Whilst it may seem fairly innocuous, the fuel cap on your car serves a few very important roles. Even though they are a simple device they can still become worn or work less effectively over time, and in some cases stop functioning altogether. We’ll explain their main uses and what to do if your fuel cap is loose and what problems it can cause. 

Why is a fuel cap needed? 

The main purpose of a fuel filler cap is to prevent the fuel from spilling as well as evaporating from the tank.  

Petrol, if left open to the air, will evaporate all its volatile chemicals within hours, causing the fuel to become stale and much less energy dense. The fuel filler cap prevents the liquid from evaporating and therefore retains the fuel's energy.  

What happens if your fuel cap is loose? 

If your fuel cap is broken or worn out, and the rubber seal is not making good contact with the filler neck, your fuel will evaporate from the tank much faster than normal.  

The more obvious consequence is that the fuel will also spill out of the tank, onto your car's paintwork. 

You may notice that if you don’t drive many miles, and your fuel cap is loose, the engine runs rough after a month or so, or fails to start at all. This would be due to the volatile compounds evaporating and the petrol becoming stale.  

How to fix a loose fuel cap 

If you notice your fuel cap is loose, the only real way to fix it is to buy another one. The mechanism that locks the fuel cap in place, and that seals the fuel from spilling, is all built into the cap itself, so there should be no issue with the filler neck or tank. 

The best place to source a filler cap would be the main dealer parts department, they will be able to supply an exact replacement of the original, ensuring there are no issues with sealing for years to come. 

Can you drive without a fuel filler cap? 

It’s strongly recommended that you don't drive if your car is missing its fuel filler cap, not only would your fuel evaporate, but spills onto the road can be dangerous for multiple reasons.  

Other than the obvious flammable dangers of petrol spilling from your car, fuel on the road can be very dangerous for road users such as motorcyclists or cyclists, as it can make the surface slick as ice.  

What to do if you lose your filler cap? 

If you're in a position where you’ve lost your fuel filler cap, and cannot get home without driving your car, you could try a couple of short-term solutions.  

Buy a universal filler cap in a garage or supermarket - lots of petrol station shops sell ‘universal’ fuel filler caps. Borrow one from another car - ask your friends and family if they have a similar car or make. 

We don't recommend: 

Don’t use a rag or item of clothing (socks are quite common!) as a makeshift cap. 

The porous material can allow the vapour to still pass through, and petrol can still seep out when the fuel sloshes around.

Don’t stuff a plastic bag in the cap. 

Most of us will have some supermarket bags stowed away in the boot, in a pinch these can be used as a cap, but make sure they don't fall into the tank, fall out or allow dirt into the system. 

How much is a new fuel filler cap? 

Most fuel filler caps are inexpensive, between £15 - £30. If however, your fuel cap has a lock built into it, you may find it can cost upwards of £100 to get one that will work with your car key.  

In the event you lose your locking fuel filler cap, an alternative is to buy a universal locking cap, or one from a different car with the key supplied.  

This may mean you need to keep the fuel cap key in your car separately (don't lose it!) but will be cheaper than getting one made to suit your key.

Are fuel caps universal? 

Most manufacturers have their own variation of filler caps, the main differences being the way the devices lock into the filler neck, with some using a half turn, some by the key and so on.  

However as many manufacturers are sharing parts nowadays, you may find some brands fit others, common ones include Volkswagen, Seat and Audi - BMW and Mini - Jaguar, Land Rover and Range Rovers - and more! 

You can buy universal fuel caps, usually, each one will be advertised at a specific set of car makes so make sure you’re getting the right one! 

How often should you replace a fuel cap? 

Modern rubber seals and tough plastics mean most drivers will never need to replace a fuel cap in their car’s lifetime.  

Exceptions may be if the cap is damaged by yourself or a previous owner or if you notice fuel leaking from the cap after filing.  

Why is my petrol leaking? 

If you’ve noticed petrol around the filler cap of your car after filling, it might be leaking from the fuel cap or from the tank itself: 

Petrol leaking from underneath the car 

Noticing petrol on the floor underneath your car can be indicative of a serious issue, as a leaking tank can be extremely dangerous.  

Firstly, check the location of the leak, if it’s to the rear, on the same side as the petrol cap, there’s a good chance it's from the petrol tank itself.  

Modern cars often use plastic fuel tanks, and there are a few failure points, one being the connection between the filler neck, the tube that runs from the filler cap to the tank, and another being the tank itself. Sometimes the metal straps that hold it in place can wear through the plastic. 

Issues with the fuel filler neck can sometimes be rectified by removing and refitting the connection with new fasteners. However, issues with the main fuel tank will most likely require a new tank assembly.  

Petrol leaking down the side of the car 

If you notice fuel running down the side of your car, particularly shortly after filling up, it may be a sign your fuel cap is not sealing properly. 

Open the fuel filler door and inspect the area for signs of fuel leaking.  

It may be hard to detect leaking fuel in anything but very cold weather, as petrol evaporates readily above freezing temperatures. The best solution may be to buy a new fuel cap and so if that resolves the problem. 

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