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. This is not true for today's port fuel injected engines whose injector’s tips extend into the intake manifold just above the intake valve. To compensate for the fluctuations in vacuum pressure, a vacuum controlled fuel pressure regulator is used.
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 vacuum will assist the valve's opening, allowing more fuel to flow back to the tank through the return line.
When the engine is under acceleration fuel demand is high, vacuum is low and the regulator is closed; not providing fuel flow through the return line. This 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 its vacuum line is removed. This is because it is now mimicking a low intake manifold vacuum condition, like sudden acceleration.