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1991 Ford F150 4.9L Missing

I have this Ford Truck, Less than 2000 miles ago, I had replaced engine with a rebuilt long block. I also replaced distributor, rotor, cap, spark plugs and Belden plug wires. Was running good for approx. 1000 miles, But, engine light would go off and on when I stepped on gas. Check all the vacuum, lines, nothing (note: I had replaced EGR valve and sensor before engine replace) finally found that the EGR vacuum Solenoid was the problem and replaced that. Now, I have a miss, also sometimes stumbles on acceleration. I took back to the original Mech that replace engine, he replaced wires and spark plugs and did the, take the wires off one at a time, could not find any constant miss firing. He also sprayed the intake, to see if any of the gaskets he replaced leaked. He told me to take to local Ford dealer. I did, they had it for 4 hours, checked and cleaned fuel injectors, also check spray pattern, they could not find anything wrong, and gave up. Took to another Mechanic in town, he had for 2 weeks, he thought it was blow back through EGR (I have aftermarket on it). He replaced with Motorcraft parts no luck. Put aftermarket back on, then had exhaust check for back pressure and that was okay, also check computer to make sure no vacuum at idle on EGR and that was ok. He finally gave up. I have tried to call show, but never can get in, so I thought I would ask here. Got any Ideas?

One lesson. With the money you have spent on this clunker, you could have bought a new truck…

Find or make an EGR block-off plate, or just disconnect your EGR valve and see it that cures the acceleration missfire. Check fuel pressure under acceleration…

Started this way before the clunker deals where around. Body is in great shape and so is the interior. Everything works. I did unplug the vacuum to it and that did not fix it. Missfire is also in idle. Ford dealer checked fuel pressure.