In a gasoline fuel injected system, if the vacuum line to the fuel pressure regulator is disconnected, rail pressure will do what?

• A. Increase rail pressure
• B. Rail pressure remains constant
• C. Decrease rail pressure
• D. Rail pressure oscillates

Answer: (B)

The fuel pressure regulator uses the vacuum from the intake manifold to adjust the force on its diaphragm and keep the rail pressure felt by the injectors at the desired level. That vacuum signal lets the regulator bleed off extra fuel when load is light (high vacuum) and reduce bleeding when load is heavy (low vacuum).

If the vacuum line to the regulator is disconnected, there is no manifold vacuum signal for the diaphragm to respond to. The regulator then operates with only the spring force and the rail pressure on the diaphragm, settling at its baseline setting. With no signal to modulate against, the rail pressure stays at that fixed value rather than rising, falling, or oscillating in response to engine conditions. Hence the rail pressure remains constant.