Modern Fuel Delivery Systems
To sell fuel delivery system parts in today’s market requires a high degree of product familiarity. Understanding how modern fuel delivery systems work can improve your sales as well.
Fuel Pump Relays
Producing adequate fuel pressure and volume is the “bottom line” for fuel pump performance. The fuel pump relay is an electric switch that’s commanded on or off by the Powertrain Control Module or PCM when the ignition key is initially turned on for approximately three seconds to pressurize the system, then again once the key goes to start, the PCM activates the fuel pump relay again. Once the engine starts, the PCM get input from the crankshaft sensors it either turns the pump off or keeps it running. Many modern vehicles have an inertia switch to turn off the fuel pump in the event of an accident. Anti-theft devices can deactivate the fuel pump if it detects tampering.
Modern Fuel Pumps, Filters, and Regulators
The pressure required to vaporize the fuel in modern engines is generated by a high-pressure electric fuel pump in the fuel tank. Depending upon the type of fuel injection system, most fuel pumps generate between 15 to 90 psi of fuel pressure. Many fuel pumps are protected from dirt by a fuel strainer attached to the fuel inlet. The fuel injectors are protected by a fuel filter downstream of the fuel pump. It’s important to remember that in-line fuel filters are frequently neglected and should be changed at the first sign of fuel delivery system trouble. In contrast, many modern fuel filters are located inside the fuel tank on the inlet side of the fuel pump and last the life of the pump. The fuel pressure regulator ensures accurate fuel delivery by precisely controlling fuel pump pressure. There are two types of pressure regulators: dual-line, and single-line. Dual-line systems return the excess fuel using a second line. Single-line fuel injection systems contain the fuel pressure regulator into the fuel pump assembly and rely on the ability of the Powertrain Control Module (PCM) to maintain fuel control by changing the amount of time the fuel injector remains open.
Pulse-Modulated Fuel Pumps
Pulse-Modulated fuel delivery systems eliminate the need for a fuel pressure regulator by controlling the fuel pump speed to change fuel pressure. In pulse-modulated systems, the PCM uses a fuel pressure sensor mounted on the fuel line to monitor fuel pressure. Pulse-modulated systems also require a professional-grade scan tool to retrieve diagnostic trouble codes and to test the system by commanding the fuel pump to produce various pulse rates.
Fuel Delivery System Sensors
Sensors are used to obtain the optimum mixture of air and fuel in the combustion chamber. The data they collect allows the PCM to change the mixture to optimize performance in various conditions such as cold starts.
Fuel System Diagnostics
To help mechanics quickly diagnose common fuel delivery system problems, auto manufacturers have built some very sophisticated on-board diagnostic systems into their PCMs. A scan tool is required to diagnose modern fuel delivery problems because, the PCM has the ability to detect a defective sensor by comparing its electrical output with that of at least two other sensors or by detecting open, shorted, and grounded sensor circuits. Last, the PCM constantly monitors fuel delivery system performance by monitoring oxygen sensor voltage. If fuel pump pressure is low or if a vacuum leak develops on the engine, the PCM begins holding the fuel injectors open longer to maintain correct oxygen sensor voltage. When this length of time exceeds 25 percent of normal, most PCMs are programmed to store a “lean” fuel trouble code in their diagnostic memories. Similarly, if the PCM reduces fuel injector on-time to compensate for a rich air/fuel mixture caused by a defective fuel pressure regulator, it will store a “rich” code in its diagnostic memory. With this information in mind, it’s important to remember that modern fuel delivery system performance is more about on-board electronics than it is about the mechanical aspects of the system.