Random Misfire, No Codes

Ran a can of Seafoam through the gas. Engine's running better, but no change to the misfire.

Does anyone have any ideas? At this point I'm struggling to find things to try. I'd rather not just throw random parts at it (fuel injectors would be next), but I don't know what else to do.
 
Reread some of this thread.

You replaced the crank position sensor but we never got the brand you used...

Either has to be Mopar, Standard Motor Products and I got away with an Echlin from Napa.

I would be highly suspect of a $20 coil from Amazon and would go back to your original if it didn't solve the problem. I used a Holley Accel coil, cap and rotor on my 97.

CPS is my bet.

-Mac
 
macleanflood said:
Reread some of this thread.

You replaced the crank position sensor but we never got the brand you used...

Either has to be Mopar, Standard Motor Products and I got away with an Echlin from Napa.

I would be highly suspect of a $20 coil from Amazon and would go back to your original if it didn't solve the problem. I used a Holley Accel coil, cap and rotor on my 97.

CPS is my bet.

-Mac
Click to expand...

Oh, thought I had. I replaced the CPS with a Mopar earlier this week. My engine tone deepened and it feels more responsive (all unexpected), but no change at all to the miss.

The coil was one suggested by a mechanic that I grabbed from a local parts store. Should I spring for a Mopar of that?
 
Last edited:
  • Like
Reactions: macleanflood
I'd like to voice my skepticism that it's really misfiring as much as you say without tripping a code. I don't question you're hearing what you're hearing, but whether it's actually a miss.

When it happens under power, can you feel it?

What do you run for a muffler? Are all your cats still in place?
 
  • Like
Reactions: Mercury and williambmac
So this is a complete wild guess...this video, Dex has been fighting a rough idle on this 4.0 for a while...turns out the cam was a Mexican cheapy and it wasn't allowing the valves to close. Not saying it's the cam...or the valves...but maybe?


-Mac
 
  • Like
Reactions: Mercury
macleanflood said:
So this is a complete wild guess...this video, Dex has been fighting a rough idle on this 4.0 for a while...turns out the cam was a Mexican cheapy and it wasn't allowing the valves to close. Not saying it's the cam...or the valves...but maybe?


-Mac
Click to expand...

I haven't watched the video but that's kinda where I'm going, especially if he's got a deleted or gutted cat or running a louder than stock muffler, which amplify all the little burps and hiccups that get hidden by a stock exhaust.

Every true misfire I've had was felt more than heard. Like power cutting out for an instant and coming back. When my E90 BMW started missing it threw a code almost immediately and my wife (who was driving at the time and is...liberal with throttle application) thought she'd blown the motor.

I wouldn't tolerate misfires, but if I chased after every little imperfection I heard in the exhaust note I'd go crazy. I just don't expect it to run like a brand new Toyota.
 
  • Like
Reactions: Mercury and macleanflood
freedom_in_4low said:
I'd like to voice my skepticism that it's really misfiring as much as you say without tripping a code. I don't question you're hearing what you're hearing, but whether it's actually a miss.

When it happens under power, can you feel it?

What do you run for a muffler? Are all your cats still in place?
Click to expand...

When the miss is bad it does feel like my power is less. I can feel the miss, the Jeep shakes a little when it hits. The code thing makes no sense, and my life would be a lot easier if there was something there that I could chase. I ran the scanner again last night after 5 hours of driving and still no trouble codes.

Multiple people have heard and it called it a miss, and my mechanic said it was a miss, but so minor he thought of it more as a "pre-miss".

No cats, muffler bought off I think Rough Country, but those were changed several years ago. The only "recent" change to my exhaust was having the pipe hung again (old hangers had broken), and there's a tiny leak near the muffler.
 
Mercury said:
When the miss is bad it does feel like my power is less. I can feel the miss, the Jeep shakes a little when it hits. The code thing makes no sense, and my life would be a lot easier if there was something there that I could chase. I ran the scanner again last night after 5 hours of driving and still no trouble codes.

