Fuel Pressure Regulators
When manufacturers first switched from carburetors to fuel injection, most of these engines contained TBI throttle body injection systems. The fuel injectors were located above the throttle plate and unaffected by the intake manifold vacuum.
Today's engines are port fuel injected, and the fuel injector's tips extend into the intake manifold just above the intake valve. A vacuum controlled fuel pressure regulator contains a diaphragm that moves with fluctuations in vacuum pressure.
When fuel pressure at the regulator is high enough, it overcomes the spring's pressure without any vacuum assistance. However, at idle, the engine's vacuum is high and assists the valve's opening. The opening valve lowers the fuel pressure at the rail by allowing more fuel to circulate back to the tank through the return line. A kinked and restricted fuel return line raises pressure at the fuel rail.
When the engine is under acceleration, fuel demand is high, and the intake manifold's vacuum is low. The fuel pressure regulator is closed under these conditions, restricting flow through the return line increasing fuel pressure at the fuel rail, providing more fuel to the injectors.
With a fuel pressure tester attached to the service port on the rail, disconnect the vacuum hose from the regulator. There should be an increase in fuel pressure. Reconnecting the hose should return fuel pressure to specifications.