I've owned my truck for a few years now and since day one, the truck has always seemed under-powered. As time progresses, this issue seems to get worse. I have a early 97 obs F250 with the 5.8l/351W. What was originally thought to be a transmission issue, has now changed to a motor issue after a trans inspection. My truck idles ok but, with what feels like a very slight miss. Barely noticeable unless you really focus. Under light throttle, the truck seems sluggish, as if you were pulling a trailer and then gets noticeably worse when more throttle is applied. Under heavy throttle, the motor screams but the truck barely increases speed. This is made worse with any sort of incline. The more throttle that is given, the more the engine feels like it's missing but, very rarely does the check engine light come on.

My first thought is timing/distributor. My timing is currently running at 8 degrees at idle. I've replaced just about every major component under the hood except for the distributor.

Has anyone else had this issue before or have any ideas where to start looking?

 

So what codes are present?
Factory base timing spec is 10deg BTDC with the spout plug removed to disable computer advance but some motors will tolerate 12 or 14 deg.
Is your fan clutch seized?
Are any of the brakes dragging?

 

I have no idea what codes are present. If the check engine light comes on at all, it's only for a few short minutes and then shuts off. I've never been able to put a scanner on it while the light is active. The fan clutch seems fine, brakes are all new and in proper working order, as is everything else I can find. The timing is a little lower than ideal and may be contributing to the overall issue. Like I said, I suspect a tired distributor may be the culprit but, I want to get an idea of what else could be causing this if anything.

 

What makes you think you have to do this while the light is on? The computer will store codes and that is what you want to see because it indicates an ongoing problem, if you can't get any codes or it comes back all clear (codes 111/111) that indicates something is wrong with the PCM because codes should be stored if the CE light comes on.

The stumble you mentioned could be from any part of the ignition system, the PIP module inside the distributor, the TFI module, the coil, or the plug wires if they are routed too close to one another or against a metal engine part. However it could also be caused by a lean condition caused by a leak in the smog(air injection) system upstream of the O2 sensor or an exhaust leak upstream of the O2 sensor.

 

I have a similar issue. Mine started this randomly 3 weeks ago. Note mine has a HO engine from a 89 stang with all the truck accessories and injection system but HO firing order.
When I would get on the throttle, it would be fine until I got to about 2,500 to 3,000 rpm and the exhaust note would get really soft (straight threw exhaust) and loose power and would barely accelerate. The only thing done to it before this was an ignition coil, oil change, and fixed the oil pressure sending unit. This first happened when a guy in a ram was revving at me and passed me and I hammered it and went into 2nd @ 45mph and it just fell on its face... This is new. Ok. Later on that day, same thing going up a hill. Full throttle and fell on its face. If I stayed on it long enough, it would go raaaaaa Ta Ta Ta Ta Ta like im hitting a rev limiter (@3,400 rpm..). I already had two codes for a TPS so I changed it out. Truck ran better. But still did this. I remembered the time my truck wasn't getting any air because I shoved an aftermarket air filter on too far and remembered that exhaust sounded the same now as it did then when I got on it. It would go raaaaAAAAAA!!!!AAaaaaahhhhhhhh(soft like its whispering) and when i let off the has it'll get loud again for a split second and jerk real hard as it shifts. All gears btw. So I thought since it's getting good air flow, maybe it's the fuel. Tried fuel filter. It worked! For a day. Problem came back when I tried to hammer it into traffic. It started doing it ALL THE TIME. half a pedal it would do it.. So I check codes again. Running lean (I can't remember the number but I think 41 or 42). TPS code gone. So I replace fuel pump. Little more performance but nothing. Replaced O2 sensor. Fixed other issues it had but not the one i'm currently trying to fix. Grandpa kept thinking it was the ICM and that it wasn't advancing timing. Well ok. Broke the ICM some how and still same issue with new replacement one. Replaced fuel pressure regulator. Nothing new. But with each part getting replaced, it's getting better and just slowing down less....lol put a MAP sensor on it.. Nothing still.. Idles better than it used to, runs better than it after did, but once you get on it.. It's Doing it a whole lot less now. New plugs, wires and MSD Cap & Button about 2-
3 months ago. Also new IAC valve. Note that when I unplug the IAC, it chugs and idles at 300 and doesn't die. Used to die when I unplugged it but now it doesn't.
Any help is appreciated. Suspecting now Distributor (PIP or something) or ECM. No noticeable vacuum leaks under hood but hissing inside (not booster tho).

 

Your problem sounds like you still have crud in the fuel lines jamming into the filter. Can you install a remote pressure gauge on the fuel rail and witch it while you're accelerating? A lot of times, monitoring the pressure at idle will never reveal anything, since the engine is not really consuming a lot of fuel. But if it's not getting enough fuel pressure and flow under load, your engine will not run well.

I had a similar problem with my van after changing out the fuel filer. That turned out to be a defective filter. Something in it would block flow if I hit a bump just right. The engine stumble would coincide with the bump, so I didn't notice it until it got really bad. This would only happen at highway speeds, where the engine is pulling significant amount of fuel. One time it got so bad that the engine kept losing power even when I had the pedal to the floor. But it recovered after I shut the engine off, and I was fortunate enough to be able to drive home with no further incident. However, I replaced that filter as soon as I could.