CEL code P0303 #3 cyl misfire

Hello ... I have a 2005 Expedition limited, 5.4L. I'm having a constant #3 misfire issue, CEL code P0303. Usually happens while going uphill and the power drops way off. Sometimes will clear itself after getting up the hill but will eventually come back. Gas mileage drops way down too.

I've changed out all 8 COP's and all 8 spark plugs within the last 2 months. I've swapped the #3 cylinder COP with another cyl and the problem always stays on the #3 cyl. Swapped #3 spark plug and the #3 fuel injector with another cylinder too hoping to see the problem would move to the different cylinder, but always stays P0303.

I've ohm'ed out the wiring between the PCM computer and the COP and fuel injector and all seems fine (shook the heck out of the wiring bundles to see if there's an intermittant wiring problem but all seemed good).

I've run Seafoam through the engine to see if there's a sticky valve or something. I haven't tested compression yet in that cylinder but that may be next.

Any idea anyone? I can't think of anything left besides COP, plug, and fuel injector that is specific to the #3 cylinder. Maybe a bad PCM?

Thanks for any help!!

Are you getting any kind of knock or anything from that side? Like something from the valvetrain? Any backfiring? I had a problem that was similar to yours. Rough going uphill and if I gunned the engine going down the highway it would backfire. Turned out it was a bad rocker and it had worn a groove in the came lobe. Replaced the cam and all rockers on that side and it was fixed. Hope this isn't your problem!!

What was your fix???? I'm having the exact same issue.


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How's the connector on the #3 cop? Sounds like it's not getting enough voltage/amperage which is why it wouldn't start misfiring till it was under load. The contacts on my '98 expy went bad from coolant dripping on them so I just spliced it in with bullet splices. Worked great.