Lawn Mower Gas Cap Leaking – Why + What to Do

You expect the smell of gasoline when you’re refueling your mower, but you don’t expect to smell it when you’re out on your lawn mowing. But it can happen. It could be due to a fault with the gas tank itself, the fuel line or it’s possible that you might notice your lawn mower’s gas cap is leaking. If this is the case you need to fix this immediately, as leaking gas is never a good sign, and even less when it could potentially come into contact with an engine or muffler that gets incredibly hot during use.

The main reasons for a lawn mower gas cap leaking include a gas tank that is overfilled, a cap that is damaged, and a replacement cap that doesn’t seal the gas tank properly.

Why is My Lawn Mower Gas Cap Leaking? (+ Solutions)

There are a few different reasons why your lawn mower gas cap may be leaking, which I’ll outline for you below. It could be a combination of more than one of these factors, so be sure to check them all, one by one.

Your Gas Tank is Too Full

This is probably the simplest cause of a leaking gas cap to remedy. Every single mower has a recommended “max” fill line in the gas tank. Have you put more gas into the tank than you should have?

If you have, it’s possible that as the gas heats up, pressure builds in the tank, the gas expands and leaks out of the tank via the vent on the cap.

Solution: If this is indeed the problem, you’re going to need to remove some of the gas to bring it down into the range that the manufacturer recommends. But if you’ve had your mower running, make sure to leave it to cool down first. And not just for a couple of minutes. I’m talking about half an hour or more.

Once cool enough to work on, you have a few different options to remove some of the gas. A lot of folks will use a fuel pump to help them siphon off the gas, but you can also drain gas from a lawn mower without a siphon by disconnecting the fuel line.

The Cap is in Poor Condition

Everything succumbs to wear and tear sooner or later. It’s possible that this is the reason for the leaking gas cap on your mower. Remove it from your mower and look for the following:

• Do you see any cracks in the cap?
• Is the seal in good condition or has it become worn?
• Are there any holes in the cap?

Another thing to check is whether the thread is still in good condition. When you fasten the cap onto the mower, does it keep spinning or are you able to tighten it up? A cap that no longer tightens up fully may have worn thread, as this could be responsible for the leaking.

Solution: If upon inspection you find that your gas cap is damaged, there’s just one solution; buy a replacement. Just make sure you buy one that will fit the make and model of mower you have. Lawn mower gas caps are not all the same. It may be possible to source a universal cap that fits a variety of models, but I always prefer to try and get one that is made specifically for the mower that I have. Perform these same checks on the new cap before fitting.

You Bought a Replacement that Doesn’t Seal Properly

Did you buy a replacement cap recently? Maybe you lost your previous one, or you broke it. Whatever the reason, if you just replaced your gas cap and it started leaking right after, you don’t need to be a detective to figure out that this may be the cause.

I’ve seen people have issues with the rubber gasket on their new cap, whereby it just wasn’t sealing the opening of the tank properly. At this point, I should probably ask…did you definitely buy a gas cap that was designed for use with your particular lawn mower? If you didn’t, that might explain the lousy fit. The user manual for your mower should provide specific details about the cap you need.

Solution: I’ve seen folks fix this by using taking the rubber gasket off of their old cap, and fitting it on their new one. This sometimes helps to create a better seal and can fix the problem. Otherwise, it’s probably back to the drawing board. You’ll likely need to buy a 2^nd replacement gas cap. Just make sure you buy one that is designed for your mower model this time.

Excessive Vibration

This is unlikely, but I thought I’d include it anyway. If your gas tank is overfilled or even just right up to the max fill line, and your mower has a big problem with vibration (I’ve seen some horrendous examples of this where the owner thought it was “just what lawn mowers do”), it’s possible that this vibration could be causing gas to crash around in the tank and leak out of the gas cap vent.

In this case, it’s not the gas cap or the tank you need to worry about fixing, it’s that vibration. And not just to prevent leaking gas. Using a mower that vibrates excessively is no good for your hands either.

Solution: Two things I’d look at right away if your mower is vibrating way more than it should be are the blade and the crankshaft. If the lawn mower blade is loose/bent or the crankshaft is bent (how to tell if your lawn mower crankshaft is bent), this could be the root cause of the problem. While you’re at it, take a wrench and make sure nothing else is loose.

Check the Filler Neck isn’t Cracked

This is another thing that I’d check as it might be that you’re under the impression your lawn mower gas cap is leaking, but actually the problem lies elsewhere. And the prime candidate here is the filler neck of the tank. You may have banged it and without realizing cracked it. Even a very tiny hairline crack is probably going to cause gas to leak out.

So when you see gas leaking, check that it’s definitely coming from the gas cap before you start throwing money at the problem. If it does turn out to be the filler neck that is cracked, you’re probably going to have to replace the whole tank. But look on the bright side – at least there will be no more leaking gas!

A Leaking Mower Gas Cap is a Problem that Needs Fixing Right Away

Ignoring a leaking gas cap is as bad as running your lawn mower without a gas cap. Both turn it into a fire hazard. And swapping it out for a temporary lawn mower gas cap that you create isn’t going to fix the issue either. You need to get to the bottom of the issue right away by using the potential causes I’ve talked about above as a checklist. Go through them and check them off one by one, until you find what is causing the leak.