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A Short Course on
Fuel Systems

by Charles Ofria

This article is broken down into four sections:

What is a Fuel System?


The modern automobile fuel system has several tasks it must perform, from safely storing the fuel with minimal impact to the environment, keeping the fuel clean, delivering the fuel to the engine, and mixing it with air in the exact proportions necessary so that it will burn completely for any given driving conditions.

The most common fuels in use today are gasoline and diesel, however a number of new  fuels are hitting the market or show promise for the future. More on  those alternative fuels at the end of this article.

The components that make up the fuel system include:

  • Fuel Tank
  • Gas Cap
  • Fuel Lines
  • Fuel Pump
  • Fuel Filter
  • Fuel Pressure Regulator





 Gasoline and diesel are liquids and are stored in a fuel tank. A fuel pump draws the fuel from the tank through fuel lines and delivers itthrough a fuel filter to either a carburetor or fuel injection system.  The carburetor or fuel injection system is responsible for sending the exact amount of fuel required to each cylinder in order to produce the power to move the vehicle.

How Does a Fuel System Work?

The fuel system is made up of the fuel tank, the fuel pump, fuel filter, and interconnecting tubes that carry the fuel from the tank to the engine where it can do some work.  Since the dawn of the auto industry, the task of taking fuel and controlling it in a way that will allow the driver to make the car go was the job of the carburetor.

Carburetors have been around since the dawn of the motor vehicle, but reached their limits in the mid 1980s when emission regulations forced a switch to the more precise and efficient electronic fuel injection system.  By 1991, fuel injection systems were on every car sold in the US, and by 1995, on all gasoline powered trucks as well.

Before jumping into fuel injection systems, I thought it might be a good idea to take a brief look at the carburetor and see how it works.  With few exceptions, carburetors are purely mechanical devices that are simple to understand and should give you a good idea of what is required to distribute the exact amount of fuel and air to extract the desired performance out of an engine.

There have been many different types of carburetors over the years, but for the sake of this discussion, we will touch on a small handful of carburetors and then move on to fuel injection.

A carburetor for a small 4 or 6 cylinder engine is usually a one barrel unit and is the simplest to describe, so let's start there.



Larger engines might have a two barrel or four barrel carburetor.  Let's stop here and discuss what these terms mean, beginning with what is a barrel.


circuits of the typical carburetor

Float system

Idle System

Main-metering System

Power System

Accelerator-pump System

Choke System


1 barrel carb

2 barrel w one primary & one secondary

4 bbl.


Let's start this section by talking about the fuel itself

Gasoline is a complex blend of carbon and hydrogen compounds. Additives are then addedto improve performance. All gasoline is basically the same, but no two blends areidentical. The two most important features of gasoline are volatility and resistance toknock (octane). Volatility is a measurement of how easily the fuel vaporizes. If thegasoline does not vaporize completely, it will not burn properly (liquid fuel will notburn).

If the gasoline vaporizes too easily the mixture will be too lean to burn properly.Since high temperatures increase volatility, it is desirable to have a low volatility fuelfor warm temperatures and a high volatility fuel for cold weather. The blends will bedifferent for summer and winter fuels. Vapor lock which was a persistent problem yearsago, exists very rarely today. In today's cars the fuel is constantly circulating from thetank, through the system and back to the tank. The fuel does not stay still long enough toget so hot that it begins to vaporize. Resistance to knock or octane is simply thetemperature the gas will burn at. Higher combustion chamber temperatures require a higher octane fuel toburn properly. As compression ratio or pressure increases so does the need for higher octane fuel.Most engines today are low compression engines therefore requiring a lower octane fuel(87). Any higher octane than required is just wasting money. Other factors that affect theoctane requirements of the engine are: air/fuel ratio, ignition timing, enginetemperature, and carbon build up in the cylinder. Many automobile manufacturers haveinstalled exhaust gas recirculation systems to reduce cylinder chamber temperature. Ifthese systems are not working properly, the car will have a tendency to knock. Beforeswitching to a higher octane fuel to reduce knock, make sure to have these other causeschecked.

