting for my daughter, and the temp was over 220 within a few minutes.

We replaced the water pump, upper and lower radiator hose, and the radiator. I had the pump and the radiator on hand for a bit, glad I did. It hasn't reached that outside temp again yet so I'm not sure if it's helped the issue or not.




It was checked late last year, but it's been six months.

I've checked fuel pressure, but how would I check flow? None of the vids/write-ups I've gone through have mentioned this and now I'm interested.





Would 10 - 12% difference in compression lead to an intermittent misfire like this? To me it seems like with the number of misses that happen (it can be several per second), if was localized to one cylinder wouldn't that be enough to trip the CEL? Is there a problem that can exist on one cylinder that can cause misfires in other cylinders? I'm just trying to wrap my head around the logic.

At this point I wonder if I should just pull the head. Growing tired of messing with it at this point. If there is a compression issue it's going to require this anyways from what I understand.

 

[lBasket]

You can just barely hear mine misfire every now and then when it idles. I have ignored it for years and things have gone well. In my head, the misfire thresholds weren't just pulled out of someone's ass and will let me know when it actually is enough to matter

 

[LostintheForest]

Try snugging up your intake manifold bolts, this just fixed my high idle and actually smoothed out a slight miss, similar to this

 

[Strizzo]

Funny enough I've had overheating problems off and on for years. Last year if the ambient temp was over 88 it would overheat at idle. Spike up over 220. I ran two coolant flushes through it, and while they pulled out some stuff it was overall pretty clean before. Squeaky clean now. Then last weekend it was 80, I was waiting for my daughter, and the temp was over 220 within a few minutes.

We replaced the water pump, upper and lower radiator hose, and the radiator. I had the pump and the radiator on hand for a bit, glad I did. It hasn't reached that outside temp again yet so I'm not sure if it's helped the issue or not.




It was checked late last year, but it's been six months.

I've checked fuel pressure, but how would I check flow? None of the vids/write-ups I've gone through have mentioned this and now I'm interested.





Would 10 - 12% difference in compression lead to an intermittent misfire like this? To me it seems like with the number of misses that happen (it can be several per second), if was localized to one cylinder wouldn't that be enough to trip the CEL? Is there a problem that can exist on one cylinder that can cause misfires in other cylinders? I'm just trying to wrap my head around the logic.

At this point I wonder if I should just pull the head. Growing tired of messing with it at this point. If there is a compression issue it's going to require this anyways from what I understand.

overheating at idle points me to a bad fan clutch. If you can stop it with a rolled up newspaper at idle, its bad.

 

[Steel City 06]

Have you inspected your catalytic converters for damage or fouling? These can eventually give up the ghost, and when they do, the exhaust restriction results in a ton of issues.

 

[TerryD]

I've heard of purge valves causing a miss-type idle change because of the sudden drop in manifold vacuum when they operate if the orifice in them wears out and allows too much flow or you have a slight leak in the system behind them.

You will most likely get a CEL for the purge valve if you do this, but unhook and plug the vacuum lines going back to the purge system as close to the engine as possible. That should allow you to eliminate it as a cause if you still have the same symptoms while driving with the system disabled.