Engine has been misfiring, should I replace parts bit by bit, or look at a system overhaul?

[MyExplorer03v8Lim]

OK.

This explorer is like an abusive girlfriend with redeeming qualities.

I met her at 153k, and took her home for $3500. Limited trim, 4.6L V8 sounded good. Engine seized at 155k and has been replaced with a junkyard mountaineer motor with ~75k.

There have also been suspension and exhaust parts replaced, as well as tires.

It had been running pretty rough and finally threw a CEL P0308, and a couple others that I don't have at the moment. The mechanic who did the engine swap ended up replacing the coil on #8 and the plugs on #8 and #5 even though he said it had all new plugs from the swap. I guess the plugs got pretty beat up from driving it for a while on 7 out of 8 cylinders. whoops. To my credit, another mechanic also drove it and didn't identify the misfire either. I'm not sure what that says about myself or the other mechanic.

So today the CEL started flashing and staying steady intermittently on my way to autozone to read the code. it read:

• Confirmed: P0303: Cylinder #3 Misfire
  Read more at: https://www.obd-codes.com/p0303
• Pending: P0203: Cylinder 3 Injector Circuit Malfunction
  Read more at: https://www.obd-codes.com/p0203
  
• Pending: P0172: System Too Rich (Bank 1)
  Read more at: https://www.obd-codes.com/p0172

My first thought is to replace all of the coils, but the guy behind the counter was trying to talk me into buying fuel injector cleaner. I truly have no idea what's going on with this vehicle. I wonder if there's a bad ground from the engine swap. I wonder if the coils are just old and it's time to do them all.

And for that matter is it also time to overhaul the fuel injection system? Has anyone been here before? Howd you handle the misfiring issues? I'm really worried that I'll replace another coil and it'll drive fine for a while, and then another cylinder will start misfiring, and I'll never know what the real issue is.

 

[Cashcrazy]

I have the 4.0 so I can't help with specifics, but the procedure to test the coils with a multimeter should be floating around online. I would test the coils before replacing. Running a can of fuel injector cleaner with a tank of gas won't hurt anything and it's cheap.

I like to go to the junkyard on a sale day to pull parts for testing. I think I paid $7 for a Motorcraft coil that I used for testing.

 

[drdoom]

You have the codes pointing you to bad wiring or connection correct? I assume you have checked the #3 connection, the snap can break on them sometimes. I have a Taurus here that has a couple of those injector connections rigged with cable ties. Anyway if I was you I would fix the codes first, and of course always stay on top of stuff like plugs, fuel filter, MAF, throttle body, PCV, etc. YMMV

 

[lincolnshibuya]

the obvious failure I see is the #3 injector wiring. Swapping it with the adjacent injector will tell you if it needs replacement. If it stays the same then it could be a wiring issue.

don't drive it too long with the current misfires, a new cat is expensive to replace.

 

[SyberTiger]

You have the codes pointing you to bad wiring or connection correct? I assume you have checked the #3 connection, the snap can break on them sometimes. I have a Taurus here that has a couple of those injector connections rigged with cable ties. Anyway if I was you I would fix the codes first, and of course always stay on top of stuff like plugs, fuel filter, MAF, throttle body, PCV, etc. YMMV

Yup, I had recently removed all my injectors and noticed that on three of them the connector tabs had broken off even though I was very, very careful when I pushed the release tabs and pulled the connectors. For all I know, they just broke off before I even touched them. Anyway, I'm going to replace all my injector connectors with THESE using water tight heat-shrink butt splicers. None of my COP connectors have that problem but they are not identical connectors.