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ASE Test Prep: Fuel Systems

There will be three questions on the ASE P2 test that deal with the fuel system and its components.


Electronic fuel injection systems come in several versions. Throttle Body Injection (TBI) was used in the 1980s on many vehicles as an intermediate step from electronic carburetion to multiport fuel injection. TBI uses one or two fuel injectors mounted in a throttle body to fuel the engine. Multiport fuel injection (MFI), which is used on almost all late-model engines, has a separate fuel injector for each cylinder. The injectors are usually mounted in the intake manifold and spray fuel into the intake ports, but on some of VW’s “direct injection” applications, the injectors spray directly into the combustion chamber. First-generation MFI systems fire all the injectors simultaneously or each bank separately, but most of the newer MFI systems are “sequential” fuel injection (SFI) systems that fire each injector separately just as the intake valve is about to open.


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. The CPI system works much like an old Bosch K-Jetronic fuel injection system, except that the electronic Maxi injector replaces the complicated mechanical K-Jetronic fuel distributor.

Most injectors are electronic and have a solenoid valve at the top to open the nozzle. The Powertrain Control Module (PCM) determines the on-time of each injector pulse to regulate fuel delivery (a longer on-time means more fuel and a richer mixture). The PCM uses inputs from the oxygen sensor in the exhaust, as well as throttle position, engine speed, load and airflow to control the injectors.


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. A shorted injector may rob voltage from the other injectors and cause the engine to stall.

Fuel pump pressure ratings vary depending on the application, but typically range from 35 to 85 psi. Pump designs also vary and include single- or double-vane, roller vane, turbine or gerotor style pumps. Most have a one-way check valve to maintain pressure in the fuel system when the engine is shut off.


Fuel pumps can fail for a variety of reasons: old age, loss of voltage, ground at the power relay, wiring connections and pump motor or bearing damage. Running the fuel tank empty may damage the pump because it relies on fuel for lubrication.

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. The pump is usually part of the fuel sending unit and may be replaced separately or as a complete assembly. The fuel inlet strainer sock should also be replaced when the pump is changed.


When fuel reaches the engine, it enters a fuel rail and goes to the injectors. A “fuel pressure regulator” on the fuel rail maintains a certain operating pressure. Inside is a spring-loaded diaphragm attached to a source of intake vacuum. As engine load (vacuum) changes, pressure is adjusted as needed to maintain proper fuel delivery. Excess fuel is routed back to the fuel tank through a return line. Many newer vehicles have “returnless” systems that do not have a regulator on the engine fuel rail. The regulator is in the fuel tank with the pump. Regulator problems that alter fuel pressure can hurt engine performance and emissions.


Airflow into the engine is regulated by a “throttle body” attached to the intake manifold. Air first flows through an air filter, then through the throttle body before passing through the manifold and into the engine. The PCM must monitor the amount of air entering the engine, so some fuel injected systems have a vane or mass airflow sensor ahead of the throttle body. Other systems estimate airflow based on throttle position, RPM, temperature and engine load.

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