ASE P2 Test Prep: Fuel System

ASE P2 Test Prep: Fuel System

Information to help parts professonals pass the ASE P2 test.

The fuel pump delivers fuel from the fuel tank to the engine. There are two basic types: mechanical and electrical. Mechanical pumps are only used on older vehicles with carburetors. Electric pumps are used on all newer vehicles with electronic fuel injection and some older vehicles with carburetors.

Mechanical pumps are low-pressure pumps (typically four to eight psi) mounted on the engine and driven by the camshaft with a lever or rod. Inside the pump is a rubber diaphragm and pair of one-way check valves. The diaphragm moves up and down to pump fuel. Fuel leakage or failure of the diaphragm or check valves requires pump replacement.

Electric pumps may be high or low pressure depending on the application. Most are mounted inside the fuel tank, but some are mounted outside the fuel tank. Some vehicles even have two pumps, one inside the tank and one outside.

Older carbureted engines with electric pumps typically use a vane in-tank, bellows style or solid-state oscillating pump. A spring-loaded pressure switch inside the pump turns it on and off to maintain a steady supply of fuel to the carburetor. The pump’s voltage supply is usually routed through a relay that may be connected to the oil pressure sending unit or a safety switch that shuts off the flow of fuel in an accident. A pump failure requires pump replacement. An OEM low-pressure electric pump can often be replaced with a "universal" electric pump. The pressure rating of the replacement pump should be the same range as the OE pump.

On engines with electronic fuel injection, a high-pressure (35 to 85 psi) pump is usually used. Designs vary and include single or double-vane, roller vane, turbine or gerotor style pumps. Most of these 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 quitting or electrical problems in the voltage supply, relay, safety switch or wiring. In many instances, a "no fuel" problem is not the pump but a failed pump relay or wiring problem. 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 can also be caused by debris in the fuel tank. This applies to mechanical pumps as well as electric pumps. 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. A new inlet strainer should also be installed when replacing a tank-mounted pump, along with a new fuel filter.

The fuel filter protects the fuel system against dirt and debris. Located in the fuel line or at the carburetor, the filter should 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 EFI vehicles have "spring lock" fittings that require a special tool to release.

The fuel hose is made of rubber and carries fuel. It 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 low-pressure carbureted systems. It should be replaced if the hose is leaking, cracked, hard, brittle or damaged. New clamps should also be used. It is usually sold by the foot from large rolls.

Fuel injectors are spray nozzles on fuel-injected engines that deliver fuel to the engine. Older Throttle body injection (TBI) systems have only one or two centrally mounted injectors. Multiport (MFI) injection systems have a separate injector for each cylinder. General Motors Central Port Injection system uses both, an electronic main injector that routes fuel to mechanical injectors for 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 can also leak fuel, causing an increase in fuel consumption and emissions. An injector failure will result in a dead cylinder and power loss. Dirty injectors can often be cleaned to restore performance. Fuel cleaning additives help, but the best results are obtained with on-car flushing or off-car cleaning. Leaky or defective injectors must be replaced.

New injectors are very expensive, so aftermarket remanufactured injectors may provide a more affordable alternative. Injector flow rates vary by application, so replacement injectors must have the same calibration as the original.

The 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 (Chrysler) have "returnless" systems with the regulator mounted in the fuel tank with the pump.

The throttle body controls airflow into the engine on fuel-injected engines. It 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.

The airflow sensor is 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 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 driveability problems if defective, and they are expensive to replace.

The carburetor supplies air and fuel to the engine. Carburetors use intake vacuum to siphon gasoline from the fuel bowl into the venturi (throat) of the carburetor. A needle valve and float inside the fuel bowl controls the fuel level inside the carburetor (important for proper fuel metering). Metering jets regulate fuel flow and the air/fuel mixture. Jets can become clogged with dirt and fuel varnish causing drivability problems. In the early 1980s, electronic solenoids were added to many carburetors so the fuel mixture could be adjusted to reduce emissions. These carburetors are very complex and expensive to replace.

A "choke" on top of the carburetor is used to restrict airflow when starting a cold engine. Problems here can cause hard starting and stalling. The choke also increases idle speed during warm up.

Replacement carburetors must have the same metering calibration and hookups (fuel line, vacuum, emissions and throttle linkage) as the original. The gasket under the carburetor should be replaced if the carburetor is removed or replaced, or if the original gasket is leaking.

The 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.

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