Fuel Pressure Regulator

Older systems use a diaphragm regulator on the rail with a return line to the tank. Many modern systems have the regulator in the tank or control pressure electronically. Direct injection engines use a pressure control valve on the high-pressure pump. Some regulators are referenced to intake vacuum to keep pressure difference across the injector constant.

A spring-loaded valve opens and returns excess fuel when pressure is too high.

A leaking diaphragm can let fuel into the vacuum line, causing a rich mixture and smoke. Measuring fuel pressure with a gauge is the standard test.