There were four basic fuel injection designs used in the 4.3L over the years.
The first was a simple TBI unit. Used in the first gen S/T trucks. Easy, simple, reliable. A caveman could do it.
The second was the CPFI - Central Port Fuel Injection. Bridged the gap from late first-gen to early-second gen S/T trucks. This was a *hybrid* multi-port design. It used a single centrally located fuel injector that fed a type of fuel distribution block that channeled the fuel through a plastic tube to the poppet nozzles at each port. Only one injector means it is a simple batch fire injection, with each cylinder receiving fuel all at the same time.
The thrid design is SCFI - Sequential Central Fuel Injection. Used in pre-2002 S/T trucks. This system used a block of six individual injectors located in a block near the top of the upper intake plenum. Each of these injectors fired sequentially, meaning fuel was pulsed individually to each cylinder. The fuel traveled from the centrally located injector block to each cylinder through a plastic tube and poppet nozzel.
The final system is the MPFI - Multi Port Fuel Injection. Used after 2002. This is a true multi-port injecton system. Retained the same intake manifold from the prior SCFI injection system, but uses 6 Multec injectors located directly at the port instead of the centrally located injector block and poppet nozzles.
My opinion:
TBI - Idiotproof. Not much help to you though, since you're looking for a 2nd gen.
CPFI - I hated them. Upper plenum gasket would deteriorate and lead to vacuum leaks. Since you've got the plenum off to replace the gasket, may as well replace the injector too since the failure rate was pretty high as well. I really just hated working on these. Maybe it was just the age. Most were out of warranty and the owner either tried to fix it themselves or took it to an indie shop who sold them a full tune-up right off the bat, without even trying to diagnose the concern, before bringing it to the dealer in a last ditch attempt.
SCFI - These, along with the Vortec 5.7L, kept food on my table for a few years. Poppet nozzles would stick leading to a single cylinder misfire. GM had multiple fixes and bulletins, culminating in a single, definitive, and expensive fix.
The first fix was to just run diluted top engine cleaner through the fuel injector cleaner. If that didn't clear it up, replace the affected injector and nozzle. The customer was also sent home with a bottle of GM fuel injector cleaner (rebottled Techron) in an effort to try and clean up any other nozzles which may not have started sticking yet.
The second fix was the same as the first, except instead of replacing the injector and nozzle, GM made a tool that connected the high pressure nitrogen tank used for EVAP diagnoses to the injector in an attempt to *blow* the nozzle open, eliminating the cost of replacing the injector. If it was successful in freeing the nozzle, a second cleaning was performed. If it wasn't successful, the injector was replaced.
The third, and final, fix was...
MPFI - This cured all the nozzle problems with the SCFI system because it did away with the nozzles. GM makes a kit that will retrofit the newer MPFI system to an older SCFI vehicle. I've done one or two of the retrofits, but I can't comment on longevitity because this was about the time I was exiting the dealership world and moving on with my career.
Really, the SCFI problems were overblown. I did alot of them because I worked driveability at a GMC store and all we had coming through the door were trucks equipped with either the 5.7L or 4.3L SCFI. I have many friends and realitives who've owned both engines and have had no issues. Run good fuel and maybe some Techron or Redline SL-1 through every once in a while and you'll have no issues.
