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I've been dealing with this on and off problem for 7 months now. This is a 2003 WJ 4.0l. I sometimes get a misfire code for cyl 3, after being cleared a month goes by and it shows up on cyl 1. Every morning when it first starts, it's rough like an old ford thats half dead. I give it a litle rev and it idles fine but never as smooth as a year ago. The injectors have been checked and are OK. Fuel pressure tested OK, plugs are only a year old, evap system is OK and i am ready to look at either the CPS or pump relay. I do sometimes over think a problem so the coil is not totally out of mind but not very likely. Now when the DTS' are cleared, the readyness cycles and remains good for a month or so. Oh, there's 91K on this and I do push it hard sometimes but not beat.

I read a similar post and an almost successful answere was to remove, check or replace the valves becuse they tend to not seat properly at this age. I've has this since 20K, never use 87 octane and always syn-blend or all syn oil. Thanks...
 

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What motor do you have?

I had a similar problem on a customer vehicle and it ended up being a worn through wiring harness on the back of the motor. A few wires would ground out on the engine block causing a misfire code.
 

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Sorry this reply has taken so long... thanks AndyZJ for your suggestion. I did check the harness where it's bunched up behind the coil pack and it was OK. I Tested the MAP and crank sensor... but before spending too much $$ and time I tried the simple and logical route. I replaced the plugs from the iridium I3's, that have such a great reputation and did give me a huge boost, back to stock NKG's and wallah! no more skips and the mpg shot back up to normal from 14. There's still a slightly rough idle but there are over 90k. Not on the another annoying problem... can't keep it aligned!
 
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