I have 1994 4.9L (90,000 miles) that idles OK but misses when accelerating or under load while driving. It has never stalled. I have done the simple things like checked for spark, replaced plug, wires and cap/rotor. Cleaned throttle body. Changed fuel filter and PCV. Fuel pressures sits at 45psi. The only diagnostic trouble code recorded is 556 - Fuel Pump Primary Circuit Failure. Any suggestions additional troubleshooting?

 

Is that 45psi at idle or when driving? You can have excellent pressure at idle but it is not accurate of what may be going on under load....

 

At idle. Increased RPM, stayed on 45, dropped to 42 when released. I'll check pressure under load and let you know.