The Mystery of High Fuel Pressure at Idle in Electronic Fuel Injection System

Electronic fuel injection (EFI) system replaced carburetors back in the mid-1980s as the preferred method of supplying air and fuel to engines. The basic difference is that a carburetor uses intake vacuum and a pressure drop in the venturi, to siphon fuel from the carburetor fuel bowl into the engine. Whereas fuel injection system uses pressure to spray fuel directly into the engine.

However, under light load or at idle, a relatively high vacuum exists in the intake manifold. This means less fuel pressure is needed to spray a given volume of fuel through the injector. Under heavy load, engine vacuum drops to near zero.

Therefore, an engine with an electronic fuel injection system has high fuel pressure at idle because of high manifold vacuum. This high vacuum level causes the system to require a higher fuel pressure to ensure proper fuel delivery to the engine during idle.