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Discussion Starter · #1 ·
I have a engine light with the misfire cylinder 3 code. I changed out the coil pack, still on. Changed out the injector, still on. New plug. Engine has 20,000 miles on it. What could this be?
 

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Same thing happened on my 98, could be similar situation; there was a TSB a while back regarding inferior valve springs. I had to replace all with newer springs. Something to look into.
 

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try this:

Swap so called bad whatever on Cylinder 3 to another Cylinder. Vice versa, swap said good whatever to cylinder # 3.

Does problem follow the companants or stay on Cylinder #3 regardless.

If the problem stays the same, possible Ignition control module Problem?

Does the injector fire? go get a node light test kit or a cheap low amperage light and plug into the injector plug to test

Fuel? -injector harness not firing that injector? - ECU?
Spark? -coil pack, spark plug, - Ignition control module?
Air? - faulty valve springs. Valve stuck open during compression stroke?
 

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Discussion Starter · #6 ·
There is nothing left to swap, and the coil is all one unit. So, replaced the coil unit, injector, spark plug. See a note on springs, would never have considered that. That will be a pain...
 

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Discussion Starter · #7 ·
Same thing happened on my 98, could be similar situation; there was a TSB a while back regarding inferior valve springs. I had to replace all with newer springs. Something to look into.
And did that fix the problem? Your light would not go off until you did that?
 

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And did that fix the problem? Your light would not go off until you did that?
Yes. Essentially what's happening is the valve spring is weak, and doesn't let the valve close all the way. This creates a misfire condition that won't go away by throwing parts at it. I'm not saying that's definitively what YOUR issue is, only that I was in the same boat as you with identical systems and did all the same things.

I pulled the head, sent it off to the machine shop for a new valve job and valve train (whatever you want to call it). Ran perfect after that but sold it a few months later, trans went out on the diesel and I'm not sure if you've priced one of those lately 😁

Edited to add: Keep in mind mine was a 98', however i'm confident those shit springs sat around in stock for while before they were pulled. Could very well have made it into the heads of later models. Look up Technical Bulletin 18-22-98. I gave up and started calling around to different shops but nobody would listen, kept saying it was electrical so I finally just said screw it and yanked the head. If it's bad enough you might be able to just crank it over with the valve cover off, sometimes you can see the difference easily.
 

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Discussion Starter · #9 ·
Yes. Essentially what's happening is the valve spring is weak, and doesn't let the valve close all the way. This creates a misfire condition that won't go away by throwing parts at it. I'm not saying that's definitively what YOUR issue is, only that I was in the same boat as you with identical systems and did all the same things.

I pulled the head, sent it off to the machine shop for a new valve job and valve train (whatever you want to call it). Ran perfect after that but sold it a few months later, trans went out on the diesel and I'm not sure if you've priced one of those lately 😁

Edited to add: Keep in mind mine was a 98', however i'm confident those shit springs sat around in stock for while before they were pulled. Could very well have made it into the heads of later models. Look up Technical Bulletin 18-22-98. I gave up and started calling around to different shops but nobody would listen, kept saying it was electrical so I finally just said screw it and yanked the head. If it's bad enough you might be able to just crank it over with the valve cover off, sometimes you can see the difference easily.
I have a spring compressor, probably easy enough swapping both springs on 3. Will try it, did the other stuff. Thanks for the tech bulletin number.
 

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Bad coil on a 2000 4.0l engine will kill two cylinders.

A Compression test will show you if you have low compression in that cylinder, a follow up with a leak down tester will show from where.

Flat cams are becoming more common on 4.0l engines due to the reduced zinc in the modern oil. You can pull the valve cover and measure the lift and observe the valve action.

Most bad 4.0l springs are found broken, also easy to see if you pull the valve cover. A broken spring usually will show up with a running compression test, but that's hard to do with the coil rail. A vacuum gauge will show a wide fluctuation.

Hope this helps!
 

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I bought a 72 cj 5 and was told it had 2 bad cylinders and it ended up being two push rods were came off the rockers. I put it on its side and I think he lost oil pressure and popped off. . It was a 4.2 . Easy fix. Do a leak down test, maybe check for flat cam lobe
 
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