7.4 Vortec idles great but hard misfire while downshifting

[bighoss]

Any help is appreciated! Trying to diagnose what the heck is wrong! Refrence: My Dad owns a ‘98 2500 with the 7.4/454. We just did a tune-up (new cap, rotor, plugs, wires, spark plugs, and fuel pump) after replacing the ignition switch since it went out. Afterwords, the pickup is hard starting but idles and goes down the road great. Here’s the big issue: when you add more throttle and try to get a downshift, there’s a very hard misfire likely in multiple cylinders. What in the world could be wrong?

 

[Schurkey]

What is the fuel pressure at prime, and under load? How old is the fuel filter?

How old are the O2 sensors? Air filter? PCV valve/PCV system inspected? EGR system inspected?

Connect a scan tool, verify EVERY sensor and computer output. Check fuel trims, and the misfire counts for each individual cylinder.

454s are known for faulty fuel injectors.

Vortecs are known for faulty distributors including the gear at the bottom and the bushings. The distributor caps are infamous for failure.

Are the plug wires routed EXACTLY like GM intended, or are they bunched together because the looms broke, and now they're either held together with tyraps, or scattered all over at random? Maybe even burning on the exhaust?

Will the coil reliably fire a spark-tester calibrated for HEI when the coil is fully-warm from being driven for twenty minutes, and misted with water from a squirt-bottle?