Engine sputtering

i have a 94 gmc jimmy with the 4.3L v6 vortec with AOD . just recently one of my friends told me to put seafoam in threw my pcv line. even since then my truck has been acting like it is misfiring but i change the plugs, wires, cap and rotor and the pcv, and the oil. all this helped for bout 5 min then it started to do it again and the exhaust smells really weird and blows out black smoke at high rpm and at idle when i first start it and will turn the grass black after bout 5 min of idleing. when i drive the truck it sputters all the way threw first gear and whenever i make it downshift. and it backfires some what often.. i dont know if it could be timeing or what but it is gettting on my nerves... so any help would be great... thanks

Welcome to AF

Do a search on here for "spider" and/or " nut kit". this sounds like the classic injector spider going bad or its related nut kit. It could be the fuel pressure regulator on the injector too, and that can be replaced seperatley instead of buying the whole assembly, you save a lot of money that way.

Get a fuel pressure guage on it and see whats happening. If it has low fuel pressure, if it bleeds off fast, these are signs of usually a injector/regulator/fuel line(nut kit) problems.

how do i test the fuel pressure with gauge??? this is the only problem i have ever had with the engine

There is a shrader valve on the back of the intake, drivers side, take the cap off(if it has one still) and throw on a guage there.