Fuel Injection

Step by step illustrated explanation of an automotive fuel injection system.

Step 1
- Fuel injection is used to supply an internal combustion engine with the proper amount of fuel at any given engine speed. Once fuel enters the fuel tank its picked up by the fuel pump which is situated inside the gas tank. This fuel pump is controlled by a relay located in the PDC power distribution center which is activated by the computer.

Fuel Pump In-Tank

Step 2 - Once pressurized, the fuel travels along the bottom of the vehicle through fuel lines toward the fuel filter.

Fuel Lines

Step 3 - Fuel then travels to the fuel filter where its cleaned before continuing to the injection rail.

Fuel Filter

Step 4 - After being cleansed by the fuel filter, pressurized fuel enters the fuel rail where its distributed to individual injectors.

Fuel Rail

Step 5 - A fuel pressure regulator is used to maintain and adjust pressure while the engine is in use, this device is only used in fuel return style injection systems. Some fuel injected vehicles are fitted with a regulator at the fuel pump.

Fuel Pressure Regulator

Step 6 - A fuel injector is used to disperse fuel at precise intervals which is controlled by the computer and adjusts as engine speeds vary.

Fuel Injector

Step 7 - The computer is responsible for producing and adjusting voltage signals which open and close the injector, allowing fuel into the engine.


Helpful Information

The quantity of fuel that enters the engine is determined by the engine management system while using feedback data which is gathered by various sensors. Oxygen sensors are used to monitor the fuel mixture in the exhaust system. A fuel injector can designed to enter the intake system or the combustion chamber of the engine. The open duration of an injector is called, pulse width.

Once an engine runability problem has materialized, its usually accompanied by a check engine or service engine soon light. Perform a code read or scan to determine the problem, and the subsequent repair needed to fix the problem. A fuel system can hold pressure for several hours so caution should be used when working on these systems.

Best Practices


Written by
Co-Founder and CEO of
35 years in the automotive repair field, ASE Master Technician, Advanced Electrical and Mechanical Theory.


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