I’m having a weird misfire issue I can’t diagnose. I’m going to detail everything I’ve had to deal with this year in case it might be related, so I apologize in advance for boring everyone. This is a 2005 SCab 4x4 XLT with the 4.6L 2V.

This summer the #5 plug went out. No big deal, I installed 8 new Motocraft plugs and it seemed to fix everything. While I was working on the plugs, I noticed an idler pulley was starting to squeak, so I went ahead or swapped both idler pulleys and put on a new belt the same day.

Everything was great for 2 weeks, then it spit out the #5 plug. Did a little research and settled on the CalVan kit and did all 8 cylinders since the kit came with inserts for 8.

Everything seemed to be fine for about a month. Then I got a 0355 and a misfire on #5 again. Swapped coils with #6 and the misfire followed the coil. Bought a new coil and everything was good again.

Then the fan was on non-stop every time the engine was running. I spent a few weeks checking out every part of the cooling system, to include the trans oil and trans oil cooler, and installed a new thermostat. Eventually my Dad was over helping me look at it and figured out the fan clutch was locked up. I guess a bearing went out and instead of spinning freely it locked up and was running 1:1 all the time. I just assumed since the fan was running then the clutch was good. Never seen one lock completely up like that.

Now we’re at last week. Got a P0305 again. Swapped the plug, which looked fine honestly, and got a P0355 again. I was pulling codes from the ScanGauge, but my Dad has a better quality scanner. We hooked his up and saw pending codes P0171, P0174, P0305, P0302, and P0300 in addition to the P0305. I swapped the coils with #6, the code followed the swap again, so I put another new coil on.

Here’s where things get weird. When I start the engine, there’s no codes and no misfires. I can accelerate however I want, including putting the pedal to the floor, with no issues until I get in the range of 60-65 mph. At that point, the ECU pulls the timing back, the check engine light starts flashing, it throws the above codes as pending and stores the P0305, and the engine starts missing on #5. If you kill the engine and start it back up, the misfire is gone again until you get to 60-65 mph and then it comes back. I had some time to mess around verifying this weirdness and it eventually fried the #5 coil again. Once the P0305 code is stored, it has to be cleared as usual. I’m hoping this means it isn’t an ECU problem.

There was a death in the extended family last Monday so instead of getting to spend time with all my family, I planned to just mess around out in the boonies when I wasn’t doing funeral planning and just being in mourning. Instead, I messed with this. I have newer truck to drive, but I honestly just like this old beater better.

I plan to check compression, but the compression tester my Dad has isn’t long enough for the deep plug wells on this engine. I’m going to get a different compression tester this week. I also plan to get a new connector for the coil on #5. I don’t honestly think this is going to fix the problem, but I feel like it’s better to eliminate the common problems before I go looking for less common things. Sort of a “when you hear hoof beats, think horses not zebras” situation.

Does anyone smarter than me have any ideas? Thanks in advance!

 

I’ll bump this once to see if anybody who hasn’t seen it yet has any ideas. Haven’t been able to work on it yet due to work. Maybe next weekend.

 

Just in case someone else has a similar problem, I fixed the problem. Installed a new fuel injector on cylinder 5, reset the computer, and the misfire is finally gone.

 

I'd carefully check the chassis wiring harness COP connector for a broken locking tab or bad wire leading to it. I had a similar issue that turned out to be this. NAPA has nice single connector replacements for about $20 eac. IIRC the part mumber EC-259 verify that with them though.

Hope this helps!