Fuel Pressure

• The vacuum hose allows pressure from the intake manifold, or lack of it, to be applied to the diaphragm inside the fuel pressure regulator. When an engine is idling, high vacuum levels exists within the intake manifold. The vacuum forces the diaphragm against a spring, reducing the fuel pressure.
  
  

Acceleration

• When a driver depresses the accelerator pedal, the pressure inside the intake manifold increases, typically by between 5 and 10 lbs. per square inch. This increases the fuel pressure, allowing a proper pressure differential to be maintained within the fuel injection system, and acts as an accelerator pump.
  
  

Problems

• If the vacuum hose is clogged, leaking or incorrectly connected it can lead to incorrect fuel pressure. This in turn can lead to high fuel consumption, poor power and erratic behavior of the engine when idling.