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ASE P2 TEST PRIMER: Fuel Systems


10/18/2013
By Larry Carley

Multiport Fuel Injection (MFI) systems have one fuel injector for each of the engine's cylinders and spray fuel into the intake port in the cylinder head. Newer engines with ultra high-pressure "Gasoline Direct Injection" (GDI) spray fuel directly into the combustion chamber.
 

The fuel system consists of an electric fuel pump (usually mounted inside the fuel tank), fuel filter (inline or part of fuel pump module assembly), fuel lines, fuel injectors, fuel pressure regulator (located on engine fuel rail or in the tank if EFI system is returnless), throttle body, fuel tank and gas cap.

Multiport Fuel Injection (MFI) systems have one fuel injector for each of the engine’s cylinders and spray fuel into the intake port in the cylinder head. Newer engines with ultra high-pressure “Gasoline Direct Injection” (GDI) spray fuel directly into the combustion chamber. Direct injection systems deliver better performance and fuel economy, but the intake valves tend to get dirty because there is no fuel spray into the intake ports to keep the valves clean. This may require special cleaning chemicals to remove the deposits.

Common fuel system problems include dirty fuel injectors (which can cause lean misfire and poor performance), dirty fuel filters (may cause fuel flow restriction, drop in fuel pressure, stalling or no start), leaky or defective fuel pressure regulator (causes drop in fuel pressure, stalling, hard start or no start), fuel pump failure (causes stalling and no start, often occurs without warning) and fuel leaks (very dangerous because of fire hazard).

Gasoline contains detergent to keep fuel injectors clean, but some fuel brands may not contain enough detergent and allow deposits to build up. Recommend using a fuel system cleaner additive regularly to keep the injectors and fuel system clean. 

Many fuel filters have no recommended replacement interval, and some are “lifetime” (until they plug) filters. Filter clogging is often due to rust or sediment inside the fuel tank. Cleaning or replacing an aging fuel tank may be required to prevent repeat problems with the filter or pump.

Fuel pump failures are often misdiagnosed because electrical faults in the fuel pump wiring circuit, relay or ground connections may prevent the pump from running. Replacement fuel pumps must have the same pressure rating and flow characteristics of the original, but do not have to be the same type as the original. On many newer vehicles, the pump is part of the fuel pump module assembly and should be replaced as an assembly (though the pump can be replaced separately on many applications). It also is important to replace the fuel inlet sock when the pump is changed.

Throttle bodies can also become dirty, and may require cleaning with an aerosol throttle cleaner product. Varnish buildup in the idle bypass can cause idle problems and stalling.

Required to pass this section of the P2 test:
1. Identify major fuel system components.
2. Identify component function and common reasons for replacement.
3. Identify related items, including hoses, tools and service chemicals.
4. Provide basic use, maintenance, installation and warranty information.















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