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Lawnmower Spits Gas Out of Fuel Cap: Reasons and Fixes


Why is my lawnmower spitting gas from the fuel cap?

You’re mowing your lawn, and gas starts spitting out of your fuel cap. What causes fuel to sputter from a lawnmower’s gas cap, and can the issue be fixed?

One of the most common reasons gas spews from a lawnmower’s cap is overfilling the fuel tank. Other reasons fuel can eject from the gas cap include problems with the cap itself and more serious internal engine problems.

Read on to find out if your specific fuel issue can be fixed quickly and cheaply or if it’s a sign of more serious hidden damage.

Fuel Cap Leaks Caused by Overfilling the Tank

Every lawnmower comes with a manual that tells you how much gas you can put into your tank. Pay close attention to what your user manual says, as well as any raised or printed indicators on or in the fuel tank if you don’t; you can run into more problems.

Like any gas-powered tool, Lawnmowers have extra space in their fuel tanks because fuel expands when heated (sourceOpens in a new tab.). If you overfill your tank, the gas will expand, causing immense pressure that becomes so great that it pushes gasoline out of the fuel cap’s vent.

If you check your fuel level and it’s above the fill line, and you have fuel escaping from the cap, your problem is most likely due to an overfill. 

To fix your problem, you will have to:

  • Turn your mower off
  • Let the fuel in the tank cool down
  • Place a bucket or gas can under the fuel line
  • Unhook the fuel line from the gas tank
  • Drain the excess gas from the tank until levels fall under the fill line
  • Reattach the fuel line 
  • Move the excess gas to a safe place
  • Start the mower up again

This YouTube video gives a great demonstration of how to check the fuel tank for proper venting:

Lawnmower Fuel Cap Venting Test

Fuel Cap Leaks Caused by a Dirty Vent

Another reason gas may be streaming out through your fuel cap is the condition of the cap itself. If you’ve had your cap for a while, it could have built-up dirt and grime in and around the vent. The best way to see if a dirty gas cap is at fault is by testing and cleaning it. 

To test your cap you’ll want to follow these measures:

  • Make sure you have a good amount of fuel in your tank
  • Make sure the cap is screwed on securely
  • Turn your mower off
  • Let the fuel in the tank cool down
  • Place a bucket or gas can under the fuel line
  • Unhook the fuel line from the gas tank
  • Drain all of the gas from the tank 
  • Unscrew the cap and check the tank

If there’s gas still trapped in the fuel tank, your vented cap is not working correctly. The reason the gas doesn’t empty all the way is a vacuum gets created due to the blocked cap. Air can’t replace the fuel, so it stays suspended. Thankfully, cleaning a cap isn’t difficult.

You can clean your lawnmower cap with:

  • A spare cloth
  • Brushes with stiff bristles
  • Water and heavy duty cleaner*
  • A combination of all three

*Note: Let the cap soak in the water and cleaner mixture for a while. Then only after the cap is cleaned and dried can you put it back on your lawnmower.

After you’ve tested and cleaned your cap you can screw it on and get back to using your mower.

Pro Tip: Use an ethanol-free fuel to avoid many small engine issues.

Make sure your fuel tank cap is not broken or has clogged vents. This can cause the mower to spit fuel from around the gas cap as pressure builds.

Fuel Cap Leaks Caused by a Broken Cap

If you always fill your gas tank according to your manual and keep your gas cap clean, then your problem may lie with the integrity of the fuel cap.

The best way to check if you have a bad gas cap is by examining it closely. Do it have:

  • A crumbling seal?
  • Cracks in the cap?
  • Only a plastic pin on the underside of your cap? (Tecumseh Brand)
  • Problems with tightening the cap?
  • A smell of gas coming from your mower?
  • No clicking sound when you screw the cap into the fuel tank?

If you encounter any of the above issues, then your cap is due to be replaced. Check your owner’s manual to find information on the correct cap to use or tell your local mechanic the make and model of your lawnmower.

Depending on the model of your mower, some gas caps will make a clicking sound when sufficiently tightened.

Remember, even if your fuel cap is new, it can still have problems from the manufacturer so double-check the above signs even with cap is brand new..

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Other Causes of Gas Spitting From A Lawnmower Fuel Cap

The fuel-spewing issues can also occur due to an increase in vibration when using the mower. Usually, the problem is simply compounding the existing issue with a gas cap but let’s go over these quickly.

Any lawnmower will have some degree of vibrations. Naturally, the problem occurs when the motion is exacerbated by things such as:

  • Loose bolts or mounts
  • Broken parts
  • Unbalanced blades
  • A bent crankshaft

These parts can range from cheap to really expensive to fix, but you don’t want to keep running your mower without fixing these issues. Allowing your mower to vibrate excessively while you use it can further damage it, and you can put yourself at risk of bodily harm since parts can dislodge from the mower.

Loose bolts and mounts are easy to fix if you have a socket wrench, so you can tighten those at home. You can also fix unbalanced blades yourself as long as they’re not bent. 

To fix your blade problem, check and make sure the blades are screwed onto your mower tightly and correctly. If the issue is not with how the blade is installed, you will have to do the following to fix it:

  • Make sure your lawnmower is off
  • Unplug your spark plug
  • Take the blade off
  • Hang your blade off a nail on your wall
  • Sharpen the side of the blade that hangs lower on the nail
  • Reinstall your blade
  • Plug your spark plug back in
  • Turn your mower on

This should immediately fix the vibration issue.

If you’re experiencing issues with other broken parts of the mower, like the crank shaft, contact a mechanic. Crankshafts are hard to fix if you don’t have the right tools.

Can You Use Your Mower Without a Fuel Cap?

After reading all of the above, you might be wondering, can you use your mower without the fuel cap? It definitely seems like a quick solution to most of the pressure issues we’ve talked about! While some people say you can run your mower without a gas cap, we do not recommend it.

If you run your mower capless, you’re more likely to spill gas as it sloshes around in the tank, and it might go undetected as you mow. This is a fire hazard (sourceOpens in a new tab.).

Gas caps also keep sediment and debris out of your gas tank. Sediment in the tank can keep your lawnmower from starting or even damage the engine.

Finally, gas caps keep harmful fumes from escaping.

Overall it’s just best not to risk it. 

Conclusion

A problem of gas spitting out the gas tank of a lawnmower is almost always an indicator that there is an issue with the gas cap. Even when the tank is overfilled, a properly fitting cap should prevent this.

Clean it, make sure it isn’t cracked, and if necessary, replace it. Do not ignore this issue or simply run the mower without a gas cap. It’s there for a reason. Purchase a replacement gas cap onlineOpens in a new tab. (link to Amazon) or check your manufacturer’s website.

Paul Brown

Paul has a two-acre yard on red clay soil in Southeast Texas. He knows exactly what the challenges are to nurturing a thriving yard in difficult soil. He takes a practical approach to yard improvement and enjoys putting best practices and “golden rules of lawn care” to the test. Click here for Paul’s author page

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