Random Misfire. I think I've tried everything.

I'm having an issue with my 96 XJ. Jeeps runs like crap at idle but great when driving. Everytime I come to a stop it stumbles worse and worse until I take off. It sounds and smells like it's loading up with fuel. Fuel pressure is at 53 psi at idle. When I rev it sometimes the presure will jump around. I've replaced the fuel pump, filter, distributor, iac, o2 sensors, crank sensor, cam sensor, water temp sensor, map sensor, cap, rotor, plugs and wires. No vacuum leaks anywhere. At this point I'm going crazy. Anyone have any suggestions?

At this point seems like fuel injectors might be the issue. Disco each FI connector one at a time and see if anything changes. If you happen to pull one that doesn't change then you have your culprit.

Each one causes a change in idle when unplugged. I swapped in the injectors from my 97 that ran perfect. Before I put them in, I cleaned and tested them and threw on new o rings. I also cleaned out the fuel rail real good.

No cel?

I had a similar experience with my 96 xj. Mine ended up being loose wires going to the fuse panel on the passenger fender. Drove fine down the road, but loaded up at idle until it would die. Then I would have to hold the throttle wide open to get it to start back. Good luck

xjchrisc said:

No cel?

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Yes random cylinder misfire

Do that long enough and you will cook your cat.When my 96 did that I would find the electrical connection loose at the injector. Temp fix was to wire tie the connection together after a good cleaning.
During the motor swap,new connector ends were installed.

IAC most likely, since it controls airflow at idle, and disengages when you are pushing the pedal. Remove throttle body, remove IAC, remove IAC housing, clean everything, scrape the old gasket and replace with black RTV, reassemble

Another thing, since you have a 96, maybe get an OBDII scanner from somewhere and look at your Air/Fuel reading

My scanner won't link to the 96, is there something I'm missing?

I'm troubleshooting about the same thing on my 96 this weekend. It seems to be running way rich at idle. Noticed the floorboard above the CAT was way hot yesterday. About a week ago it loaded up very bad and was blowing soot. A run up the road in low gear seemed to clear that up, best guess was carbon fouled plugs.

It's been acting up for months now, it wants to stall or stutters after about the first minute of driving, then clears up. Seems to make no difference hot or cold. No codes.

I'm leaning towards an IAC issue, but before I check that I'm going to make sure my voltage, charging system and grounds are in order. I've had IAC issues in the past that always turned out to be a voltage issue, something in the charging system, either a battery failing or alternator failing. The IAC seems to be real sensitive to charging issues and seems to often be the first indicator of charging issues.

I've got some sort of suspicion it has something to do with my O2 heater and when it transitions from open loop to closed loop. But that is just a guess.

Seems to idle OK, not good but OK, Just that stutter after about a minute of running and the rich condition at idle.

You may want to pull a plug and check it for carbon fouling, could be causing your miss. Maybe check spark for weakness, first thing I'm going to do after I clean the IAC and check my grounds and charging system.

Iac is brand new and the housing was cleaned when I replaced it.

Wayne Sihler said:

Do that long enough and you will cook your cat.When my 96 did that I would find the electrical connection loose at the injector. Temp fix was to wire tie the connection together after a good cleaning.
During the motor swap,new connector ends were installed.

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It's frowned upon here but the cat is long gone.

Maybe the O2 sensor. Sometimes they are bad out of the box.

I would check for injector leak too.

I've tried 2 upstream. Might end up trying another

xjchrisc said:

I had a similar experience with my 96 xj. Mine ended up being loose wires going to the fuse panel on the passenger fender. Drove fine down the road, but loaded up at idle until it would die. Then I would have to hold the throttle wide open to get it to start back. Good luck

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I'm gonna have to look into that today

Why don't you acquire a scan tool that will talk to your Jeep in "Chrysler" language, and actually diagnose the problem, by looking at whats going on? You can find used Snap-On MT2500's and OTC 4000E's on ebay for less than $200. Those are older scan tools but they support the chrysler specific J2610 protocol (with the right cartridge/key/cable) which is what the DRB2/3 uses. This way, you can see all the data that you would see with the chrysler tool, check sync. actuator tests, etc, etc. This is NOT generic OBD2 which is junk, especially on a 96.

A bad IAC won't cause a misfire, but it will cause stalling and low or high idle issues.

Didn't know something like that existed. Thought they were all the same. I'm a carpenter not a mechanic that's why I'm here asking. Thanks for the info

Several years ago I did just that....E-Bay,Snap -On "Brick" With keys,cartriges,adapters ect. I can check XJ,s up to 2000
Total cost was less than $250.
I even take it to my buddy,s shop,so he doesnt have to hook up his $5000 dollar system to look at my antique.

hexdmy said:

Why don't you acquire a scan tool that will talk to your Jeep in "Chrysler" language, and actually diagnose the problem, by looking at whats going on? You can find used Snap-On MT2500's and OTC 4000E's on ebay for less than $200. Those are older scan tools but they support the chrysler specific J2610 protocol (with the right cartridge/key/cable) which is what the DRB2/3 uses. This way, you can see all the data that you would see with the chrysler tool, check sync. actuator tests, etc, etc. This is NOT generic OBD2 which is junk, especially on a 96.

A bad IAC won't cause a misfire, but it will cause stalling and low or high idle issues.

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had similar issue on my dads 96, its turned out to be the o2 sensors, changes it and fine

hexdmy said:

Why don't you acquire a scan tool that will talk to your Jeep in "Chrysler" language, and actually diagnose the problem, by looking at whats going on? You can find used Snap-On MT2500's and OTC 4000E's on ebay for less than $200. Those are older scan tools but they support the chrysler specific J2610 protocol (with the right cartridge/key/cable) which is what the DRB2/3 uses. This way, you can see all the data that you would see with the chrysler tool, check sync. actuator tests, etc, etc. This is NOT generic OBD2 which is junk, especially on a 96.

A bad IAC won't cause a misfire, but it will cause stalling and low or high idle issues.

Click to expand...

If the IAC is stuck almost shut, it might idle, but is likely to idle rich. If it idles rich long enough you can carbon foul the plugs, which will cause a miss at idle.

Pull a plug and have a look.

Or do a moderate to higher RPM run up the interstate, which will usually clean the plugs up some (temporarily).

Weak spark can cause almost the same thing to happen, the plugs can carbon foul. Weak spark and the ignition usually craps out and starts to break up before red line, often around 3000-3500 RPM or so. Weak spark can have multiple causes. So swapping parts may or may not solve this issue.
An easy test is to put it in 1-2 and drive it at a constant speed at higher RPM's and see what happens.

I'm trying to think of things that might not cause the CEL to light up, but will still cause issues.

Another quick test is to disconnect the battery and reset the computer. If it runs better on defaults, before it relearns and the problems start to reappear, it is likely to be a sensor (or connector or wire) issue. If there is no difference, it is likely to be a component problem, coil, high voltage cable, injector etc.

Swapping out sensors without cleaning the connectors and at least visually inspecting the wiring can be a waste of time and money.

The last bend of the injector harness where it exits the cable channel is a known trouble spot. You can have maybe three strands of copper left connected under the insulation, the broken strands may or may not be making contact.. It not only takes volts to fire an injector, it takes a certain number of amps also.