Multiple people have heard and it called it a miss, and my mechanic said it was a miss, but so minor he thought of it more as a "pre-miss".

No cats, muffler bought off I think Rough Country, but those were changed several years ago. The only "recent" change to my exhaust was having the pipe hung again (old hangers had broken), and there's a tiny leak near the muffler.
Click to expand...

I wasn't thinking of the exhaust as a cause for a miss, rather that it could amplify normal fluctuations as to make them sound like a miss. I don't think the leak should play a role since it's well downstream of the pre-cat O2 sensor(s).

If you can feel a shake or momentary drop in power, then I guess it's a miss. I just don't know why it's not popping a DTC. Some more advanced scanners will show a misfire tally per cylinder that you can watch as it counts up, but if it's not triggering a DTC I have to think it must not be detecting them and the counter wouldn't help. I'm also not 100% positive that feature is there on TJ's, especially early ones so I wouldn't ask you to spend $150+ on a scanner without me being sure it even does what I'm talking about.

You said the compression test was all within 10%, but what was the actual value? Have you done a leakdown test? I didn't see in the thread whether you posted up what you saw on the vacuum test.

When you checked fuel pressure, how did you check it? Engine off but cycled the key to pressure up the system? Engine idling? Engine free revving? Engine under load? All of those situations have significantly different fuel demands and a tired pump may be able to provide pressure at low flow but fall on its face when the engine is actually drinking. This is also where you could view live data while you drive and see if your fuel trims appear to be doing anything weird.
 
freedom_in_4low said:
I wasn't thinking of the exhaust as a cause for a miss, rather that it could amplify normal fluctuations as to make them sound like a miss. I don't think the leak should play a role since it's well downstream of the pre-cat O2 sensor(s).

If you can feel a shake or momentary drop in power, then I guess it's a miss. I just don't know why it's not popping a DTC. Some more advanced scanners will show a misfire tally per cylinder that you can watch as it counts up, but if it's not triggering a DTC I have to think it must not be detecting them and the counter wouldn't help. I'm also not 100% positive that feature is there on TJ's, especially early ones so I wouldn't ask you to spend $150+ on a scanner without me being sure it even does what I'm talking about.

You said the compression test was all within 10%, but what was the actual value? Have you done a leakdown test? I didn't see in the thread whether you posted up what you saw on the vacuum test.

When you checked fuel pressure, how did you check it? Engine off but cycled the key to pressure up the system? Engine idling? Engine free revving? Engine under load? All of those situations have significantly different fuel demands and a tired pump may be able to provide pressure at low flow but fall on its face when the engine is actually drinking. This is also where you could view live data while you drive and see if your fuel trims appear to be doing anything weird.
Click to expand...

I've been on the fence about a better scanner. Someone suggested getting one that can show injector pulse width, but I can't confirm a scanner can actually ready that on mine. I would budget for it if I thought there was any utility in upgrading from my little $20 Amazon cheapie.

I've only had the compression test done, and I'm not sure on the specific values, he just told me it was all within 10%. I'm having trouble finding another local Jeep mechanic (mine went on sabbatical), but if I can find one I can see about the leakdown test.

When I tested the fuel pressure I did so just keying the pump and while idling, but not under load. Would I just need to hook the tester up to the rail and press the accelerator? Would I need to actually drive it? I could rent that tool again pretty easily. And what specs should it be? Everything I found in Google was just having it in idle.
 
Mercury said:
I've been on the fence about a better scanner. Someone suggested getting one that can show injector pulse width, but I can't confirm a scanner can actually ready that on mine. I would budget for it if I thought there was any utility in upgrading from my little $20 Amazon cheapie.

I've only had the compression test done, and I'm not sure on the specific values, he just told me it was all within 10%. I'm having trouble finding another local Jeep mechanic (mine went on sabbatical), but if I can find one I can see about the leakdown test.

When I tested the fuel pressure I did so just keying the pump and while idling, but not under load. Would I just need to hook the tester up to the rail and press the accelerator? Would I need to actually drive it? I could rent that tool again pretty easily. And what specs should it be? Everything I found in Google was just having it in idle.
Click to expand...

