Random Small Misfire

M

Mercury

TJ Enthusiast
Original poster
Joined
Jul 18, 2018
Messages
274
Location
Louisville, KY, USA
I won't go into a huge description as I've posted this elsewhere on here, but a new piece of data.

I've had a random misfire for nearly two years now. No codes, but definitely there and confirmed with multiple mechanics.

I have replaced:
Plugs, wires, cap, rotor
Distributor
CPS (mopar)
TPS (mopar)
IAC (mopar)
Throttle body
MAF (non-mopar, but since it didn't improve anything I switched it back to the factory one)
O2 sensors (NTK/NGK)
Computer
Timing chain/sprockets
Smoke tested and fixed all vacuum leaks (a hose, throttle body, AC selector switch)
Smoke tested and fixed exhaust leaks (except one at the muffler, but I doubt it's involved)
Fuel pressure is one PSI below spec
Compression tested (one was a little low, but within 10% of the others)
Ignition coil (mopar)


I'm sure there's more, but I'm drawing a blank.

Where it's odd is, the first 10 - 20 seconds after starting the Jeep there's no misfire. When we replaced the timing chain/sprockets the first one to two minutes after starting the Jeep the first time there was no misfire. It doesn't change when it's hot, happens at idle or under load (though it's harder to notice when it's above idle), outside temperature doesn't seem to matter.

Two Jeep mechanics have scratched their heads and thrown up their hands. Another, who actually works at a machine shop and rebuilds 4.0s, says he isn't sure but it seems like the issue is electronic, but that was just based on listening to it, no actual diagnostics.

I'll try anything, but I'm just not sure what to try next shy of just pulling the head and driving it to a machine shop, which I'm loathe to spend the money on if it's just more from the parts cannon.

Does anyone have any ideas?
 
Last edited:
I just don't know what changes after that 10 - 20 seconds. It's not anywhere near fully warm, and I just don't know the "startup cycle" stuff well enough.
 
Usually that's open to closed loop.

The PCM starts on default fuel tables. Runs until the O2s heat up, then adjusts according to sensor data.

Unplug your O2s and force the PCM to use default tables. You'll get a code for the O2s but it'll tell you something.

-Mac
 
macleanflood said:
Usually that's open to closed loop.

The PCM starts on default fuel tables. Runs until the O2s heat up, then adjusts according to sensor data.

Unplug your O2s and force the PCM to use default tables. You'll get a code for the O2s but it'll tell you something.

-Mac
Click to expand...

That I can do! I'll try that tomorrow. Does it hit closed loop that fast? I thought that took a few minutes.

ColoJeep said:
Just curious , what spark plugs ?
Click to expand...

Originally I had the iridiums recommended on the forums, then the regular ones recommended here. Champion, I think, but I confirmed from one of Jerry Bransford's posts before I got them. Both sets, same issue, no excess deposits or wetness on any of them, including the ones I installed a few months ago.
 
  • Like
Reactions: ColoJeep
A good scan tool will take frame snapshots of all sensor data at the time of a fault and several or many before the fault.

That data should point you to the problem.

Not having a fault might be the tricky problem because it won't trigger snapshots.

So you might need to record a bunch and see if you can nail the time of the perceived miss with live data.

Your mechanic is clearly not interested in spending the time to do this. Most aren't. Troubleshooting is very time consuming.

If you really want to fix this it might be a good time to invest in a quality code reader.

-Mac
 
macleanflood said:
A good scan tool will take frame snapshots of all sensor data at the time of a fault and several or many before the fault.

That data should point you to the problem.

Not having a fault might be the tricky problem because it won't trigger snapshots.

So you might need to record a bunch and see if you can nail the time of the perceived miss with live data.

Your mechanic is clearly not interested in spending the time to do this. Most aren't. Troubleshooting is very time consuming.

If you really want to fix this it might be a good time to invest in a quality code reader.

-Mac
Click to expand...

I'm not opposed to a good one. Any recommendations? I have a few Bluetooth ones on Amazon picked out in the $100 range, but I don't know which would give the most info.
 
Mercury said:
Ooooo, how did he figure that out? Have DMM, will test.
Click to expand...

Don't back probe it it will be inconclusive and you run the risk of pushing it too far in and spreading the sleeve. You have to, pardon my french, muscle fuck the plug wiggling it to induce a fault.... just be careful not to crack the plastic as the connectors are almost non existent in the world now.
 
  • Like
Reactions: macleanflood
Ccturley said:
Don't back probe it it will be inconclusive and you run the risk of pushing it too far in and spreading the sleeve. You have to, pardon my french, muscle fuck the plug wiggling it to induce a fault.... just be careful not to crack the plastic as the connectors are almost non existent in the world now.
Click to expand...

I was looking at your thread the other day and left thinking about just that. Smart. How hard would you say your muscle fuckery level is in this situation?

Ccturley said:
the pcm is programmed to go from closed to open loop at approximately 150 degrees.

if you're down for a good read.... https://wranglertjforum.com/threads/my-tj-hates-me.80080/page-4
Click to expand...

Getting through it, just not the shortest read 😂

Okay, if that's the case then it's probably not an open-loop/closed-loop situation. I'm still going to try unplugging the O2 sensors just to see what happens.
 
  • Like
Reactions: ColoJeep and macleanflood
Mercury said:
How hard would you say your muscle fuckery level is in this situation?
Click to expand...
Hard enough to really manipulate the wires inside the plug. if youre getting intermittien contact you have to really force it to the surface. Cardinal rule if it feels like its going to break it will back off.
Mercury said:
Getting through it, just not the shortest read 😂
Click to expand...
It isn't I've been chasing my problem for over 2 weeks now. I'm a retired ASE Master Tech who does drive ability and electrical... And I think I've finally found the problem.
 
  • Like
Reactions: macleanflood
Top Bottom