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 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 is used to compensate for the fluctuations in vacuum pressure.
When fuel pressure at the regulator is high, it overcomes the spring's pressure without any vacuum diaphragm assistance. But while the engine is idling, less fuel pressure is required, the engine's vacuum will assist the valve's opening, allowing more fuel to flow back to the tank through the return line.
When fuel pressure at the regulator is high, it overcomes the spring's pressure without any vacuum diaphragm assistance. At idle, the vacuum assists the valve's opening, allowing more fuel to circulate back to the fuel tank through the return line.
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. The closed regulator increases 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, there should be an increase in pressure when the regulator vacuum line is disconnected.