What Do I Really Need to Know About My Vehicle's Fuel System?

Source: Bosch

Dirty Fuel is a Big Cost

The modern fuel system consists of many components that help to deliver precise amounts of fuel into the combustion chamber at the right time. From the 1980’s and 1990’s, most vehicles, both petrol and diesel, now use many forms of direct injection. Whilst diesel engines have always been direct injection, petrol used carburetors.

The modern fuel system is about fuel efficiency, responsiveness, and emission control. Most of the work is done through the Engine Control Unit (ECU), a computer that receives signals from various sensors, and adjusts the amount of fuel into the combustion chambers.

Some of the key components of the fuel system are discussed below.


The fuel tank

This is the reservoir for the fuel. It is normally situated on the floor or chassis. All that the driver needs to do is to ensure that the tank always has some fuel. Avoid running empty, especially for diesel engines. Running empty may lead to the vehicle drawing fuel with sediment at the bottom of the tank. Even with the presence of filters, it is something advisable to avoid.

In the tank there is the float for measuring fuel level, a sieve for first stage filtration, and sometimes the pump.


The Fuel Pump

Most vehicles have two pumps; the low and high pressure (injector) pump, especially diesel vehicles. The pumps can vary in quality and design. The old petrol pumps were diaphragm or vacuum, whereas the modern ones are electric or electronic. Most cannot be repaired. The cost is low and can therefore be easily replaced.

The challenge occurs mostly with the diesel injector pump. The purpose of the injector pump is to deliver pressurized to the combustion chambers through the injectors in the right quantity and at the right time.

The injector pump is very susceptible to dirt and running dry. The pump gets lubricated by the diesel, which means running dry or low can damage the pump. As a driver or vehicle owner, you need to ensure that you use clean fuel. Clean fuel means you are using clean containers and nozzles, and the filters are replaced on time. The diesel injector pump needs regular service by specialized workshops. Their job is to clean and calibrate the pump, as well as replace bearings. A malfunctioning fuel pump creates a lot of problems for the driver. The vehicle becomes hard to crank, becomes unresponsive on accelerating, wastes fuel, dilutes the oil, and loses power.

Fuel Injectors

These can be arranged on a common rail (so called common rail injectors), or individully connected to the injector pump. They are responsible for introducing a fine pressurised mist of the fuel at the precise moment into the combustion chamber. The injectors are sensitive to dirt and clogging, which prevents them from properly opening or closing. They need clean fuel and regular service. Common problems are dribbling, when the injectors drip fuel, due to failure to open and close properly. Sigs and symptoms can easily be checked by your mechanic. Replacement can be costly. Most service replaces the nozzles.


Fuel Filters

Petrol engines normally have one filter, whilst some diesel systems have two. Filtration is critical in ensuring that the fuel is clean. One would rather spend on frequent filter change than encounter costly pump and injector repairs later.



Various sensors send data to the Engine Control Unit (ECU) to calibrate the amount and pressure of the fuel going into the combustion chamber.


Oxygen Sensor

The oxygen sensor is one of many other sensors that send signals to the engine control unit in or order to control the amount of fuel injected into the combustion chamber. The oxygen sensor is mounted on the exhaust manifold, and monotors the amount of unburnt oxygen in the exhaust gas. The unburnt oxygen is used as a signal to calculate the amount of fuel required to adjust the mixture.

Other sensors that send signals to the ECU are:


Water Temperature Sensor

It measures the engine temperature to determine the amount of fuel required in the chamber. A cold engine needs more fuel than a hot engine.


Mass Airflow (MAF) Sensor

The Mass Air Flow sensor measures the amount of air entering the engine when driving. It also measures the air density and humidity.


Throttle Position Sensor 

This measures the extent to which the driver is stepping on the accelerator pedal, to influence the fuel quantity and timing.


Manifold Pressure (MAP) Sensor

It measures the amount of vacuum in the combustion chamber, to determine the load on the engine.


Crankshaft Position Sensor

It measures the RPM of the engine and the crankshaft position, to determine fuel delivery into the cylinders, as well as timing.


As a vehicle owner, ensure that your fuel system is clean. There are many fuel system cleaners available from spares outlets.


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