Lesson 7: The Fuel System

This lesson is about the fuel system. The fuel system is the system which pumps fuel into the cylinders of the internal combustion engine and mixes it with the perfect amount of air for combustion. There are two types of fuel systems: fuel injection and carburetors.

Prior to 1985 most vehicles (almost all of them) used a carburetor. A carburetor uses a venturi (narrowed passage within the carburetor) to draw fuel from a float bowl based strictly on the volume of air flowing through the carburetor. A carburetor is a primitive method of mixing air and fuel. It is far less efficient than modern fuel injection. That’s why all modern cars today are built with fuel injection.

How a Fuel Injection System Works:

Fuel injection is a complex electrical circuit using multiple sensors as inputs to a PCM (powertrain control module, also called the computer). The PCM then interprets the inputs from these sensors and produces a PWM (pulse width modulated) signal. A PWM signal is a digital on/off signal. This translates into how much fuel is injected into the engine.

The PCM then receives feedback signals from oxygen sensors. An oxygen sensor is a sensor that checks the air-fuel ratio by monitoring exhaust gas at or near the exhaust manifolds (review Lesson 5 if needed). Using this information, the PCM can adjust fuel trim accordingly to keep the engine running clean, long and efficient. This is one of the main reasons newer automobiles last so much longer, burn less fuel, and have cleaner emissions.

Fuel Filter and Air Filter:

There are two filters which protect the fuel system and engine: the fuel filter and the air filter. The fuel system is very sensitive to contamination and debris. A small amount of contamination can cause severe problems in any fuel system. Therefore, these filters are critical for protecting the fuel system and engine.

When you fill your vehicle with fuel from a gas station, the fuel will have contaminants. Between the fuel tank and the engine the fuel passes through a sock filter which prevents large contaminants from passing through. Then the fuel will pass through the fuel filter which prevents small contaminants from passing. Anything that passes through the fuel filter can pass through the rest of the fuel system. Refer to the owner’s manual to find the manufacturer’s recommendation for the mileage at which the fuel filter should be changed.

An air filter protects the engine from dust and dirt entering the combustion chamber through the air intake system. Any dirt in the engine can cause engine wear and eventually lead to engine failure. Therefore, an air filter, like the fuel filter, is also a critical component in any internal combustion engine. The air filter is inspected with every oil change and it is changed as needed. (The air filter was also mentioned in Lesson 2 since checking it is part of general maintenance).

Lesson 7: The Fuel System is one of twelve lessons that are also available as a free ebook download which is complete with illustrations. These lessons are based on the outline for the Boy Scout Automotive Maintenance merit badge and adapted by Crawford’s Auto Repair for a general audience. This article is for informational purposes only and the author does not assume responsibility or liability for any accident that may occur when working on motor vehicles. By reading this page you accept the condition that you are ultimately responsible for your own actions. For additional information on the topics on this page see Fuel System Repairs and Air Intake System Repairs.

Copyright © 2014, Jeff Crawford. Permission is granted to republish this article for personal or commercial use as long as the content, citation, and copyright notice all remain intact and unchanged. There must be an active “follow” link to CrawfordsAutoService.com.

Lesson 7: The Fuel System is licensed under a Creative Commons Attribution-NoDerivatives 4.0 International License.