Lawn Mower Leaking Gas (Why + How to Fix)

Over time, your lawn mower can get a little rough around the edges, and new problems you haven’t encountered before can come up. Leaking gas is one of those problems that are out of the ordinary and likely not something you’ve needed to worry about before. So, if your lawn mower seems to be going through gas faster or you can see an actual leak, you will need to fix it immediately.

Let’s look at where the gasoline can leak from and what you’ll need to do to stop it.

Why is Your Lawn Mower Leaking Gas?

Three main issues cause gas leaks on a mower: parts slowly wear out over time, parts become damaged by mistreatment, or servicing and repairs are carried out incorrectly. Whatever the cause, a lawn mower can leak gasoline from any point within the fuel system, resulting in a hazardous situation.

Diagnosing a Lawn Mower That is Leaking Gas

If my lawn mower ever has a gas leak, I always want to find out the root cause, just in case I can avoid it in the future. Unfortunately, not always but certainly sometimes, gas leaks happen because of something we have done that has caused the leak. So, let’s look at the fuel system and see where the leak could be coming from.

Fuel Tank Filler Cap

Right at the start of the fuel system is the fuel cap. You’ll find that the cap has a washer that seals the tank. As gas sloshes around while the mower is in operation, a bad seal will allow fuel to leak out of the top of the tank. Also, a leaking lawn mower gas cap could be due to something as simple as overfilling the tank and the fuel leaking out of the breather vent in the cap.

Fuel Tank

Next in the fuel system is the fuel tank. These are a little harder to damage, but it’s not impossible. Usually, the fuel tank will protrude from the side of the mower and is open to damage from objects.

I wouldn’t be surprised if you told me that an object out in the yard punctured your tank. Another likely cause of a leaking gas tank is rust. Rust just loves to eat away at the joints in metal fuel tanks, so this is a good place to look.

Fuel Lines

If you have a fuel leak, I’d put the fuel lines as my number one place to look. They are certainly the weakest part of the fuel system. 9 times out of 10, you’ll find that a split or cracked fuel line is the root cause of your gas leak.

If you haven’t been looking after your fuel lines with extra care, they will rapidly harden and crack. Additionally, if you have recently serviced your fuel filter or another part of the fuel system, you could have unknowingly damaged a fuel line or not entirely installed it correctly.

Fuel Filter

If your lawn mower has an inline fuel filter that sits outside the fuel tank, this is a good place to look if your lawn mower is leaking gas. A fuel filter is a simple and pretty cheap part of the lawn mower that can be prone to leaking, usually because it cracks easily. I’m sure you can imagine a serviceable one or two-dollar part isn’t very strong and durable.

Fuel Pump

If you follow along the fuel line past the filter, you’ll come to the fuel pump. The fuel pump has an inlet and an outlet. In a few instances, I’ve found that these inlet and outlet pipes crack and cause the mower to leak fuel. With a little push against the pipe, you should be able to see if they are cracked or not. Additionally, you can probably see the actual leak while the mower is running.

Carburetor

The carburetor is the last part of the fuel system that could be causing your lawn mower to leak gas. A carburetor is a pretty solid part of the lawn mower, but there are a couple of places on it that could leak.

The priming bulb and the fuel bowl/cup are two areas where gas could leak out from. If you look under the carburetor and see it dripping, then further inspection of these two areas could identify where the leak is coming from.

How to Fix a Lawn Mower That is Leaking Gas

If you find your leak by looking through the different parts of the fuel system, you’ll want to move on to fixing the root cause. I’ve put together the tools and parts you’ll need, plus the steps you can follow to stop the leak. Let’s start by looking at what I believe to be the most common cause of why a lawn mower leaks gas.

Replacing Damage Fuel Lines

The simplest solution to fixing a leaking fuel line is replacing it. Depending on your lawn mower, you will have two or three lines in the complete system. So, if one is leaking, I’d recommend replacing all of them.

If you use a pair of pliers to slide off the spring clips holding the fuel line in place, you should then be able to remove the line. All you need to do is replace the old line with a new one and then clamp it into position using the original spring clips.

Just make sure you cut the new fuel line to match the same length as the old line. If it’s longer or shorter, you could create some additional problems for yourself.

Parts & Tools Need to Replace Fuel Lines

• New Fuel Line
• Pliers
• Pipe Cutters

Replacing the Fuel Cap

Before you run to the store to buy a new fuel cap, remove some fuel. If you have overfilled the tank, then a new cap isn’t going to solve the leaking. Other issues surrounding the filler cap need some additional investigation, so getting the fuel down to a good level is the best place to start your further research.

But if you find that the lawn mower’s gas cap is leaking and you don’t think it can be fixed, you will need to replace it with a new one.

Parts Needed to Fix a Leaking Gas Cap

• Replacement Gas Cap

Fuel Filter

Next on my list of potential causes for your lawn mower leaking gas is the fuel filter. This is a quick fix as long as you have a replacement on hand. So, you’ll need to grab your pliers and remove the spring clips the same way you would when changing out the fuel lines.

First, pinch the spring clips and slide them down the fuel line. Then remove the fuel lines from both ends of the fuel filter. Next, grab your new filter and slide it on the fuel lines. Then reposition the spring clips using your pliers.

When installing the new filter, make sure you install it with the correct orientation, as fuel should only flow in one direction. You should find an arrow on the side of the filter showing you which way the fuel should flow. When replacing a fuel filter, be careful not to damage the fuel lines. If they are old, then they could easily crack or split.

