What does it mean?
Well-known on diesel engines, high-pressure fuel injection systems have made their way to gasoline engines, specifically those equipped with direct injection systems. Direct injection, unlike port injection, injects fuel directly into the cylinder and, depending on driver demand, may inject fuel during the intake stroke or near the end of the compression stroke, or even during the power stroke.
To make such injection possible, particularly when injecting at the end of compression or during the power stroke, high pressure is required, and the typical high-pressure fuel injection system usually runs from 200 psi to over 3,000 psi, whereas common port fuel-injection systems usually run between just 10 psi and 65 psi. Direct fuel injection systems may or may not be paired with port fuel injection systems, but one thing they all share is the presence of two fuel pumps.
Usually located in the fuel tank, an electric low-pressure fuel pump sends fuel to the high-pressure fuel pump, which is mounted on the engine. The high-pressure fuel pump is mechanically-driven, usually by a special camshaft lobe, boosting fuel pressure to over 200 psi, depending on driver demand and ECM command of the pressure-regulating spill control valve.
The engine control module (ECM), in addition to controlling fuel injection, monitors fuel pressure in the system, which it uses to fine-tune injector pulse-width. If the ECM detects that fuel pressure isn’t rising sufficiently on command, it will set a diagnostic trouble code, DTC P0087, “Fuel Rail/System Pressure – Too Low,” and illuminate the malfunction indicator lamp (MIL). Concurrent DTCs may include cylinder misfire, air-fuel ratio sensor, or fuel trim codes.