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Vehicle is 1994 Ford E-150 5.8L V8
Fuel injected, return-type fuel system
So I finally figured out that the misfire under load condition is being caused by air in the fuel lines. There was so much air in there it was causing an "air spring" effect when priming fuel pressure. So I got the schrader valve test port on the fuel rail up as high as possible and bled through there while running until no more bubbles came. Then drove around and it was great! But pretty quickly it started missing again and I stopped to bleed it again there were lots more bubbles.
I couldn't find much online... it seems like air in the fuel line isn't normally a problem on gas engines? Does that mean that its letting in copious amounts of air? I sure never bled the system after changing a fuel filter, which is right there in the pressurized part of the system.
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