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Hi everyone. I've been chasing down a P303 misfire code for over a year now. Initially the misfire was legitimate. Cylinder 3 was stuck open and pouring gas through. Entire top of the engine was rebuilt. The rest of the engine was examined thoroughly and is functioning fine.
The code keeps popping. Multiple tests have run. Compression is fine, rundown is fine, all the cylinders are working. All the wiring and pressures are on spec. My mechanic (who has 35 years experience including 15 with Chrysler) has never seen anything like this. Wiring was tested with 12V and all working fine.
We moved the injector, and the misfire chased the injector that was moved so we replaced it with a manufacturer one. I know the Jeeps can be picky. It is still throwing the code, now on cylinder 6 where the fuel injector was moved. About the only thing that hasn't been replaced is the oxygen sensors. I see several posts/videos of people chasing down similar code problems. Any ideas what might be causing it?
The new injectors do look different than the old ones. The folks at Chrysler didn't really have an answer for that other than "that's what goes in them now". I thought maybe this new type of injector they are using might run more lean than the old ones. Or maybe the oxygen sensor is malfunctioning from when the initial problem occurred. Any thoughts are greatly appreciated.
 

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P03xx misfire codes are produced when the crankshaft position sensor sees a slowdown on the crank when it's that cylinder's turn to drive it. O2 sensors, etc. have nothing to do with it, it is purely a measure of a mechanical process.

The most common causes of a single cylinder misfire will be spark, fuel injection, or in extreme cases your valves. If you moved your injector and it followed, the injector is the problem. Especially since you moved banks from 3 to 6, that eliminates the less likely things it could have been. A new injector ought to be plug and play, I'd be almost certain you replaced it with the wrong injector or something wasn't hooked up right. Take an old injector to the parts store and compare before you leave.
 
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