I lost my fuel cap a couple months ago and haven't bothered finding a replacement. I haven't noticed any issues with it, but I'm wondering whether it's causing any damage or reducing gas mileage or anything.

  • What kind of car is it (year, brand, model)?
    – race fever
    Commented Dec 18, 2015 at 14:35

4 Answers 4


It might cause issues with the evaporative emissions controls as the tank is currently permanently vented to the atmosphere and that can trigger a check engine light. In general it shouldn't be affecting the fuel mileage, though.

The other concern is that you'll probably end up clogging the various fuel filters sooner because the lack of fuel cap means all sorts of dust might enter the tank while you're driving. You'll also see more fuel evaporate over time, especially in hot climates.

Of course it also makes it a lot easier to stick a hose down the filler neck and siphon some fuel out for those who like to circumvent the legal process of purchasing motive fluids.

  • 4
    I know it was probably meant as a joke, but I doubt the 2-5 seconds it takes to unscrew the cap would stop a hypothetical fuel siphoner, especially since they'll need to flip the hinge lid thingy to see that a cap's missing, but +1 for the rest of the answer.
    – Davy8
    Commented Aug 12, 2011 at 19:59
  • Locking fuel cap? Commented Aug 12, 2011 at 20:09
  • You may not notice the amount that evaporates in your fuel mileage, but if you have emissions inspections in your state your vehicle will fail and you'll have to make another trip for the retest!
    – JayL
    Commented Aug 15, 2011 at 4:16
  • @Brian I thought that was a joke, I didn't realize they actually made those, or that fuel theft was an actual problem. Haven't heard of anyone in my life that's had that happen.
    – Davy8
    Commented Aug 15, 2011 at 16:52
  • @Davy8 Not sure where you're from, but in the USA MidEast, fuel theft is an on-going issue ever since fuel passed $2/gal. Gets a LOT worse when it's over $4/gal. Not just siphoning from cars that don't have locking caps/latches, but people have even been drilling holes directly into the bottom of gas tanks (on trucks, where they can slide under the vehicle for easy access) to steal the gas! Commented Aug 15, 2011 at 17:13

In the UK, most petrol (gas) stations sell temporary fuel caps in the appropriate colour, for very little money. They are universal fitting and usually made of plastic, so just press into the hole making a snug fit.

In addition to what Tomo suggests though, I would say that it also depends on the vehicle and the fuel that you use.

A big problem for motorcyclists is spilled diesel from vans and lorries. On many lorries there is very little height between the high fill level of the tank and the filler cap, so if the cap is not refitted correctly, diesel can slosh out of the tank and onto the road, causing a major hazard for other vehicles, especially two-wheelers.

Petrol leaks do not pose the same issue for other road users, but even so, if the filler cap has a short 'neck', then you could lose more fuel from it sloshing out than from evaporation.

Also, don't discount the threat of fuel theft. With many vehicles, it is the filler cap itself which locks the petrol tank, so going without a cap could make it trivial for people to siphon off your fuel. If the missing cap is hidden behind a flap the threat is less, but even so if someone is intent on stealing fuel, they will just flip open the flaps that aren't locked.

  • +1 however I still don't understand how having a cap protects against fuel theft? Is someone really going to flip the outer flap then see a fuel cap and decide it's not worth the extra 2 seconds to unscrew the cap? Am I missing something?
    – Davy8
    Commented Aug 15, 2011 at 15:22
  • As I said With many vehicles, it is the filler cap itself which locks the petrol tank, as an example, see the illustration in this article: green.autoblog.com/2011/03/27/… Try searching for "locking fuel cap" in a google images search and you will find loads of them.
    – Mark Booth
    Commented Aug 15, 2011 at 16:37
  • Ah, I didn't know those even existed. I guess whether it's a problem depends on where you live. I've never heard of anyone having fuel stolen before so I've never heard of protection against it either.
    – Davy8
    Commented Aug 15, 2011 at 16:50
  • @Davy8 - I suspect that fuel theft would be far more common in the US if petrol prices were close to the $8.5 per us gallon that were are paying in the UK at the moment. *8')
    – Mark Booth
    Commented Sep 6, 2011 at 13:07

Very on most cars and extremely on some.

Extremely: on my car (2004 Subaru WRX), a loose filler cap will trigger a check engine light and the limp home mode. The engine computer will drop into a mode where it won't allow revs above a low limit and disallows all boost. Suddenly, you're in a 8:1 compression ratio car and aren't able to get out of your own way on the highway. This isn't a gradual switch, either. All of a sudden, you feel like you're going backwards through rush hour traffic....

Very: the fundamental purpose of the modern fuel cap isn't to keep gas from sloshing out or to keep sudden rainstorms from getting in. It's to keep the evaporating fuel from wafting out of the tank, wasting your money and leaving a trail of raw pollution in a cloud behind you. Remember, you're driving a car that gets progressively hotter the farther you go and gasoline is happy to completely evaporate as its temperature increases. Modern caps click on because they're trying to make a gas-tight seal against some non-trivial vapor pressures.

Get a new cap. They're cheap and, if you're concerned about siphoners, you can get one with the little luggage key lock. It won't stop a determined fuel thief but it will persuade a random punk to move onto the next car, who happens to be driving around with an old rag stuck down the filler neck as a temporary cap....

  • 1
    @Mark Booth, seriously. When you see those guys, there's a definite feeling of "where are the police when you need them? Get this Molotov cocktail off the road."
    – Bob Cross
    Commented Aug 15, 2011 at 17:52
  • Luckily, you don't get many of them in that the green and pleasant land of middle England. *8')
    – Mark Booth
    Commented Aug 15, 2011 at 21:07

There is another important function of the fuel cap, it helps control flammable vapors.

An example would be if you parked a car inside a hot garage baking in the summer sun. There would be an ignition risk from the enclosed place filled with gasoline vapors leaking out of the open gas tank.

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