Fuel Systems and Fuel Rail Construction
17-02 Analyze fuel injectors and fuel pressure regulation.
The fuel rail is used to feed each injector with pressurized fuel from a central location. This component is made in a particular shape to help limit the effect of the pulsations from the fuel pump and noise from the fuel injector (FIGURE 17-18). Along with minimizing the pulses, the fuel rail allows for a common attachment point for the fuel injectors. The fuel rail is made of a fuel-resistant material, which could be plastic, aluminum, or stainless steel. The type of fuel rail that the engine needs depends on which fuel system it has. In an electronic returnless system, the fuel rail has only one pressure input to the fuel rail. Along with only one input, the rail also has a fuel pressure sensor so that the PCM can control the operation of the fuel pump (FIGURE 17-19). The mechanical returnless system uses a fuel rail with only one input, but it does not regulate the fuel pressure electronically based on a pressure sensor but rather regulates with a fuel regulator in the fuel filter (FIGURE 17-20). The return port fuel system uses a pressurized feed line and then a return line attached to the fuel pressure regulator (FIGURE 17-21). This system has the fuel pump operate continuously and maintains fuel pressure by using a vacuum-operated fuel pressure regulator, which allows excess fuel to return to the fuel tank.
FIGURE 17-18 The fuel rail provides the fuel pump with a distribution system to feed each injector with pressurized fuel. Along with feeding the fuel injector, the fuel rail can either direct fuel back toward the fuel tank or use an electronic fuel pressure sensor to maintain the fuel pressure by using the PCM to control fuel pump’s on time.
FIGURE 17-19 An electronic returnless fuel systems uses a fuel pressure sensor to tell the PCM when it needs to increase or decrease the fuel pump’s on time. Always feeding the fuel system the correct amount of fuel minimizes wear and tear on all of the fuel-injection components.
FIGURE 17-20 The mechanical returnless fuel system uses a fuel filter with a mechanical fuel pressure regulator integrated into it. If the fuel pressure overcomes the spring in the regulator, the fuel is then directed back to the fuel tank. When the fuel filter is replaced, the technician must make sure it is of the same quality as the original equipment manufacturer’s (OEM’s) fuel filter.
FIGURE 17-21 The port fuel-injection system consists of a fuel feed line and a return line. This system enables the fuel pump to operate at full power all of the time, and when the system pressure is not needed, the vacuum diaphragm in the fuel regulator opens the valve to allow excess fuel to go back to the fuel tank.
Fuel Pressure Regulator
The fuel pressure regulator is used to regulate the fuel pressure within the fuel system (FIGURE 17-22). There are different types of fuel pressure regulators: the vacuum-controlled type and the mechanically controlled type. The vacuum-controlled type uses a manifold vacuum source to open the valve. When the engine is at idle, there is high vacuum, which causes the valve to open, lowering the fuel pressure within the fuel rail. When the engine is under a load or as it is is accelerated, the vacuum lowers, which increases the fuel pressure in the fuel rail. A mechanically controlled regulator is a calibrated spring that keeps the fuel valve closed until the fuel pressure overcomes the spring pressure (FIGURE 17-23). When the spring pressure has been overcome, it opens a valve and allows fuel to go back to the fuel tank.
FIGURE 17-22 The vacuum-controlled fuel regulator uses the engine’s load to control how much fuel the fuel rail needs to maintain engine operation at the determined rpm.
FIGURE 17-23 The mechanically controlled pressure regulator has a calibrated spring in the regulator that will be overcome by the pressure if it gets too high in the fuel rail. Once the pressure has built up over the spring pressure, the fuel will be exhausted back to the fuel tank.
In an electronically controlled returnless system, the fuel pressure is monitored by a fuel pressure sensor that feeds information to the PCM (FIGURE 17-24). Once the PCM receives this information, it can control the fuel pump module to modulate the fuel pump’s on time. By controlling how long the fuel pump is on, the PCM can control fuel pressure in the fuel rail. As the engine is accelerated, the PCM can keep the fuel pump on longer to increase the volume and the pressure needed for the engine to continue to operate at that level.
FIGURE 17-24 The fuel pressure sensor tells the PCM the amount of pressure available to the fuel injectors when the engine is operating. If the PCM determines that the engine at the current rpm needs more fuel pressure, it can command the fuel pump to stay on longer to increase fuel pressure in the system.
Fuel Injector
The fuel injector is the component that injects the fuel into the combustion chamber or intake port (FIGURE 17-25). It is an electric solenoid that moves a pintle within the injector that allows pressurized fuel to be sprayed out the nozzle (FIGURE 17-26). The operation of the injector is contingent on the PCM receiving the correct inputs from the engine’s position sensors, and those inputs allow the PCM to time the event to when it can be most efficiently used. This is a 12 V component on conventional fuel-injection systems, where the injector is provided key-on voltage of 12 V and the PCM controls the ground portion of the circuit (FIGURE 17-27). The PCM controls the ground side of the circuit because the ground side doesn’t create as much of an electric spark as the positive side when it is controlled and if there is a short to ground it will most likely cause a voltage spike to the PCM unlike controlling the positive side of the injector. Another benefit to controlling the ground side of the injector is that if there is a short to ground, the PCM will not be spiked by excess voltage.
FIGURE 17-25 The fuel injector disperses fuel into the intake tract to allow it to enter the combustion chamber to support the combustion event.
FIGURE 17-26 The 12-volt fuel injector is a basic solenoid that uses an electromagnetic field to move the pintle, which is used as a valve to control fuel flow.
FIGURE 17-27 The fuel injectors are controlled on the ground side by the PCM so that the PCM can determine the proper time to actuate each one to coincide with the combustion cycle.
The orifice that is exposed when the pintle is moved sprays the pressurized fuel into a fine mist. This atomization of the fuel allows it to mix thoroughly with the air in the combustion chamber and enhances the combustion process. The location of the port fuel injection is so close to the combustion chamber that the heat of the process carbonizes the tip of the fuel injector. This carbonization can cause the injector to clog, which can reduce engine performance. To help combat those issues, the injectors are designed with wider seats for the pintles or at different angles to minimize the effect of the heat-fuel mixture on the tip.
- When testing conventional fuel injectors, pull the wiring diagram up on the service information to determine which wire is power and which is control.
- Gain access to the fuel injector connector. This may require disassembling some of the upper intake plenum or, if it is equipped, a main harness connector for the entire fuel rail.
- Using a multimeter, set it to voltage, turn the key on, set the meter to voltage, and probe the power wire to the injector to determine whether battery voltage is present.
- Once the power has been determined to be correct, check the pulse of the ground circuit. This can be done by using a noid light or a conventional test light.
- After determining that the power and ground side of the circuit are good, perform an ohm test on the injector to determine whether it is within specifications. Using an ohmmeter, probe the two terminals on the injector to get a reading.
- Compare the reading with the service information. If the specification is not available, compare it with the other injectors on the vehicle. If one injector is out of specification, it can affect the power side on all of the injectors.
