Hey guys,
I need some opinions/advice...
I have an '01 4.0 XJ Classic.
I started getting a p203 code (missing on cylinder 3/ faulty injector circuit) about a year ago, but was running fine for a week, then a week later it started running on 5 cylinders and engine light flashing instead of solid. It would also "redline" or cutoff and rpm gauge bounce at 3500 rpm, which I'm assuming was the computer detecting an issue and protecting the motor from higher rpm's? At first I was able to unplug injector 3, plug back in and then would run on all 6 and engine light go solid for a day or 2. I had good voltage at that harness, was getting a good connection and there was good continuity through that pair of wires. I decided to reset the ECU and see if that fixed my issue. It did for a week and engine light went off and ran normal. After a week, engine light back and a day later flashing and running on 5 again. At that point, the unplug the harness to cylinder 3 and plug back in trick was not working and didn't do anything. I reset the ECU again and engine light went off and ran on all 6 for a day, then back to flashing trouble light and running on 5.
At that point I decided to take to a shop, as I didn't want to just start guessing and throwing parts at it that it may not need. I figured they could diagnose further and and get to the bottom of the issue without having to replace every part concerning that injector circuit... So they gave it back and said they found a fuse out, which a few days earlier I thoroughly checked every fuse not just visually, but for continuity even (I'm an electrician) and they were all good. They also said they reprogrammed the ECU to "normal running conditions", which from my understanding, the ECU programs itself over its first 50 starts and I didn't even know that it was programmable. Whatever they did, it was running on all 6 again and engine light off, so I paid and was satisfied enough.
Fast forward to a week ago and one day engine light comes back on and later the same day, before I could even check codes, she starts running on 5 again, flashing light and all same symptoms. So I do the reset the computer trick again and engine light goes off and runs fine for the past week, so I thought maybe a fluke and it was okay again. Then last night I'm driving my girlfriend to Portland for a convention (I live outside Seattle) and halfway there engine light comes on solid and then 2 mins later flashing and running on 5. I pull over, reset the computer, engine light goes off and I get back on the road. 5 mins down the road, lights back on, flashing and back running on 5. I decided to criss my fingers and just keep going. Found a sweet spot between 70-80 mph and the shaking and vibrating from running on 5 would stop. It also sucks a lot more fuel running on 5 (I'm guessing fuel is just dumping info that cylinder and not firing?) and would run slightly warmer and oil pressure seemed to go up a little.
So now I'm kind of screwed here in Portland for the weekend, auto shops are closed, I don't have the extra $ to start just guessing at replacing parts myself (I have my electrical tools, as well as a basic socket set and some wrenches with me). I disconnected the positive cable last night when we got to the hotel and rested on the starter bolts overnight to fully drain the capacitors and fully reset the computer and when I started it this morning, the reset dis nothing and immediately started on 5 and engine light came on and flashing within 30 seconds.
Please help! I don't know what to do and really don't want to risk driving 4 hrs back with it running like that and risking doing permanent damage to my baby's heart and soul... or the motor. Any help, opinions and advice would greatly appreciated! I have to be back at work Monday morning and am supposed to be driving back tomorrow and I'm sweating it and don't know what the truck to do!
Sorry for the novel of a post, I just wanted to give as much detail as possible in hopes someone could lead me in the right direction.

Someone? Anyone?
I'm assuming the issue wood lie with PCM or the actual injector itself if I'm getting 14.1V to #3 injector harness? And I'm really confused wtf the auto shop did with the "reprogramming" of the computer (if that's really a thing? )there and why it fixed it for a year and if I may need to do that again after resetting it... I'm currently trying to pull the injector so i can just put voltage right to it and make sure its opening and closing, but I'm trying to disconnect the fuel line going to the rail and thinking i need a special tool for that? Almost like it might take a sharkbite/quick connect (plumbing fitting) sort of tool - like a sleeve that slides in there possibly?

Hmmm. The ECU fires each injector by grounding it. 12v is supplied as long as the key is on, and each injector fires when the ECU provides ground.

Have you checked out the ground side of that circuit, from the flaky injector to the PCM?

Could also be a bad injector. Swap it around and see if the problem follows that injector.

Mark,

Thanks for the response and advice!
No, I have not checked the ground side of that injector harness. I had 14.1V to the power wire when running, but yeah, didn't check the other. Any idea of the ohm range and for how long when PCM pulses the ground?
Another question: When i get the old injector out and put 12V to it and it opens and closes, then its good and that's not my issue correct? Or can it still open/ close and be bad?

I don't know what the spec is, but I'd guess just a few ohms. A dozen or less, maybe? Pulse width will be determined by the engine running condition. It's how the ECU adjusts the fuel/air ratio. Need more fuel = wider pulse. You'd need an o-scope to see it.

BTW, the ECU grounds over where your distributor would be on an older Cherokee. Where the battery main ground bolts up to a plate on the engine block. Probably not your problems since it's only one injector, but.....

Second question, yes, I can't see how it could open and close and still be bad, unless it's opening partially, mebbe?

The FSM (for 97 atleast) states resistance across the injector terminals should be 12 ohms +/- 1.2 ohms at 20*C (68*F).