You go to start your lawn mower on a Saturday morning, and instead of a healthy roar, you get a sputter or just silence. Often, the culprit is water in gas lawn mower – dangerous engine contamination. It’s a common problem that can stop your yard work in its tracks and, if ignored, lead to costly repairs. This guide will help you understand why it happens, how to fix it, and most importantly, how to prevent it for good.
Water and gasoline don’t mix. In your mower’s fuel system, water sinks to the bottom of the tank and carburetor. Since engines can’t run on water, this causes starting failures, rough operation, and rust. Left unchecked, it can destroy vital metal parts inside your engine. Let’s look at how water gets in there in the first place.
How Water Gets Into Your Lawn Mower’s Gas Tank
Knowing the sources is the first step to prevention. Water usually enters through condensation, poor storage, or contaminated fuel.
- Condensation: This is the big one. When temperature changes cause the air inside a partially empty gas tank to cool and warm, moisture condenses on the tank walls. This water then drips into your fuel. A full tank leaves less air space for this to happen.
- Old or Improperly Stored Gasoline: Gas left in the mower over winter, or fuel stored in a vented container outside, absorbs moisture from the air. Ethanol-blended fuels (most pump gas) are especially good at attracting water.
- Contaminated Fuel Source: Sometimes, the gas can itself or the station’s underground tank has water in it. Using a dirty or rusty funnel can also introduce water.
- Exposure to Rain: A loose or missing gas cap, or simply leaving the mower outside uncovered for long periods, can let rain water seep directly into the tank.
Water In Gas Lawn Mower – Dangerous Engine Contamination
This heading isn’t an exaggeration. Water in the system does more than just prevent starting. It creates a chain reaction of damage. The water itself corrodes steel and aluminum parts, like the carburetor bowl, fuel lines, and even the inside of the cylinder if it gets that far. But the ethanol in modern gas makes things worse. When water is present, the ethanol can separate from the gasoline in a process called “phase separation.”
The ethanol-water mixture sinks, forming a corrosive layer at the bottom of your tank. This layer attacks rubber and plastic components, like fuel lines and seals, causing them to degrade and fail. The resulting gunk and varnish can clog the tiny jets and passages in your carburetor, which are finer than a human hair. This contamination is why a simple drain might not always be enough.
Signs Your Lawn Mower Has Water in the Gas
Your mower will give you clear signals. Watch for these symptoms:
- Failure to Start: The engine cranks but won’t fire up at all. This means the fuel reaching the cylinder is mostly or all water.
- Rough Running & Sputtering: The engine starts but runs unevenly, surges, or sounds like it’s choking. This indicates a mix of gas and water.
- Stalling Under Load: The mower starts and idles okay but dies as soon as you try to engage the blade or move forward.
- White or Blueish Exhaust Smoke: While some smoke is normal at startup, excessive white smoke can signal water being burned in the combustion chamber.
- Visible Water in the Fuel: If you can see separation or droplets in a clear fuel line or in the carburetor bowl, it’s a sure sign.
Step-by-Step: How to Remove Water from Your Lawn Mower
If you suspect water, don’t keep trying to start the mower. You’ll just make it worse. Follow these steps carefully.
Step 1: Safety First
Work in a well-ventilated area, away from sparks or flames. Let the engine cool completely. Disconnect the spark plug wire to prevent any accidental starts. Have a container ready for old fuel.
Step 2: Drain the Old Fuel Tank
Place your container under the fuel line or tank outlet. Most mowers have a fuel shut-off valve; turn it to “off.” Use a siphon pump or carefully tilt the mower to pour the old fuel from the tank into your container. Get out as much as you possibly can.
Step 3: Remove and Clean the Carburetor Bowl
The carburetor bowl is where water collects. Locate the bowl (a small metal or plastic cup under the carburetor). Place your drain pan beneath it. Unscrew the bolt at the bottom of the bowl to drain it. Then, remove the bowl completely by taking out the screws holding it on. Inspect for water, rust, or gunk. Clean it thoroughly with carburetor cleaner and a soft brush. Check the float and needle valve for stickiness.
Step 4: Dry the System
Use a clean, lint-free rag to wipe out the inside of the empty gas tank. You can also use a hair dryer on a cool, low setting to blow air into the tank for a few minutes to evaporate any remaining moisture—just be absolutely sure no fuel vapors are present. Let everything air dry.