Parts Need to Replace a Fuel Filter

• Pliers
• New Fuel Filter

Fuel Pump

If you find your fuel pump leaking, then there’s no need to worry. You can very easily complete this repair without help from the repair shop. If you can change a fuel filter, you should have no problem with a fuel pump.

The first thing you want to do is grab yourself a pair of pliers. The fuel pump has three attached pipes, usually held on by spring clips. So, use your pliers to slide the clips along the pipes to remove them. Next, take a marker pen and label the attached lines 1,2, and 3. This is going to make it less likely that you’ll mix up which pipe is which.

Next, using a socket wrench, remove the bolts holding the pump to the mower body. Next, you can go ahead and remove the pipes attached to the pump. Then set the old pump to one side. Next, position the new pump onto the mower and install the bolt previously removed. Then, install the pipes that were removed and reposition the spring clips. That’s really all there is to it.

Tools & Parts to Replace a Fuel Pump

• Pliers
• Marker Pen
• Socket Wrench Set
• New Fuel Pump

Carburetor

A leak in the carburetor, as I said before, usually stems from a primer bulb or a fuel cup. So let’s start with the primer bulb.

Replacing a Primer Bulb

Primer bulbs come in all different designs and are attached to lawn mowers in various ways. Fortunately, they all seem to have very simple designs that usually consist of rubber bulbs and mounting washers. All designs attach to the lawn mower’s side and are used to prime the engine on cold starts.

First, you’re going to need to remove the old primer. Depending on the design, you will carefully remove a washer that clips on or forcefully remove a washer that is pressed on. Clipped versions are usually made of plastic, and pressed versions are usually made of metal.

For you guys with clipped-on primers, like on Briggs & Stratton engines, push in the clips and slide off the primers. Using a couple of screwdrivers should help. If you are replacing the primer on a B&S engine, then they actually sell a tool for this exact purpose. Once removed, you’ll need to clip on the new primer. Companies like B&S have made this a simple process: you can replace a primer in a matter of seconds.

For those with metal washers, say on a Tecumseh mower, you’ll need to use a hook to pull out the washer. Once you get the washer out, you can pull off the rubber primer. Now it’s time to install the new primer.

These kinds of primers can be a little tricky. The tooth lock washer needs to fit into the circular groove it came out of. The fact that the washer is slightly larger than the hole makes them tricky to install. To make this easier, find a socket in your socket wrench set that is the same size as the tooth lock washer. Then place the socket over the washer and tap it with a hammer. This seems to be the best method to get a tooth lock washer into place without damaging the bulb. Once you get the washer snuggly into place over the bulb, you’ll have completed the repair.

Tools & Parts to Replace a Priming Bulb

• Replacement Bulb
• Screwdrivers
• Socket Wrench Set
• Hammer

Replacing a Carburetor Fuel Cup

Carburetors are usually well protected on your lawn mower, but that doesn’t mean they can’t be damaged. If you find that your cup is leaking, you’ll need to get hold of a replacement fuel bowl kit.

So, start using a wrench to remove the bolt holding the cup. Then, remove the bolt and cup from the carburetor and set it to one side. Next, remove the o-ring between the carb and the cup and install the new one. Then, position the new cup onto the mower. Grab the original bolt, attach the new seal washer, and thread it through the new cup.

Finally, use your wrench to tighten the bolt. This is all you need to do. You’re basically replacing the seals where the cup is leaking. If you think your cup is okay, you could just replace the seal washer and the O-ring. This could save you a dollar or two.

Tools to Replace a Carburetor Fuel Cup

• Fuel Cup Kit (Cut, O-ring, Sealing Washer)
• Socket Wrench Set

Gas Tank

Finally, the last repair on the list is the gas tank. Now, you can do repairs to the gas tank with various products on the market that seal any holes. However, I’m not confident that they work all that well. So, to be on the safe side, I’d go ahead and replace the tank with a new one.

First, you’ll want to remove the fuel from the lawn mower’s tank. Next, using a pair of pliers, remove the spring clip holding the fuel line onto the fuel tank, and then remove the fuel line. Once the fuel tank is disconnected, you’ll need to remove the lawn mower engine cover. A small socket set or screwdriver should work just fine for this.

Next, remove the screws holding on the tank with your screwdriver, and then remove the tank from the lawn mower. Now, take your new tank and position it on the mower. Next, take the fixing screws and fix the new tank into position. Next, reinstall the engine cover and the fuel line. Finally, position the spring clip on the fuel line and fill the fuel tank.

Tools & Parts to Replace a Fuel Tank

• Pliers
• Socket Wrench Set
• Screwdrivers
• New Fuel Tank

What About a Riding Lawn Mower Leaking Gas?

You may be wondering if there are any differences between a riding lawn mower leaking gas and a regular walk-behind mower. The fact is that the causes and fixes are very similar for both types of mowers.

The slight differences are that riding mowers may have longer fuel lines, double fuel tanks, and probably won’t have priming bulbs. However, the basics of the fuel systems will be the same for both types of mowers, and you’ll need to investigate the root cause just the same. The parts will look different, but they do the same job.

So, if you work through the same list of potential causes, you should find the reason why your riding lawn mower is leaking gas.

Why It’s Dangerous Not to Fix This Problem Right Away

Failing to fix leaking gas on a lawn mower right away is dangerous for two main reasons. First, leaking gasoline will evaporate and vaporize and your lawn mower can potentially catch fire, and can cause serious bodily harm. Second, gasoline will kill grass and any vegetation it comes into contact with and contaminate the soil.