ASE PS2 Test Preparation Guide: Fuel System

ASE PS2 Test Preparation Guide: Fuel System

● FUEL PUMP — Delivers fuel from the fuel tank to the engine. Fuel injected engines usually have a tank-mounted, high-pressure electric pump. Older carbureted vehicles used an engine-mounted, low-pressure mechanical pump. Most EFI fuel pumps operate at 35 to 85 PSI or higher. Designs vary and include single- or double-vane, roller-vane, turbine or gerotor style pumps. Most pumps are mounted inside the fuel tank and are part of a modular assembly that also includes the fuel level sending unit, pickup tube and filter sock or strainer.

High-pressure electric pumps usually have a one-way check valve to maintain system pressure when the engine is shut off. Leaks in the check valve can contribute to hard starting.

Pump failures may be due to wear, the motor failing or electrical problems in the voltage supply, relay, safety switch or wiring. Running out of fuel can also damage a high-pressure EFI electric pump because the pump relies on fuel for lubrication.

In some instances, a “no fuel” problem may not be a bad fuel pump but something else such as an electrical fault, plugged fuel line or filter. Accurate diagnosis is needed to prevent unnecessary pump replacement and returns. A new pump is required if the original pump is not working or fails to deliver an adequate supply of fuel (low-pressure and/or volume).

Fuel pump failure also can be caused by debris in the fuel tank. If there is evidence of debris or rust in the pump or filter, the fuel tank should be cleaned or replaced as needed to prevent repeat pump failures. Replacement pumps must have the same pressure and flow ratings as the original, though the style of pump can differ.

● FUEL FILTER — Protects the fuel system against dirt and debris. Located in the fuel line, or fuel pump assembly inside the fuel tank. Most late model vehicles have no recommended filter replacement interval (replace as needed). But if the filter becomes plugged, it may cause a loss of power or prevent the engine from running (no fuel). Many in-line filters on EFI vehicles have “spring lock” fittings that require a special tool to release.

● FUEL HOSE — Rubber hose that carries. Must be approved for fuel system use and pressure rated for the application. Fuel hose for fuel injection has a higher pressure rating than that for older, low-pressure carbureted systems. Hose should be replaced if leaking, cracked, hard, brittle or damaged. New clamps also should be used.

● FUEL INJECTORS — Spray nozzles on fuel injected engines that deliver fuel to the engine. All late model engines have some type of Multiport Fuel Injection (MFI) system. With this setup, a separate fuel injector is provided for each cylinder. The injectors are usually mounted in the intake manifold and spray fuel into the intake ports. On some newer engines, such as VW’s “direct gasoline injection” engines, special high-pressure injectors spray directly into the combustion chamber similar to a diesel engine. Some older engines have a “Throttle Body Injection” (TBI) setup that uses only one or two injectors in a throttle body. Another variation is General Motor’s “Central Point Injection” (CPI) system. Here, a centrally-located “Maxi” injector routes fuel to mechanical poppet valve injectors at each cylinder. Injectors on most electronic fuel injection systems are electronic and have a solenoid valve at the top that opens the nozzle. When the solenoid is grounded or energized, the valve opens allowing fuel to spray into the manifold or cylinder head port. Injector duration (on time) is controlled by the PCM using inputs from the oxygen sensor and other engine sensors.

Dirty injectors are a common problem. Injector nozzles can become clogged with fuel varnish over time, causing a loss of engine performance and misfiring. Injectors also can leak fuel, causing an increase in fuel consumption and emissions. An injector failure will result in cylinder misfire and power loss. Dirty injectors can often be cleaned to restore performance.

● FUEL PRESSURE REGULATOR — Controls the pressure within the fuel injection system and routes excess fuel back to the fuel tank. Usually mounted on the fuel rail on the engine, the regulator has a spring loaded diaphragm attached to a source of intake vacuum. As engine load (vacuum) changes, pressure is adjusted up or down as needed to maintain proper fuel delivery. A problem here can result in lower than normal fuel pressure and poor performance. Some newer vehicles with “returnless” EFI systems, the pressure regulator is mounted in the fuel tank with the pump.

● THROTTLE BODY — Controls airflow into the engine on fuel injected engines. Usually has a throttle bypass device for regulating idle speed. Wear around the throttle plate shaft may allow air leaks and require replacing the throttle body. Some throttle bodies have a special coating inside that can be damaged if an incorrect cleaning solvent is used to clean the unit.

● AIRFLOW SENSOR — Used on many fuel injected engines to monitor airflow into the engine. Vane airflow sensors (VAF) use a spring-loaded flap to measure airflow. Mass airflow sensors (MAF) use a heated grid or wire to measure airflow. Some import applications (Toyota and Lexus) use an airflow sensor that measures air turbulence to monitor airflow. Vehicles that have a “speed-density” EFI system do not use an airflow sensor but estimate airflow based on other sensor inputs (throttle position, intake vacuum, engine rpm and air temperature). Airflow sensors can cause a variety of drivability problems if defective, and are expensive to replace. Cleaning a dirty mass airflow sensor with a product approved for this use may restore normal operation.

● AIR FILTER — Protects the engine and fuel system against dirt. The filter is mounted ahead of the throttle body or on top of the carburetor. Flat panel air filters are used on most newer vehicles while round filters are used on older carbureted engines. Filters should be inspected regularly and replaced once a year or as needed. Replacement filters must be the same size as the original. Low restriction aftermarket filters are available for performance applications.


Cooling System

Electrical System

Exhaust Parts

Ignition System

Manual Transmission/Transaxle Parts

Suspension & Steering Parts

Fuel System

Engine Parts

Emission Controls

Driveline Components

Heating, Ventilation & Air Conditioning (HVAC)


Automatic Transmission/Transaxle

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