Should you leave gas in lawn mower over winter?

Quick answer

The quick answer is that you should not leave gas in your lawn mower over the winter. Here’s why:

  • Gasoline can deteriorate and gum up over time. This can clog up the carburetor and fuel lines.
  • Old gas has less volatile compounds that help with ignition. This can make it hard to start your mower in the spring.
  • Gas left over winter can separate into lighter and heavier compounds. This can also clog up the mower’s fuel system.

The best practice is to empty the gas tank before storing your lawn mower for winter. Run the engine dry to use up any remaining gas in the system.

Should you leave gas in a lawn mower over winter?

Leaving gasoline in your lawn mower over the winter months is generally not recommended. Here are some key reasons why you should avoid storing your mower with gas still in the tank or carburetor:

Gasoline can go bad over time

Gasoline is a complex blend of hydrocarbon compounds such as pentane, hexane, heptane, octane, and nonane. Some of these light volatile compounds give gasoline its ability to ignite and combust inside your mower’s engine.

Over time, especially if stored for months over winter, these volatile compounds start to break down and evaporate. The gasoline essentially starts to go stale, leaving behind the heavier, thicker, and more complex compounds.

This stale old gasoline becomes less volatile and combustible. Trying to start your mower’s engine with old deteriorated gas can be difficult or impossible. The thickened compounds can also clog up jets and passages inside the carburetor.

Most gas starts to deteriorate after about a month of storage. Letting it sit for 3-6 months over winter makes it much worse.

Old gas has less energy content

As the lighter and more volatile compounds in gasoline evaporate over time, the energy density of the fuel decreases. This is measured as the BTU (British Thermal Unit) content.

Fresh gasoline has around 115,000 BTU per gallon. After prolonged storage, this can drop as low as 87,000 BTU per gallon.

This reduced energy content translates into poorer engine performance. Your mower will run sluggishly or not start at all when trying to ignite old low-BTU gasoline.

Phases separation occurs in old gasoline

Another problem with storing gasoline long term is phase separation. This is where the gasoline starts to separate into distinct lighter and heavier layers.

The lighter phase contains those volatile, combustible compounds needed for proper ignition. The heavier layer contains unstable oxidizing agents that can corrode and clog up fuel lines and jets.

If you leave gas in a mower over winter, this phase separation can render the fuel completely unusable come spring.

moisture can accumulate over time

Even small amounts of moisture contamination in gasoline can cause major problems. Water tends to sink to the bottom of the tank or carburetor bowl.

If moisture accumulates over winter, it can freeze and completely block fuel flow when you go to start your mower after months of storage. This moisture can also cause corrosion issues.

Leaving gasoline in a partially full tank allows air space where condensation can form. This promotes moisture buildup over time.

Oxidation and varnish residues form

Gasoline left sitting for extended periods is susceptible to oxidation reactions. This causes thick, paint-like varnish residues to form.

These oxidation deposits can completely clog up jets, ports, and passages inside your mower’s carburetor and fuel lines. Trying to run this varnished old gas through the system is asking for trouble.

The oxidation rate doubles with every 20°F increase in temperature. So gasoline stored in hot conditions like a shed or garage over summer will go bad very quickly.

Rust and corrosion can occur

Certain compounds in stale gasoline can corrode and rust metal components it contacts, especially if moisture is present.

Small amounts of sulfur compounds, peroxides, and organic acids can promote corrosion inside the tank, carburetor, and fuel lines when left over winter.

This rust and corrosion requires costly repairs and can permanently damage engine components. Removing all old gas before storage helps avoid this problem.

How to properly prepare lawn mower before winter storage

To avoid all the problems caused by leaving gas in a stored mower, here are some tips for proper winterization:

1. Run the mower dry

The first step is to run the engine until it dies from lack of fuel. This uses up all remaining gasoline left in the tank, fuel lines, and carburetor.

Let it sputter and stall out completely to make sure no liquid gas is left anywhere in the system.

2. Drain the fuel tank and lines

After running it dry, disconnect the fuel line from the carburetor inlet and drain any remaining gas from the tank. Capture this in an approved gas container, not just on the ground.

You can also siphon or remove any old gas manually using a turkey baster or small hose. Remove as much as you possibly can.

3. Empty the carburetor bowl

There will still be some gasoline left in the carburetor bowl after draining the tank and lines.Consult your owner’s manual on how to properly drain the carburetor.

Often it involves loosening or removing the bowl drain screw. Empty the bowl contents into your gas container.

4. Dispose of old gasoline properly

Make sure to responsibly dispose of the old drained gasoline. Don’t just dump it on the ground or down a household drain.

Some municipalities have special waste oil/gas recycling centers that accept old fuels. Or search for a hazardous waste disposal site in your area that takes gasoline.

5. Add fuel stabilizer before final run

For even better winterization, add fuel stabilizer to a fresh tank of gas and run that through the mower before storage.

