In a gasoline fuel injected system, what happens if the vacuum line to the pressure regulator becomes disconnected?

• A. Engine shutdown
• B. Rail pressure remains constant
• C. Rail pressure increases to its highest rate
• D. Rail pressure drops

Answer: (B)

The fuel rail pressure regulator uses a diaphragm with a spring on one side and a vacuum reference from the intake manifold on the other. The vacuum signal lets the regulator vary the rail pressure as engine load changes. If that vacuum line becomes disconnected, the regulator loses its reference to manifold vacuum and can no longer modulate pressure with engine demand. The spring then sets a fixed baseline on the diaphragm, so the rail pressure stays at that set value rather than changing with engine conditions. In short, without the vacuum signal, the regulator can’t adjust the pressure, so the rail pressure remains constant at the regulator’s baseline.