On my 2000 Jeep Cherokee I have a cylinder 1 misfire, I thought it to be an issue with a spark plug and or the coil pack and have replaced all 6 spark plugs and the coil pack yet I still have the misfire. Using a scanner to monitor the misfire's the misfire will randomly go and start with about 1-3 second intervals. Any ideas? I wanna say it's a wire or injector but I'm unsure. This is not a recurring issue my XJ is at 280k miles and this issue just occurred out of nowhere, same time as when my power steering pump began leaking.

 

Should also mention that the misfire only occurs on idle, thus when I rev above 1000 rpm the misfire does not occur but when I let the engine sit under 1000, say 750rpm at idle the engine will be fine but then begin to misfire after a few seconds

 

My opinion? You should be pleased you got 280K out of those crappy Siemans injectors!

 

My guess would be a clogged or heat soaked injector. You can test that the voltage getting to all of the injectors is that same and that the connectors are well connected before replacement. Also, you could try a fuel additive to clean the system.
I replaced mine as a set with remanufactured Bosch 4-holes. I might go with new Standard Motor Parts 1-holes provided that I have to do this again. They aren't much more expensive than the used/reconditioned old ones. It is recommended to replace as a set like spark plugs for what that is worth. Be prepared to replace all of the o-rings provided that you will just replace the one fuel injector. BWD has a box of them.

 

Try swapping the #1 injector to a different cylinder. See if the misfire follows to that cylinder. The "swap and follow" technique is perfect for this problem

 

Just a note. All the injectors are supplied with 12v whenever the engine is running and are signaled to spray by the PCM grounding them thru digital drivers. These signals come in nanosecond pulses so the valve pindle is almost in constant motion.

 

I got 6 new injectors and checked the wiring harness for faulty connections or low voltage and nothing was wrong with the harness. I still have a cylinder 1 misfire and with all new spark plugs, coil pack, and 6 new injectors. The only thing I could think of is it being either a lifter issue or being a different connector.

Update ETC:
I just poured some seafoam directly into the throttlebody and reved the engine over the course of a couple minutes and the misfire on cylinder 1 has completely vanished and is dead straight, however I now have a serious misfire on cylinder 5.

 

How much is some?
I prefer to use water to remove deposits from the valves and cylinders and pistons. Drive for a while so that the temperature is nice and hot. Pour slowly.

For Seafoam you want to slowly pour a bit, then pour enough to kill the engine. Wait about 20 minutes so that the chemicals have time to continue removing deposits.

Perhaps, you have an issue with the seating of the valves. Cleaning them well, then re-gapping them would be a good thing to do. You are welcome to try both above methods for cleaning with the least amount of effort/work involved. Re-gapping will require removing the rocker arm cover and use a feeler gauge.

 

I'm with you on the water deal. That was the only thing that fixed my buddy's 99 XJ.

Not so on the valves. They're hydraulic......