2002 Mustang 3.8 Automatic 198,564 Miles
Current mods: MAC OffRoad H-Pipe, K&N CAI, MSD Wires

I got a code for misfire on cylinder 5 (P0305) a month after rebuilding the upper end of the engine (had a blown head gasket). The car ran extremely smooth and got great gas mileage before the problem occurred. I’ve replaced the following parts: Fuel Filter, Coilpack (Motorcraft), fuel injectors, IAC, Throttle position sensor, camshaft sensor, coil wires, spark plug, all vacuum lines, egr, dpfe, all gaskets, all O2 sensors.
A few months before, the misfire would happen would occur when going uphill very randomly and funny enough it would happen at an exact road and spot that I used to take when going to work. It happened every morning at the same place. Recently, I just replaced the Upstream O2 sensors (got them directly from a Ford dealership) and the misfire only happens during idle when in Drive (Sometimes neutral but hardly ever happens). When coming to a stop, the idle would drop from 680 to 430 rpms then slowly come back up to normal. This happens every 15-40 seconds. My father in law (he’s ICAR certified and used to be a master tech at Chevy and Pontiac) already did a compression test for me and all cylinders have good compression both wet and dry, and same as the fuel compression. I can’t really think of anything else but a bad pcm or either a bad ground somewhere. When I replaced the fuel injectors, the code went away for 300miles, the O2 sensors got around 61miles.

Note: recently got off work really late and the sun was already down (random but hear me out). With the fog lights and headlights turned on, the idle misfire occurred a lot more than usual. And the code eventually came back as a pending code. Drove the next day and it drove extremely smooth and fine in the morning.
Also when testing the coilpack, when I pull the coil wire on cylinder 1 it doesn’t misfire but every other cylinder misfires when I pull their corresponding wires.
Any thoughts?


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Misfire codes can be from either fuel or spark issue. You changed the injectors and 300 miles later issue came back. Check the injector harness for cylinder 5. Make sure the connector is clicking in properly and/or if the wire going back into harness is damaged anywhere. I have had injector and COP connectors not work right on my F150 because the connector didn't fully engage and click (or tabs were broken preventing the click).

This is a 6 coil coil pack, correct? I have had new coil packs (same part) on my Contour not work right when new (aftermarket part). I ended up grabbing a used Motorcraft unit from a V6 F150 truck at the wreckers.

Also check the spark plug wire for cylinder 5. I have had many aftermarket "performance" wires work like crap and short/jump a spark where it shouldn't. With it idling, look and follow it back to coil pack looking for shorts/arching.