back story, I picked up a 2000 f-250 ccsb last fall for 1300 bucks with 258k on the clock, knowing that it will be just used to haul my 10k fifthwheel a few times a year and haul my lifted explorer as needed. fast forward to last weekend. Truck was pulling awesome minus wanting to stay locked out in OD so I just left it in tow haul. I was stopped by a traffic light then proceeded to accelerate up to the speed limit running up to about 4k rpm's in 1st and 2nd gear. shortly after shifting into 3rd gear I developed a miss (about 5 minutes before this I commented to my wife about how well it was running) I pulled over to make sure I didn't have a plug spit out which i did not. i then proceeded to finish the haul home for the next 25 miles with it running a cylinder down. After getting home and much to my surprise I have no CEL lights for codes I had a friend scan it and his Snap on anner failed to detect any misfires. the engine almost never misfires at idle just under load. So yesterday I had it scanned by another friend and the live data showed fuel trims running lean on bank 1 and running a bit fat on bank 2. pulled the first 3 plugs on bank one for inspection and plug one was finger tight no need for a ratchet to break it free. all of the 2pulled looked white and crusty from running lean. so then I pulled #5 on the driver side and this too was white and crusty, not as crusty as the other 3 but still was. it has autolight double platinum plugs in it. I swapped out the first 3 COP's on bank 1 with spare's that I had from the 5.4 V-8 doner truck I bought with a bad oil pump so rule out COPS for them 3. but still have to chase this misfire down. Could a plugged fuel filter perhaps be the cause? Not enough fuel going in and spark being blown out from the vacuum when under load? looking for suggestions. also used ether to attempt to find any vacuum leaks but no prevail.
Thanks

 

I'd check the fuel pressure, especially under load.

Even though you didn't find any vacuum leaks, did you check the PCV elbow that's known to crack?

 

I found no obvious cracks to the PCV on the passenger side hose that leads to the center of the intake. pulled it all off and looked at it good/wiped all the grime off. it is a bit loose (stretched) on the connector hose to the PCV though.

 

I also disconnected the battery to clear the ECU memory to see if I can get a new baseline for it to learn from and perhaps throw a code.

 

Sounds like fuel pressure being a bit low or containing the spark when under load would be the first things to check out.
Get a fuel pressure gauge (Harbor Freight 62623 $20) and find out what it's doing.
If it's been over 50k since the last plugs/boots change you might consider do that - Motorcraft plugs SP-479 and Denso boots/springs 671-0001 work fine - under $60 total.
Since one of your plugs was finger tight I'm guessing they were not torqued properly or they have been in there a long time, torque them to 20-25 ft-lbs.
You can have a lot of "pending" misfires before a code gets set.
A good OBDII bluetooth adapter and software like Forscan can help identify which cylinder/s are misfiring from the "pending" data.
Has the MAF been cleaned lately?
I had a bank 1 lean issue with never a bank 2 indication which to me means something is happening on bank 1 after the cylinders since a problem before the cylinders such as intake, throttle body, air intake would usually affect both banks. Turned out I had loose bolts on the exhaust pipe to manifold connection as the code stopped getting set after tightening the bolts. Try swapping the O2 sensors and see if the lean code changes to bank 2.

 

I went to hook my Snap-on fuel pressure gauge up but seems I lost that adapter for the line so now I have to order a ford one. unknown when the last plug change was as the previous owner passed away 3 years ago and it pretty much was minimally used for the past 2 by the family of the owner. maf was just cleaned while troubleshooting it. we scanned it with a Snap-on Modis but I never knew about Mode 6 till just about 30 minutes ago. I will have to drive it for a bit for the ECU to relearn and also put some fuel in it since its less than 15% capacity which will stop mode 6 from what I just read.
Hopefully I will know more in a day or two.

 

I understand the PCM relearn part, but stopping Mode 6 with less than 15% fuel in the tank is a new one ???
I wonder what the reasoning behind that is.
Your misfire may be just a boot that is degraded.
I had lots of misfires on #6 & #7 on our Exped from small amounts of water dripping off the firewall heat shield.
The springs inside the boots were rusty.
Also, had a coil crack at the base of the connector that took awhile to locate.