You need to check fuel pressure and fuel pressure regulator?
FUEL PRESSURE REGULATOR
The fuel pressure regulator is attached to fuel supply manifold assembly, downstream of fuel injectors. It regulates fuel pressure supplied to injectors. Regulator is a diaphragm-operated valve with one side responding to fuel pressure and the other side to intake manifold vacuum.
When intake manifold vacuum is low, an internal spring increases pressure on diaphragm, blocking off fuel return passage and increasing fuel pressure. When manifold pressure is high, spring pressure is overcome by vacuum, opening fuel return passage and lowering fuel pressure. Excess fuel is by-passed through regulator and returned to fuel tank.
The regulator also controls fuel line vapor formation, allowing for rapid restarts, assistance in engine idle stabilization and maintenance of fuel pressure when engine is turned off. Pressure is adjusted at factory to compensate for fuel flow differences between injectors.
1991 FUEL PRESSURE SPECIFICATIONS (EEC-IV - TRUCKS)
Engine. KOER psi (kg/cm2 ). KOEO psi (kg/cm2 )
4.9L EFI.45-60 (3.2-4.2).50-60 (3.5-4.2)
All Others.30-45 (2.1-3.2).35-45 (2.5-3.2)
Monday, December 15th, 2008 AT 5:36 AM