Fuel stabilizers contain antioxidants and corrosion inhibitors that leave a protective coating inside the fuel system.

6. disconnect and remove the battery

The final step is to disconnect and remove any lead-acid battery used with electric start models. Take the battery indoors and connect it to a trickle charger over winter.

A full battery charge prevents electrolyte freezing and permanent battery damage at cold temperatures.

Can you leave gas in a lawn mower for short term storage?

Properly prepared stabilized gas can generally be safely left in a mower’s fuel system for short term storage up to 1-2 months. Here are some tips:

– Add fuel stabilizer to fresh gasoline and run it through the mower. This coats the system to prevent corrosion and oxidation.

– Keep the mower stored in a cool, dark place. Sunlight and heat hasten fuel deterioration.

– Keep the gas tank full to minimize air space and moisture condensation. Consider adding a fuel cap stabilizer.

– Use a fuel additive like Sta-Bil or Briggs & Stratton Fresh Storage to prolong shelf life. Avoid alcohol-based additives.

– Check fuel levels and monitor color. Drain tank if gasoline shows signs of fouling, separation, or discoloration.

– Run engine every 4-6 weeks to circulate stabilized fuel. This prevents varnish deposits inside the carburetor or lines.

– Drain all fuel and run the engine dry prior to longer term storage beyond 2 months.

So with proper fuel stabilizers and periodic operation, gas can be left safely over shorter storage stints. But for winterizing or long term storage, complete fuel drainage is still strongly recommended.

How to get old gas out of a lawn mower

If trying to resurrect a lawn mower with old gas still left inside, here are some tips for removing it:

– First drain the tank, lines, and carburetor completely as previously described. Capture old gas in an approved container.

– For varnish or residue in the carburetor, remove and soak it in carburetor cleaner solution. Rinse thoroughly after soaking to remove all cleaner.

– Run several tanks of fresh fuel mixed with quality fuel injector cleaner. This can help dissolve gum and varnish. Use cleaner made for 2-cycle oil mixes if needed.

– Disconnect the fuel line from carburetor inlet and blow compressed air through it to remove lodged debris. Wear eye protection.

– If the fuel system is severely fouled, replacement of affected components may be needed. New fuel lines, filters, diaphragms, needles, etc.

– Severely varnished carburetors may need to be disassembled and cleaned by a small engine shop with an ultrasonic bath.

– Make sure to drain and properly store your mower at end of season going forward to avoid needing to remove old gas again.

How do you maintain a lawn mower for winter?

Here is a summary checklist for complete lawn mower winterization and storage prep:

– Run engine until it dies from lack of fuel. Use up all gasoline left in system.

– Disconnect fuel line and drain tank. Drain carburetor bowl. Dispose of old gas safely.

– Change oil and filter if due. Dispose of used oil properly.

– Remove battery if electric start model and place on trickle charger.

– Wipe mower body clean. Remove built-up grass clippings under deck with putty knife or brush.

– Inspect and replace worn parts like blades, belts, filters, spark plug, tires, etc.

– For riding mowers: Grease all fittings and lubricate linkages, cables, etc.

– For walk behind mowers: Sharpen blade, lubricate cable and wheel gears.

– Spray with penetrating lubricant on bolt threads and moving metal components.

– Touch up any scratches/rust spots on metal with matching paint.

– Store mower cleaned, dry, and covered indoors – detached garage, garden shed, etc.

– Avoid storing in damp locations. Use fuel stabilizer if storing for short term 1-2 months.

Following this thorough winterizing checklist will keep your mower running like new after storage. Taking proper care prevents problematic deterioration over the dormant winter months.


Leaving gasoline in a stored lawn mower over winter is clearly not recommended, no matter how inconvenient proper drainage may be. The potential downsides of gummed carburetors, phase separation, moisture contamination, and varnish buildup simply outweigh any perceived convenience.

With some effort, you can completely empty the mower’s fuel system and avoid all stabilization or corrosion concerns. This prevents difficult starting, poor performance, or the need for expensive repairs come springtime.

If you don’t have time for full winterization, at very least add fuel stabilizer and run the mower dry. Though not ideal, this avoids some of the worst problems with leaving plain gas sitting unused for months.

Just remember that taking a little time up front to properly winterize your equipment saves much more time, money, and headache down the road. Your mower will reward you with years of reliable service when properly cared for both in season and during downtime.

Fuel Issue Description Effects
Deterioration Breakdown of light volatile compounds over time. Hard starting, poor performance
Lower energy content BTU rating drops as compounds evaporate Lack of power, misfiring
Phase separation Gas separates into varnish and oxidizing layers Blocked jets, clogged filters
Moisture contamination Condensation from air space in tank Icing, corrosion, rust
Oxidation Thick varnish deposits inside fuel system Restricted flow, stuck components
Corrosion Rust inside tank, carburetor, lines Poor starting, reduced performance

Leave a Comment