You might have experienced the check-engine-light coming on, along with a sinking feeling wondering what has now gone wrong with your car. Don't panic, however, the problem could be as simple as a missing or loose fitting petrol cap.

You might wonder how such a simple thing can cause problems. Read on to find out why.

How The Evaporative Emission Control System Works

Petrol is a volatile substance and readily evaporates. If the petrol cap is not fitted properly, the vapours exit from the fuel tank into the atmosphere, and this is recognised by your car as a leak in the fuel system which causes the check engine light to come on. 

The Evaporative Emission Control System (EVAP) in your car is designed to prevent fuel vapours escaping from the fuel tank and lines, polluting the atmosphere. The EVAP siphons the vapours in the petrol tank to a storage canister, and after the car is started and warm, these vapours are condensed and fed into the engine to be used as fuel.

The petrol tank and fuel lines are a closed, pressurised system, and the EVAP constantly monitors the pressure in the fuel tank and lines to check for fuel leaks and evaporation. However, it is not straightforward to create these pressurised conditions because petrol is continually being removed from the tank as you are driving. In a completely sealed pressurised environment, the siphoning of petrol from the tank would create a degree of suction that could cause the tank to collapse inwards as it empties. So a method is needed to allow the venting of air to replace the fuel being removed.

In older  Evaporative Emission Control Systems, the petrol cap has a spring loaded valve that allows air to vent as the fuel empties. In newer systems, the petrol cap is completely sealed, and air is vented via the storage canister. Because of this, when replacing a petrol cap, you must replace it with the same kind as the original, either a spring-loaded or sealed model.

Consequences And Signs Of Fuel Evaporation

It is important to prevent fuel evaporating into the atmosphere because these vapours are polluting twenty-four hours a day, even when your car is not running. If fumes are evaporating from your tank into the atmosphere, this also means that your car's fuel efficiency will decrease.

The end shot is if you notice a petrol smell around your car, a loose fitting or missing cap could be the cause; take the cap off and put it on again securely. Also, if you are using more petrol than usual and you have not changed your driving habits, make sure to check the petrol cap.

So while the check engine light can come on for a variety of reasons, if it is your day, the problem will be a missing or loose petrol cap. 

However, make sure to check with your car mechanic, someone like Quickstop Brake & Clutch Centre, if you have any concerns regarding the fuel system, as it is important to capture and fix any leaks as quickly as possible. A regular car service is always a good preventative measure.