Diesel fuel
Diesel fuel, like gasoline is a complex blend of carbon and hydrogen compounds. It toorequires additives for maximum performance. There are two grades of diesel fuel used inautomobiles today: 1-D and 2-D. Number 2 diesel fuel has a lower volatility and is blendedfor higher loads and steady speeds, therefore works best in large truck applications.Because number 2 diesel fuel is less volatile, it tends to create hard starting in coldweather. On the other hand, number 1 diesel is more volatile, and therefore more suitablefor use in an automobile, where there is constant changes in load and speed. Since dieselfuel vaporizes at a much higher temperature than gasoline, there is no need for a fuelevaporation control system as with gasoline. Diesel fuels are rated with a cetane numberrather than an octane number. While a higher octane of gasoline indicates resistance toignition, the higher cetane rating of diesel fuel indicates the ease at which the fuelwill ignite. Most number 1 diesel fuels have a cetane rating of 50, while number 2 dieselfuel have a rating of 45. Diesel fuel emissions are higher in sulfur, and lower in carbonmonoxide and hydrocarbons than gasoline and are subject to different emission testingstandards.






Fuel Tank
Tank location and design are always a compromise with available space. Most automobileshave a single tank located in the rear of the vehicle. Fuel tanks today have internalbaffles to prevent the fuel from sloshing back and forth. If you hear noises from the rearon acceleration and deceleration the baffles could be broken. All tanks have a fuel fillerpipe, a fuel outlet line to the engine and a vent system. All catalytic converter cars areequipped with a filler pipe restrictor so that leaded fuel, which is dispensed from athicker nozzle, cannot be introduced into the fuel system. All fuel tanks must be vented.Before 1970, fuel tanks were vented to the atmosphere, emitting hydrocarbon emissions.Since 1970 all tanks are vented through a charcoal canister, into the engine to be burnedbefore being released to the atmosphere. This is called evaporative emission control andwill be discussed further in the emission control section. Federal law requires that all1976 and newer cars have vehicle rollover protection devices to prevent fuel spills.

Fuel Lines
Steel lines and flexible hoses carry the fuel from the tank to the engine. Whenservicing or replacing the steel lines, copper or aluminum must never be used. Steel linesmust be replaced with steel. When replacing flexible rubber hoses, proper hose must beused. Ordinary rubber such as used in vacuum or water hose will soften and deteriorate. Becareful to route all hoses away from the exhaust system.

Fuel Pumps
Two types of fuel pumps are used in automobiles; mechanical and electric. All fuelinjected cars today use electric fuel pumps, while most carbureted cars use mechanicalfuel pumps. Mechanical fuel pumps are diaphragm pumps, mounted on the engine and operatedby an eccentric cam usually on the camshaft. A rocker arm attached to the eccentric movesup and down flexing the diaphragm and pumping the fuel to the engine. Because electricpumps do not depend on an eccentric for operation, they can be located anywhere on thevehicle. In fact they work best when located near the fuel tank.

Many cars today, locate the fuel pump inside the fuel tank. While mechanical pumpsoperate on pressures of 4-6 psi (pounds per square inch), electric pumps can operate onpressures of 30-40 psi. Current is supplied to the pump immediately when the key isturned. This allows for constant pressure on the system for immediate starting. Electricfuel pumps can be either low pressure or high pressure. These pumps look identical, so becareful when replacing a fuel pump that the proper one is used. Fuel pumps are rated bypressure and volume. When checking fuel pump operation, both specifications must bechecked and met.

Fuel Filters
The fuel filter is the key to a properly functioning fuel delivery system. This is moretrue with fuel injection than with carbureted cars. Fuel injectors are more susceptible todamage from dirt because of their close tolerances, but also fuel injected cars useelectric fuel pumps. When the filter clogs, the electric fuel pump works so hard to pushpast the filter, that it burns itself up. Most cars use two filters. One inside the gastank and one in a line to the fuel injectors or carburetor. Unless some severe and unusualcondition occurs to cause a large amount of dirt to enter the gas tank, it is onlynecessary to replace the filter in the line.


Fuel Injection

Throttle body injection

Port injection

Direct injection






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