Step 5: Add Fresh Fuel and a Treatment
Once everything is dry, reassemble the carburetor bowl. Add fresh, high-octane gasoline (which often has less ethanol) or ethanol-free fuel if you can find it. Immediately add a fuel stabilizer and water remover like HEET or Sta-Bil. These products bind to small amounts of water and allow them to be burned safely through the engine.
Step 6: Prime and Start
Reconnect the spark plug wire. If your mower has a primer bulb, press it 5-7 times. Set the choke, pull the starter cord. It may take a few extra pulls to get the fresh fuel through the system. Once it’s running smoothly, let it operate for 5-10 minutes to circulate the treated fuel.
Prevention: How to Keep Water Out for Good
Fixing the problem once is good, but preventing it is better. These habits will save you time and money.
- Use Fuel Stabilizer Year-Round: Add stabilizer to every gallon of gas you put in your can, not just for storage. It prevents oxidation and helps resist moisture.
- Practice Proper Storage: At the end of the mowing season, either run the tank completely dry or fill it to the brim with stabilized fuel. A full tank leaves no room for condensation air.
- Buy Quality Fuel: Use fresh gasoline from a busy station. Consider using ethanol-free fuel (often sold as “recreational fuel”) for your small engines. It’s more stable and doesn’t attract water.
- Store Your Mower and Fuel Can Indoors: Keep them in a dry, temperature-stable place like a shed or garage. Never leave a gas can outside.
- Invest in a Tight-Sealing Fuel Can: Use a modern, plastic safety can with a good gasket in the cap. This minimizes air and moisture exchange.
- Maintain the Air Filter: A clean air filter ensures proper engine vacuum and operation, helping the engine use fuel efficiently and reducing the chance of old fuel sitting.
When to Call a Professional
Sometimes, the contamination is too severe for a DIY fix. Here’s when to take it to a small engine repair shop:
- If you’ve drained and cleaned the system but the mower still won’t start or run properly.
- If you see extensive rust inside the fuel tank or carburetor.
- If the carburetor jets are permenantly clogged and need ultrasonic cleaning or replacement.
- If you’re not comfortable taking apart the carburetor yourself. Its easy to lose small parts or damage delicate components.
A pro has tools and tanks to fully flush the fuel system and can rebuild or replace a damaged carburetor if needed. The cost is usually less than buying a new mower.
FAQ: Water in Lawn Mower Gas
Can I use rubbing alcohol to remove water from gas?
Yes, but with caution. Isopropyl alcohol (90% or higher) can bind to water and allow it to burn. Commercial “dry gas” products are essentially this. However, adding too much can be harmful. It’s safer to use a dedicated fuel treatment designed for small engines.
How long can gas sit in a lawn mower before it goes bad?
Regular gasoline with ethanol can begin to degrade and absorb moisture in as little as 30 days. After 60-90 days, it’s often bad enough to cause problems. With a quality stabilizer, it can last up to 12-24 months.
What does water in gas look like?
In a clear container, gasoline and water will separate. The gasoline floats on top, and the water (sometimes cloudy if mixed with ethanol) sinks to the bottom. You might see a distinct line or just droplets clinging to the bottom.
Will seafoam help with water in gas?
Sea Foam Motor Treatment is excellent for this. It is both a stabilizer and a water remover. It can help absorb small amounts of moisture and clean varnish. For a tank with a lot of water, draining is still the first step, but Sea Foam is great for maintenance and treating minor contamination.
Can water in gas ruin an engine?
Absolutely. Beyond the immediate running issues, it causes corrosion (rust) inside the fuel tank, carburetor, and fuel lines. Severe corrosion can lead to holes in the tank or irreparable carburetor damage, requiring expensive replacements. In worst-case scenarios, if enough water enters the cylinder, it can cause hydrostatic lock, which can bend a connecting rod and destroy the engine.
Final Thoughts on Protecting Your Equipment
Water in your lawn mower’s gas is a solvable problem. The key is prompt action and consistent prevention. By understanding how condensation and bad fuel habits create the issue, you can take simple steps to avoid it. Always use fresh fuel, add stabilizer, and store your equipment properly. Your lawn mower is a vital tool, and a little preventative care ensures it starts easily every time you need it, season after season. Remember, the few minutes spent on maintenance are always less than the hours and money spent on repairs.