Getting to know the fuel injection system

The Motor Ombudsman’s Information Line handles enquiries relating to a vehicle’s fuel injection system. This week’s “Getting to Know” guide focuses on what it is, and what its role is.


What does a fuel injection system do?


A car’s fuel injection system is designed to provide fuel to the engine while reducing the level of emissions.


What is the difference between direct and indirect fuel injection?


Direct injection, which is mainly found in diesel engines, is where the fuel is injected directly into the cylinder which is filled with compressed air.


Indirect injection, used by most petrol engines, is where a pump sends the fuel under pressure from the fuel tank to the engine bay where, still under pressure, it is distributed individually to each cylinder.


What are the components that make up the fuel injection system?  


Fuel injection systems have many different components that all function together to analyse engine conditions and deliver the right amount of fuel. The core parts are as follows:


  • A fuel supply module: it contains components such as the high-pressure electric fuel pump and fuel filter.
  • An intake air adjuster: it ensures that there’s the right quantity of air for the engine.
  • An Engine Control Unit (ECU) and sensors: they make sure that the system injects the correct quantity of fuel into the intake air stream.
  • Fuel injectors: they are mounted on a fuel supply rail to deliver the fuel to the engine.


How does the fuel injection system work?


The fuel supply module drives fuel under pressure to the injectors. The amount of fuel that reaches the injector is controlled by the ECU which analyses the air temperature, throttle position, engine speed, engine torque and exhaust data gathered from sensors in and around the engine to regulate the supply at each intake stroke. Air arrives via the intake manifold and is drawn into the engine past the intake valve, or valves. How the fuel and air are introduced to, and merged with each other, is different subject to the type of fuel injection system being used (i.e. whether it is direct or indirect – see definitions above).


Where can I get my fuel injection system replaced or repaired?


To find your local Motor Ombudsman-accredited franchised car dealership or independent garage to get your fuel injection system replaced or repaired, visit The Motor Ombudsman’s online Garage Finder.