Probably had this question ask and answered however if I may, I'm requesting opinions.

Truck:
2007 F150XL 4.2 V6 with just over 420,000 on the odometer. Bought 2nd hand. Unknown prehistory.

Poblem
Developed a bad miss and loss of power by 30% with a PO305 code. Change out all the plugs and was fine. Truck ran perfectly for 5 days. Then when driving 100km/h on my local freeway my engine started missing really bad once more. Again with a PO305 code.
So I changed out the wires. However no real improvement. Next the injectors were replaced. Still has a noticeable miss and a slight decress in power, nothing like it had, but still a miss and power loss. The thing is there are no codes showing at all.

What I've been told so far
I've been given 2 different suggestions of the cause. And before I go further I'm looking for opinions. Don't want to get stuck just tossing cash at a problem.

1. Possable electricl problem at the injectors power connection plug. Not quite getting a perfectly solid connection.
2. Possible problem with the coil. Could cause a low powered spark causing a misfire yet wouldn't give any codes.

I'm thinking it wouldn't be any of the sensors, other electricl or vacume lines because, wouldn't there be more codes showing other then the original PO305?

Any Ideas or opinions ?
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Before you fire off the infamous Random Parts Cannon, let’s get some basic diagnoses done.

You need ample fuel, spark and compression or you end up with a misfire. Evidently, you are getting hard misfires in #5 to trip the CEL.

I doubt it’s a fuel issue or all cylinders would be affected. Maybe the injector. Possibly, but you switched the out, so not so likely now.

In any event, that “30 percent” power loss is drastic. If you didn’t find any abnormalities on the plugs, time to pull a compression test. 420,000 kilos or miles is a great run and most engines are tired by that point.

TL;DR. Before tossing money at the engine, be sure it is healthy, first!

 

 

^^^ BIL 4.2 had similar issue and COILpack, wires and plugs replacement fixed it totally.
his only had 112K at the time though...

 

I had a '90 Ford Escort GT that suffered intermittent shutdown issues. Took it to the dealer several times with no solution. Was on the way there for another go and it really shut down with no re-start. Found a phone I could use (pre-cell phone) and the dealer (only a mile away) came and took it in. Long story short, I had put plug wires on it with the correct grease but for some reason, even though it was right there in the box, I did not install the new COIL WIRE! The old one was corroded! So, fast forward to the fuel-injection 4.2L and you basically are having the same effect. Its either the coil pack or corrosion on wire terminals. Always use the Dielectric Grease when messing around with coil wires/spark leads.

The timing of me making a rare visit here and reading this post is actually scary, because just today I had to drive the truck about 15 miles out in the country to have a shop fix up my chain saw. Every start over the last 18 years on this truck, in all kinds of weather has been instant. Today, when I went to leave the shop and return home, the start was a little delayed. For the first time, I released the key and hit it again, and it started. Then, six hours later, I went to go somewhere else and it did it again. Just for the record, what I'm talking about is slightly extended cranking before the engine starts - something like 3 seconds instead of 2 - but after 18 years, you get used to things, and suddenly its different - trouble is on the way. The original plugs are still there, but the mileage is only 35,000 miles. I'm thinking I have a wire with some corrosion on it, the coil pack going bad, or possibly the fuel pump driver module going out. I guess I'll find out, one way or the other.

 

Even with low miles, you've had the truck since new. Coil pack, wires, and plugs are in order. I would also look at the fuel filter. Have you changed that yet?

 

No, I've never seen a fuel filter on it - I thought it was in the tank on the pump. ***Never mind, I researched it, and I'm pretty sure you're right about it. I'm going to replace it right away. I forgot to mention, I've had many cars since fuel injection came in, and most of them had 65K to 90K when I got something new. In those 35 years, I have never had to replace a fuel filter and never had any injector/intake service either. However, I never had a vehicle for 18 years before this truck. Years versus miles - the years win. Even if you never drove it at all, many things will go bad during that time. I think if my fuel filter is clogged at 35K miles, its a very good chance that I bought some bad gas with a lot of rust/dirt in it. Plus, now I guess I can look forward to putting a fuel pump in it - every particle of crap had to come through the fuel pump to get to that filter.

***Nope, I drove it to the shop Friday morning to get a fuel filter on it, and it drove fine with the normal power/acceleration. The shop turned me away because they were short-staffed for the holiday and also backed-up. On the way home I kept punching it and there is apparently no shortage of fuel to the injectors so I put the whole thing off until more symptoms show.