A little info about 96+ fuel systems

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Here's an article I found that I thought might be interesting to some of you:

Those Vortec equipped Chevy trucks are work horses, as long as you keep ’em well fed. That would be with fuel pressure and plenty of it. The Central Sequential Fuel Injection system used on 1996 and newer trucks helps make driving fun again. The system complies with the stringent emission requirements, starts good, idles smoothly, increases the horsepower, and the torque makes you feel like you were throttling one of the old high performance engines again.

Some of our troubleshooting procedures and test equipment will definitely be rearranged as the vehicles make their way into our repair shops. Troubleshooting steps that were once second on the list may be moved up front in the diagnostic procedure. For example: if I were diagnosing a no-start, hard start, surge or stalling condition with one of the mentioned vehicles, I would be looking at the fuel system pressure first. Low fuel pressure has been a real problem.

Unlike some of the earlier Central Port Systems used on the 4.3L engines, which contained a single throttle body type injector which fed all poppet nozzles, the new system includes an injector for each poppet nozzle, attached by a plastic tube. Each injector is pulsed sequentially. When the injectors are mounted in the fuel meter assembly they are surrounded by fuel with the exception of the top and bottom. When the injector coil energizes, the fuel under pressure is forced through the injector and a plastic tube attached to a poppet valve. The fuel pressure overcomes the spring tension in the poppet valve forcing it from its ball seat, thus spraying fuel for a given cylinder. When the injector de-energizes, the poppet valve spring forces the ball seat closed, cutting off fuel at the poppet nozzle. If the fuel pressure drops below the minimum, the poppet nozzles will not receive adequate pressure to overcome the spring tension, thus no fuel spray. A poppet nozzle which sticks open results in a pressure loss once the engine is shut down. This can promote dieseling, long crank times or plug fouling.


FUEL PRESSURE IS IMPERATIVE

The regulated system pressure calls for 60–66 PSI with the pump running and engine off. If the fuel pressure falls below 60 PSI the system is in trouble. Most of the time the engine will not start, especially when cold. If it does, the system will be lean and DTC codes will be set. A definite hard start or poor performance will occur. We have encountered systems that would start and idle, when hot, at 50 PSI. But idle is all it would do. It was not driveable. If the pressure drops while driving, the engine will usually surge and stall. A restricted fuel line or fuel filter would be a suspect. When performing the fuel pressure test do not allow the system pressure to exceed 75 PSI or damage to the regulator will occur.

When the ignition switch is turned to the on position, the Vehicle Control Module (VCM) energizes the fuel pump relay for 2 seconds, allowing the fuel pump to pressurize the system. If the VCM fails to see ignition reference pulses in 2 seconds the module shuts off the pump relay. As a back-up to a defective fuel pump relay or VCM signal to the relay, the oil pressure switch will activate the fuel pump when the oil pressure reaches 4 PSI. However, a long crank time would be encountered.

FUEL PULSATOR

Many complaints of hard starts or no starts have been registered on 1992-1996 GM trucks. When the technician performs the diagnostics, a low fuel pressure may be encountered. The fuel pump is usually diagnosed as the culprit. Removing the pump will often reveal a split in the pulsator/dampner which will result in a loss of fuel pressure. The mentioned pulsator is the flexible connector positioned between the fuel pump and the metal line in the fuel tank. The split is due to high concentrations of alcohol in the fuel attacking the material. GM has a new pulsator (#25175836) constructed of Viton which is resistant to the alcohol. Instead of the pulsator, some pumps incorporate a rubber hose approximately 1⁄4 inch thick which attaches the pump to the metal line. Examine the hose for deterioration or splits.

WHEN THE PRESSURE DROPS

Likely suspects for a dropping pressure condition would include:
a) Leaking fuel injectors or poppet valves
b) Damaged injector O-rings which seal the injectors in the fuel metering body
c) Leaking pressure regulator valve
d) Damaged or split fuel pulsator/dampner
e) Leaking fuel pump check valve
f) Leaking fuel lines