A friend and I are working on a 91 F250 4x4 with zf 5speed that we put all the running gear out of an F350 4x4 into. Truck is all together and ready to hit the road, except we can't get the engine to run right.
Just put in new plugs, wires, cap, rotor, new exhaust manifolds (they were cracked), new exhaust system, two new fuel pumps (high pressure pumps in tank on a 91), and battery. Starts and idles perfect, but when load is applied, the engine starts stumbling and misfiring like it is running out of fuel. Fuel pressure remains steady between 45 and 50 lbs. all the time, so it is not a fuel delivery issue. Swapped out the MAP sensor with a spare I had on the shelf and made no difference. Both of us have been working on engines for over 25 years and this one has me stumped. Check engine light isn't on either.
Anyone had this problem with a 4.9 before?

 

HELP What did you find out? Did you solve it?
I have been having the same problem with My '94 E-150 4.9L, 200k miles, for two years. Initially I replaced the usual suspects: plugs, wires, cap, rotor.
I happened to be working at Ford at the the time. A good friend there had worked the technician tough problems help line. He advised that it was ground related - the ignition module would 'reset' if it dropped voltage due to a lack of a ground.So I cleaned several grounds and added a couple.
I have replaced the following based on how others solved their issue:
Distributor, Ignition module, coil.
I have run seafoam, lucus,and other injector cleaners.
I removed the EGR and tested it with a hand vac pump.
I cleaned the terminals on the throttle position sensor.

Also, I set the timing to 10 degrees BTDC. It pinged so I backed off a bit.

Many of these tings seem to help briefly. Or sometimes the thing will drive 10 miles without the harsh misfire. but nothing has made a significant and lasting improvement.

 

OK here's a clue:
The engine does not misfire in Park or Neutral. Nor does in misfire inr Forward if I stall torque it. Unfortunately, I stall torqued it too much and caused a clutch to burn up and weld itself permanently engaged. SO, it drives in neutral, acts like it is stall torquing in park and reverse. Bad news. But it provides this interesting clue: If I drive it in Neutral, it goes as if it is in first gear BUT NO MISFIRE!. If I shift to D, 2 or 1, it misfires... what the heck?.

 

https://www.ford-trucks.com/forums/1400908-stumble.html

I've had had a similar problem with my truck; I have replaced all my vacuum lines, distributor cap and rotor, plugs and wires, IACV and the stumble is still there. At this point I think I will try throwing a TPS and O2 sensor at it once I get the specific ones relevant to my truck....