I believe the pressure should remain constant. The pump is either on or off and there's no electronic modulatin of the regulator. I'm sure there's probably some tolerance but if it drops more than a couple psi I would probably look into it.

Yeah, I would hook up the tester on a day where you can run the hose and gauge where you can see it and drive it around the block.

I had a fuel pump strand me on the side of the interstate once that had good fuel pressure with key on and even idling, for a while.
 
freedom_in_4low said:
This is also where you could view live data while you drive and see if your fuel trims appear to be doing anything weird.
Click to expand...

I can see fuel trim data, though I don't know what the data means. What should I be looking for?

freedom_in_4low said:
I believe the pressure should remain constant. The pump is either on or off and there's no electronic modulatin of the regulator. I'm sure there's probably some tolerance but if it drops more than a couple psi I would probably look into it.

Yeah, I would hook up the tester on a day where you can run the hose and gauge where you can see it and drive it around the block.

I had a fuel pump strand me on the side of the interstate once that had good fuel pressure with key on and even idling, for a while.
Click to expand...

Interesting. I'll rent that tool again and see if I notice anything. So it should just be a steady 40 PSI throughout?

Something interesting I noticed. I started the Jeep, drove a short distance, then turned it off to restart the stereo. It sputtered and acted like it didn't want to. Gave it throttle and it ran and straightened out, but it was odd.
 
Mercury said:
I can see fuel trim data, though I don't know what the data means. What should I be looking for?
Click to expand...

Base is 0%, see if they do anything consistently like go 10% or higher or any other behavior that corresponds to when the misfires are actively happening.

Mercury said:
Interesting. I'll rent that tool again and see if I notice anything. So it should just be a steady 40 PSI throughout?
Click to expand...

I believe so, yes.

Mercury said:
Something interesting I noticed. I started the Jeep, drove a short distance, then turned it off to restart the stereo. It sputtered and acted like it didn't want to. Gave it throttle and it ran and straightened out, but it was odd.
Click to expand...

That sounds like vaporized fuel or that the pressure bled down while it was off, which could point to a bad fuel pressure regulator or a leaky injector. Did you change the injectors already? I've lost track.
 
freedom_in_4low said:
Base is 0%, see if they do anything consistently like go 10% or higher or any other behavior that corresponds to when the misfires are actively happening.



I believe so, yes.



That sounds like vaporized fuel or that the pressure bled down while it was off, which could point to a bad fuel pressure regulator or a leaky injector. Did you change the injectors already? I've lost track.
Click to expand...

When I've watched recently I don't think I saw higher than 7%, but I was only spot checking at points.

I haven't changed the injectors yet. I was hoping to find a way to test them first.
 
I just started my Jeep and it's idling at about 350 and trying to stall. Pressed the gas, RPMs climbed, then as soon as I let off the gas it dropped at 350 and tried to stall again. After I held the pedal for about a minute it's now idling at 800.

After a minute of idling steadily at 800 it bottomed out again. Dropped to 350 and it kept trying to rev back up but not making it.
 
Last edited:
macleanflood said:
Damn that just sounds like an IAC or vacuum.

Fuel pressure maybe but that's just classic IAC.

-Mac
Click to expand...

It's not the only sign of a vacuum leak. Driving yesterday the defrost vent kicked on and stayed on for a few minutes before going back to dash vents. I've tested everything with a vacuum pump, it holds pressure steady, sprayed fluid all around the vacuum lines when it's running. I've fixed every leak I can find, but my mechanic found one I hadn't and now this happened. I don't know what else could have a leak, I've checked every piece of hose and tubing I can find. I replaced the vent selector switch a couple of months ago when I found a leak there, it's been fine up until yesterday.

If there's a vacuum leak, it's intermittent.
 
  • Like
Reactions: freedom_in_4low
New thing: I had trouble starting the Jeep just now. It sputtered and wouldn't start until I could give it gas. Could this be a fuel delivery problem?
 
Top Bottom