My truck is a '97 F150, with the Lightning swap done on it, tuned by JJ @ WMS. Truck was running fine, until about the beginning of this month. Started bucking and shaking slightly and periodically at higher RPMs (3000-3500) until you let off. As it warmed up, it seemed to get worse. Changed the plugs, no change. Started to get worse and worse gradually until two days ago... now I can't even get it over 10 MPH as it shakes and bucks and feels like it hits a wall. Cold or warmed up, it behaves exactly the same. ECT / IAT / TPS / CPS and CSS test good and are functioning normally. IAC is fine (new from Ford not long ago do to "Moosing"). EGR is turned off in the tune. Fuel pressure tests fine at idle and at higher RPMs. Fuel filter is only about 3k old. Tested and interchanged the regulator with another known good one. No vacuum leaks, and vacuum pressure is good. All the readings I can see with the SCT Xcal3 show everything sensor-wise operating correctly.

I try to start it, the truck fires, peaks at 1500 rpms, and immediately dies. Occurs this way with or without throttle cracked open. Unplug the MAF, truck starts, idles smoothly and is perfectly normal with no throttle applied. Give it throttle and it shakes like heck, let off and the engine dies. Restart, let it idle, and plug in the MAF, engine can be throttled smoothly to reach about 3500-4000 in park/neutral. Let off the throttle and it dies. Behavior is the same with the MAF plugged in or unplugged with a load on it (in gear). Swapped it with a buddy's known good one, same MAF, same calibration. Same issues.

I'm starting to think the computer might be fried, even though it happened gradually, because with half the sensors disconnected it should still run on the programmed defaults, which it doesn't seem to be able to do. Re-flashed it with every tune file I have, all do exactly the same thing. With 264K on it, maybe the computer's just had it. At this point, I have no idea where to keep looking. Maybe I'm missing something, any ideas?

 

Don't take this as an insult but your at a point you need to get deeper into diagnostics to find the issue.
All the simple guessing and parts swapping is not going to get at the root cause unless you would get lucky.
A dealer or shop likely won't touch the problem with the motor mods and non stock program and take responsibility for the job.
Get the use of a GOOD Scanner and look at the live data.
Get one that has a 'Trap' feature.
Plug it in, set up the trap, try a run to see what it freezes for data, then run with what it says is the trouble area.
Your into hot rodding so you have to be the one who becomes the expert on your setup.
Good luck.

 

I swapped a few parts that were available to me for the simple purpose of elimination. I've been testing everything I can think of that might cause it with a meter or with my scanner. Took it to a friends shop, set it up on the Snap-On scanner, and came up dry.

 

Was the trap feature used on a test drive?
Good luck.

 

i would try checking the coil packs these motors where noted for blowing spark olugs out and the coil packs working intermeditley and when they do it wont run shakes bad and feels like the motor is gona rip out of the mounts but shut it off and start it again 20 minutes later and it could be ok it should turn in the check engine lite and if you hit it with a scanner it will tell you if its missing and what cylinder it is. and if thats not it they also had problems with valves sticking then when they finally open it fires harder causeing no power and a nasty shake also.