ASE P2 Test Preparation Guide: Fuel System

ASE P2 Test Preparation Guide: Fuel System

WHAT YOU NEED TO KNOW: • Identify major fuel system components • Identify fuel system component functions • Identify related items • Provide basic use, installation and warranty information

* Fuel injectors — Spray fuel into the intake manifold or intake ports. Older engines use one or two injectors in a centrally-located throttle body (“Throttle Body Injection” or TBI). Most late model engines have one injector for each cylinder (“Multiport Fuel Injection” or MFI). On some newer engines with “Direct Fuel Injection” or DFI, the fuel is sprayed directly into the combustion chamber under high pressure. The injectors on most newer engines are also triggered individually just as the intake valve opens (called “Sequential Fuel Injection” or SFI). On older vehicles, they were usually triggered simultaneously.

An electronic fuel injector sprays fuel through its nozzle when the solenoid in the top of the injector is energized. The solenoid lifts the valve open, allowing fuel pressure to push fuel through the injector and out the nozzle. Some older engines and import applications use mechanical injectors and a fuel distributor. Another variation is GM’s “Central Port Injection” where a single “MAXI” electronic injector routes fuel to individual mechanical injectors for each cylinder.

Fuel injector nozzles can become clogged with fuel varnish over time, causing a loss of engine performance and misfiring. Cleaning the injectors with injection cleaner will often restore normal operation. If an injector fails to respond to cleaning, it must be replaced with a new or remanufactured injector. Injectors can also leak fuel, causing an increase in fuel consumption. Shorts, opens or connector problems in the injector can prevent it from working, causing a misfire in that cylinder.

* Fuel pump — Creates pressure to push or pull fuel from the fuel tank to the engine. On older carbureted engines, a low pressure (4 to 8 psi) engine-mounted mechanical pump was used. On newer fuel injected engines, a high pressure (35 to 90 psi) electric pump is used, which is usually mounted inside the fuel tank. If the fuel pump fails, it will prevent the engine from running (no fuel). Replacement pumps must have the same pressure rating and flow characteristics as the original, as operating pressure is critical with most fuel injection systems.

 Also, when the pump is replaced, a new filter sock or strainer should be installed in the tank to protect the pump.
* Fuel pressure regulator — Controls fuel pressure to the fuel injectors. On most engines, the regulator is mounted on the engine fuel rail that supplies the injectors. The regulator has a vacuum hose connection to the intake manifold to sense engine load, and a return fuel line to route excess fuel back to the fuel tank. Inside, is a spring-loaded diaphragm connected to a bypass valve. If the diaphragm leaks or the valve bypass fails to open, fuel pressure may be too high. If the bypass valve leaks or fails to close, fuel pressure will be too low. On “Returnless” EFI systems, the regulator is located with the fuel pump inside the fuel tank.

* Fuel filter — Protects the fuel system from dirt and contaminants. The filter is usually located in the fuel line between the fuel tank and engine. The filter can be replaced periodically (once a year) for preventive maintenance. If plugged, it can cause a loss of power or prevent the engine from running (no fuel). Some in-line filters on newer vehicles have special “spring lock” fittings that require a special tool to release.

* Throttle body — Mounted on the intake manifold, the throttle admits air to the engine. An “Idle Speed Control” (ISC) solenoid or motor allows some air to bypass the throttle plates to vary idle speed. The ISC system is controlled by the PCM.

* Throttle position sensor (TPS) — A sensor mounted on the throttle shaft that monitors the throttle opening for the PCM. The PCM uses this information to determine engine load. On engines with “Throttle-By-Wire,” the TPS sensor also helps the PCM tell if the throttle position motor is opening the throttle the commanded amount.

* Airflow sensor — A sensor used on many fuel injected engines to measure airflow into the engine so the PCM can regulate the air/fuel mixture. The sensor is located between the throttle body and air filter housing. Most late model engines have a heated filament or hot wire Mass Airflow Sensor (MAF). Older engines often used a flap-style Vane Airflow Sensor (VAF). Some Japanese applications use a “Vortex” airflow sensor. A common problem with MAF sensors is that the sensor element becomes contaminated with fuel varnish, oil or dirt deposits. This causes the sensor to under-report airflow, resulting in a lean fuel mixture and lean fault codes. Cleaning the MAF sensor with electronics cleaner often solves this problem.

* Fuel hose — Rubber hose that carries fuel from the fuel tank to the engine (and back again). Hose designed for fuel injected engines has a much higher pressure rating than fuel hose designed for older carbureted engines. Hose deteriorate with age, and leaks can be extremely dangerous because of the potential fire hazard. New clamps should be used when replacing hoses.
* Fuel additives — Such as fuel injection cleaner, octane boosters, and moisture absorbing additives to prevent gas line freeze during cold weather. Regular use of fuel system cleaner can keep injectors clean.
* Fuel pressure gauge — For testing fuel pressure.

Caution: Electronic fuel injection systems are under pressure even when the key is off. Pressure can be relieved by applying vacuum to the fuel pressure regulator. Open hose connections carefully and use a rag to catch fuel spray.

A common cause of fuel pump failure is dirt or rust inside the fuel tank. The inside of the tank should be inspected, and the tank cleaned if dirty, or replaced if rusty.

Fuel pumps are often misdiagnosed. The pump may not be working because of electrical issues (blown fuse, bad relay, poor wiring connections, or an anti-theft system issue) rather than a mechanical failure. Or it may not be delivering normal pressure or flow because of a bad fuel pressure regulator, clogged fuel filter or damaged fuel line. A new fuel filter should always be installed when a fuel pump is replaced.

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