I have found a lot of threads about misfires but I don't seem to find any that helps with my issue.

I have a 93 xj 4.0 that has a random misfire only at idle or when letting it roll at idle rpm's I don't feel any hesitation,stumble, or misfire under load or cruising at any speed. Things I have tried is new cap,rotor,plugs, and wires. I have also tested the TPS and cleaned the throttle body and IAC. I replaced the fuel pump because when I did test the rail it was only sitting at 24 psi and wouldn't hold above 20 psi for longer than 30 seconds and now it's running a solid 31 psi at idle and 36psi disconnecting the vacuum line to the regulator which i guess is low? supposed to be 39? Just as a cheap test to rule it out i swapped in a new coil but it didn't change a thing.

The only thing i could think is perhaps dirty injectors or also I had some confusing issues testing the supply to the coil where I have read I should be getting 12v to the coil with the key on but I only read 7 with key on. Was I supposed to be cranking to do the test because I have found mixed opinions on it. If its the power to the coil that could be an issue what is the cure for that all I can dig up is bad computer is that the case? Hopefully someone can shed some light on things to try or a definite this happened to me and this is how I fixed it. Oh the misfire doesn't seem to have any consistency on what cylinder or how often sometimes it will be several within fractions of seconds between each other and then be fine for 20 seconds before having a hiccup.

 

Did you check and clean/tighten all the grounds and battery cables?

 

I was chasing a rough idle for quite a while. It got worse as the engine warmed up. I had replaced everything. I was finally out of ideas and replaced the coil. I had replaced the coil with an Echlin from Napa 4-5 years prior so didn't suspect it. I went back to a spare OEM one and it solved my idle problem. the coil is not in a particularly great place by the engine block. It must get very hot in that location. I keep meaning to do a coil relocation on mine to move it up to the firewall lip.
One other time, I had mistakenly used Autolite plugs. Idled like crap. Switched back to Champion coppers and the rough idle went away. This was on a '99 so YMMV.

 

Have you performed a compression test?
With what?
Is the battery trash?

 

Try NGK ZFR5N plugs.

 

Battery is strong a year old red top and never drop below 14v running. I cleaned the terminals up as well as all the grounds that I could find and tried a new coil. The fuel pump was a precision brand from the auto store but I feel good that it wouldn't be the cause since I'm getting steady 31psi at idle. Running champion plugs already since most everyone swears by them.

I haven't done a compression test yet. A buddy recommended doing a vacuum test so I lit up a few cigarettes to relax and puffed them down the brake booster vacuum hose. I was amazed it actually worked since it sounded silly but smoke was coming out from underneath the throttle body so I ran down today and ordered a new throttle body gasket. I didn'​t see any smoke anywhere else so I will report back if that ended up finally curing it. If not I will run a compression test and report back those findings aswell if you think that might help narrow it down. Thanks for all the replies so far!​​​

 

Case closed! The throttle body gasket was a great replace because it was hard as a rock and chipped off piece by piece. This however did not solve the issue so I moved on to do a compression test and when I pulled all my plugs out the plug on cylinder six was so mashed that I'm surprised it didn't constantly misfire and that it was so random it seemed like it was on multiple cylinders. I did gap them all when putting the new plugs and wires on but I must have dinged it when I was putting it in. The thought never crossed my mind it could be that since I was pretty thorough putting them in. Another car problem caused by a rookie mistake I guess. Good news is I didn't replace anything that didn't need it anyways so atleast I'm not kicking myself much over it.

Thanks for the replies fellas!