A ghost of a misfire

[ColoJeep]

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

True , considering each plug fires 5.41 times a second at idle , some misfire is totally normal. You are correct that the thresholds are thought out.

 

[Mercury]

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.

I'll give the bogging-engine-until-it-gets-angry idea a try, that's an interesting approach. I ran Seafoam through it late last year, but doesn't hurt my feelings to do it again.

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

Cool, thank you for the instructions! The manifold bolts are possible, it was an utter bear trying to get those tightened up when we did the manifold last year, but I've done the job before and it worked fine. First time for everything, though.

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

I've dealt with it so long that I'm nearing that point. I just hate throwing in the towel on anything with my Jeep.

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

That was my thought as well, I ordered a new clutch and everything, but the OEM one still had plenty of resistance, nearly as much as the aftermarket, and the OEM was smooth where the AM had a bit of a grind, so we just didn't install it.

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.

Cat has good flow and no restrictions, I had my mechanic run tests when it was pulled for some welding repairs.

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.

You know, the purge system is something that I've wondered on. So I'm disconnecting where it's fed from the intake manifold and plugging it at the manifold? If I left it like this for a period of time, say a few hundred miles of driving, would that cause any problems?

 

[TerryD]

I'll give the bogging-engine-until-it-gets-angry idea a try, that's an interesting approach. I ran Seafoam through it late last year, but doesn't hurt my feelings to do it again.




Cool, thank you for the instructions! The manifold bolts are possible, it was an utter bear trying to get those tightened up when we did the manifold last year, but I've done the job before and it worked fine. First time for everything, though.





I've dealt with it so long that I'm nearing that point. I just hate throwing in the towel on anything with my Jeep.





That was my thought as well, I ordered a new clutch and everything, but the OEM one still had plenty of resistance, nearly as much as the aftermarket, and the OEM was smooth where the AM had a bit of a grind, so we just didn't install it.





Cat has good flow and no restrictions, I had my mechanic run tests when it was pulled for some welding repairs.





You know, the purge system is something that I've wondered on. So I'm disconnecting where it's fed from the intake manifold and plugging it at the manifold? If I left it like this for a period of time, say a few hundred miles of driving, would that cause any problems?

In a couple hundred miles, I think you'd be fine. I wouldn't leave it unhooked for a long time. If the miss is as regular as you say, you should notice if it helps pretty quickly.

 

[Strizzo]

Are you using any coolant over time? I understand that cracked heads are very common if they have been overheated. The coolant could be intermittently causing the missfire, although typically you'd have a missfire after a hot restart if there was coolant leaking into the cylinder. Something to consider i guess, maybe you could hook a pressure gauge up and then start the engine and let it get up to temp and see if it holds pressure after shutting off.

 

[Mercury]

Are you using any coolant over time? I understand that cracked heads are very common if they have been overheated. The coolant could be intermittently causing the missfire, although typically you'd have a missfire after a hot restart if there was coolant leaking into the cylinder. Something to consider i guess, maybe you could hook a pressure gauge up and then start the engine and let it get up to temp and see if it holds pressure after shutting off.

Coolant levels have been steady. Maybe every few months when I change the oil I'll add a little coolant to the overflow, but even then it's a small amount. I'm still happy to check it, I'm not discounting anything at the moment.

 

[Mercury]

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.

Thank you! It didn't fix the misfire, but this sure quieted it down. Apparently there's a leak I need to address. Can you replace the those metal-like hoses with regular vacuum line?

Replaced the vacuum canister with a universal one, the Jeep is definitely running better. I'm curious to see what happens when the other leaks are fixed as well. What gets me is that I smoke and vacuum pump tested this all and there wasn't anything to indicate leaks but there obviously were leaks.

 

[TerryD]

Thank you! It didn't fix the misfire, but this sure quieted it down. Apparently there's a leak I need to address. Can you replace the those metal-like hoses with regular vacuum line?

Replaced the vacuum canister with a universal one, the Jeep is definitely running better. I'm curious to see what happens when the other leaks are fixed as well. What gets me is that I smoke and vacuum pump tested this all and there wasn't anything to indicate leaks but there obviously were leaks.

It's doubtful it's the metal lines. Probably the rubber joints where they tie into the tank and canister.

 

[Mercury]

It's doubtful it's the metal lines. Probably the rubber joints where they tie into the tank and canister.

Still, can they be replaced with vacuum hose? I haven't pulled them to see the fittings because I'm worried it'll tear the rubber, but if I can just run vacuum hose and clamp the ends that's the route I'll go.

 

[TerryD]

Still, can they be replaced with vacuum hose? I haven't pulled them to see the fittings because I'm worried it'll tear the rubber, but if I can just run vacuum hose and clamp the ends that's the route I'll go.

I wouldn't do that myself, no. Rubber degrades a lot faster, is more susceptible to heat and abrasion damage, and will hang loose and be likely to hang on road and trail debris.

You can cap both ends of the tube and pressurize it then use soapy water to look for leaks.

 

[Mercury]

I wouldn't do that myself, no. Rubber degrades a lot faster, is more susceptible to heat and abrasion damage, and will hang loose and be likely to hang on road and trail debris.

You can cap both ends of the tube and pressurize it then use soapy water to look for leaks.

Ohhh, sorry, I meant the hoses up top, not the one that runs down to the fuel tank. They almost don't feel like real metal. If I can just replace those with hose I'll go that route.



I'm considering just changing out the fuel pump. I'm concerned about damaging any of the hoses/fittings on top, but I've replaced just about everything else in this thing without the misfire improving.

 

[macleanflood]

It's possible the charcoal from the failed canister lodged itself in the lines and solenoid. Mine did. That charcoal might be absorbing smoke or causing flow issues. I'd suggest blowing out all the lines.

-Mac

 

[Mercury]

It's possible the charcoal from the failed canister lodged itself in the lines and solenoid. Mine did. That charcoal might be absorbing smoke or causing flow issues. I'd suggest blowing out all the lines.

-Mac

Oooo, just saw this. Do you mean just removing the vacuum lines and hooking to an air compressor?



Update: replaced the fuel pump, still missing. However, it feels like it's changed. The misfire is still present at idle, but when pressing the gas my buddy and I no longer hear it. The LTFT is now finally going into the negatives! It hasn't done that in months of driving. I mean that, I tracked my LTFT on a 1.5 hour drive recently and it never dropped below 0.

Anyone have any other ideas? Soon I'll rent a compression tester and see if that shows anything, but I'll take any other suggestions. I've nearly resigned myself to pulling the head.

 

[Mercury]

Well, I was wrong. The misfire is still there under load. Now the miss is worse at idle, but better under load. Freaking weird.

I'll add that when we removed the old fuel pump the filter sock was floating around the tank. I don't know if it came off when we pulled the old one (though we were gentle as heck pulling it, wanted to save it in case it wasn't the problem) or if it's been floating around for months/years.

Would the unfiltered fuel have gummed anything up? The fuel injectors were taken out and cleaned a year ago with a machine, didn't impact the misfire at all.

The big change with the fuel pump is the negative LTFT values. That's new, curious how much fuel economy is going to change.

 

[Mercury]

I noticed that when the miss hits my vacuum drops slightly. Very slightly, but the needle drops a little and then rebounds. Driving yesterday my vent selector fought me for about ten seconds, switching to the dash vents. Don't know if that's vacuum or if I have blend door issues.