The gas cap that you remove when filling up the fuel tank of your car can sometimes wear out. This can lead to problems with the fuel in the tank and can sometimes affect engine performance.

As a general rule, the symptoms of a bad gas cap include a smell of fuel from the vehicle, trouble removing or tightening the gas cap, a check engine light on the dashboard and water getting into the fuel tank.

Luckily, changing the gas cap is one of the easiest repair jobs you can carry out, once you have established that it is the gas cap that is at fault.

What Problems Can A Loose or Bad Gas Cap Cause?

The gas cap is often overlooked when it comes to problems with the fuel tank or a smell of fuel from the vehicle. Over time, the rubber seals on the gas cap will harden and eventually wear out, causing it to fail.

A worn out gas cap won’t seal the fuel tank as it should and this can lead to more serious engine performance problems if the fuel is constantly becoming contaminated with dirt or water.

Here’s a more detailed explanation of the symptoms of a bad or failing gas cap.

1. Smell of fuel from the outside of the vehicle

A bad gas cap can allow fuel vapors to escape from the vehicle as it no longer seals the tank as it should. This will lead to the smell of gas around the outside of the vehicle and sometimes inside.

bad gas cap

There are many other (more serious) reasons why fuel vapors are escaping from the fuel delivery system of your car. Here’s a full article explaining why your car smells like gas that goes into more detail.

2. Check engine light

Sometimes a bad gas cap can cause problems with the fuel system on your car. The fuel delivery system on modern cars is a complicated, highly pressurized system that runs from the gas cap right through to the fuel injectors. If there is a problem anywhere in the fuel system then this will lead to engine performance problems and a check engine light flashing on the dashboard.

A bad gas cap can let dirt or water vapor into the gas tank, and this can contaminate the fuel in the tank, leading to problems with the performance of the car. This can cause a bad fuel filter and can damage the fuel injectors if it happens regularly.

3. Various Emissions related Error Codes

If the gas cap is no longer sealing the gas tank and is allowing fuel vapor to escape, or contaminants to get in to the fuel, this will usually lead to problems with the vehicle Evaporative Emission System (EVAP). 

The EVAP system fitted to modern vehicles is there to manage the evaporation of fuel fumes from the gas tank. If there is a leak (a bad or failing gas cap) then this will trigger an error in the EVAP system that will be stored in the ECU memory. An engine code error can lead to a check engine light on the dashboard, or changes to how the engine performs.

Common EVAP related engine error codes are explained in detail in these other published articles, these include P0456, P0440, P0441, P0452 and P0455.

4. Poor fuel economy

A bad gas cap can lead to poor fuel economy. If the gas cap is not sealing the fuel tank, the fuel can become contaminated and in some cases fuel can actually evaporate from the tank.

Fuel escaping from the tank is usually only a problem for older cars, as newer tanks have one way systems fitted that will prevent fuel and fuel vapors from escaping via a badly fitting gas cap. However, if the fuel is allowed to escape as vapor this will lead to poorer fuel economy as fuel will be literally disappearing from the tank!

Contaminated fuel can also lead to poor fuel economy as the fuel injection management system will struggle to 

5. Gas cap not fitting properly

It might seem obvious, but a very common sign of a bad gas cap is one that does not fit or tighten properly. This is more common on older vehicles, where gas caps were made from metal rather than plastic, however it can still be a problem with newer vehicles.

Newer gas caps are made from plastic and have rubber seals that ensure an air tight fit. Over time, these seals can wear out by becoming hard and brittle. They can also become clogged up with dirt and dried fuel (especially if it’s a diesel fuel tank). These can all contribute to a gas cap that is difficult to fit or that won’t tighten properly.

Next Steps

If it’s obvious that the gas cap is damaged or is not tightening properly then you should start by replacing the cap with a new OEM replacement. Gas caps are cheap and you should always replace one with a genuine OEM gas cap from a dealership.

If the gas cap doesn’t look worn out and is tightening properly, then you probably have a problem elsewhere in the fuel system. If the vehicle is old (a classic with a metal gas tank and gas cap) then it’s always worth replacing the gas cap to begin with. Older metal caps don’t seal as well as newer plastic versions, so it may not be obvious if there are problems with it.

If you have a newer vehicle, it’s unlikely a bad gas cap will cause major problems with how the engine performs, and it is less likely that the gas cap will become damaged to begin with.