You’ve seen both sides of the advice online: fill it up before storage, or drain it completely. The correct answer is to fill the tank and add a marine-grade fuel stabilizer rated for ethanol-blended fuel. A full tank leaves minimal air space where condensation can form, and the stabilizer prevents the ethanol in E10 fuel from absorbing moisture and separating from the gasoline.
The one condition worth noting upfront: this recommendation applies to standard seasonal storage. If your jet ski will sit for longer than a year, the approach changes, and that’s covered below.
The Short Answer: Fill the Tank (With One Condition)
Fill the tank before storing your jet ski and add a marine-grade fuel stabilizer rated for ethanol-blended fuel. A full tank limits the interior air space where condensation forms, and the stabilizer prevents E10 fuel from phase-separating during the off-season.
This recommendation holds for seasonal storage of roughly three to six months. For storage beyond twelve months, the guidance shifts, and you’ll find the specifics in the storage duration section.
Why Some Owners Store With an Empty Tank
Some owners drain the tank to avoid stale fuel entirely. The problem with that approach is that an empty metal tank is exposed to interior humidity, which accelerates corrosion on tank walls. Dry fuel system components, including seals, o-rings, and fuel lines, can also degrade without fuel contact over a long winter. A full tank with stabilizer protects both the fuel and the system.
What Happens to Gasoline When a Jet Ski Sits
Three specific things happen to untreated gasoline during storage:
- It oxidizes. Light hydrocarbon compounds evaporate over time, leaving heavier residues that varnish carburetor passages and fuel injectors. That varnish is what causes the hard-starting and rough-idling problems owners notice at the start of a new season.
- Ethanol-blended fuel absorbs ambient moisture through tank venting and condensation. When enough moisture accumulates, the ethanol separates from the gasoline in a process called phase separation. The resulting water-ethanol layer at the bottom of the tank won’t combust properly and corrodes metal components.
- A partially full or empty tank has more air space. More air space means more opportunity for condensation to form on the interior tank walls each time temperatures fluctuate through a storage season.
Why Ethanol Is the Specific Problem in Stored Fuel
Most pump gas in the United States is E10, a blend containing up to 10% ethanol. NMMA has been vocal on exactly this issue, warning that ethanol content above 10% poses serious risks to marine engines and fuel systems, including phase separation, corrosion, and damaged fuel lines and gaskets.
Per ASTM D4814, phase separation can begin within 30 days in humid conditions without treatment. A generic automotive stabilizer won’t adequately address this risk in a watercraft fuel system. You need a marine-specific or ethanol-compatible formula.
The Condensation Argument Explained
Condensation forms when warm, humid air inside the tank cools and water vapor converts to liquid. A full tank leaves minimal air space, which means minimal condensation potential. A half-empty tank cycling through temperature swings over a five-month storage season can accumulate meaningful moisture. Filling the tank is a straightforward way to reduce that exposure.
How to Prep the Fuel System Before Storage
Follow these steps in order to protect the fuel system correctly:
- Top off the tank at the pump with fresh fuel. Do not use gas that has been sitting in a can.
- Add a marine-grade, ethanol-compatible fuel stabilizer at the manufacturer’s recommended dose immediately after filling.
- Run the jet ski for five to ten minutes on the water or using a flushing system. Stabilizer that sits only in the tank does not protect fuel lines, the fuel pump, or the carburetor and injectors.
- Confirm the tank cap is sealed properly before storage.
For stabilizer selection, marine-specific formulas such as STA-BIL Marine or Star Tron are designed for E10 fuel and appropriate for this use. Generic automotive stabilizers may not adequately address phase separation in watercraft fuel systems. Use a marine-rated product.
Check Your Owner’s Manual First
Sea-Doo (BRP), Yamaha WaveRunner, and Kawasaki Jet Ski owner’s manuals each include fuel storage guidance specific to your model. That guidance may specify tank fill level, stabilizer type, or minimum run time after treatment. When OEM instructions differ from general advice, follow the manufacturer’s specification.
Sea-Doo’s owner resources portal and the equivalent Yamaha and Kawasaki owner portals offer free access to current operator’s guides for owners who no longer have the physical copy.
How Storage Duration Changes the Answer
Storage length is the variable that determines whether the full-tank recommendation holds or needs to be adjusted. For pricing context on different storage options, see our breakdown of how much you should pay for storage.
| Storage Duration | Recommended Approach |
| Under 90 days | Full tank with stabilizer is best practice; some manufacturers consider treatment optional in this range |
| 90 days to 12 months | Full tank plus correctly dosed marine stabilizer; the standard seasonal recommendation |
| Beyond 12 months | Drain the fuel system; fog with marine-grade storage oil to protect internal components |
STA-BIL’s published product documentation acknowledges a maximum protection window of up to 24 months under ideal conditions. Marine service technicians generally recommend draining the fuel system for any storage exceeding one year to prevent varnish buildup in injectors and carburetor passages.
What “Stale Fuel” Actually Means for a Jet Ski Engine
Stale fuel is gasoline that has oxidized past usable combustion quality, or fuel where phase-separated ethanol has contaminated the tank bottom.
Symptoms to watch for when the ski returns to use:
- Hard starting. The engine cranks longer than usual or requires multiple attempts before it catches.
- Rough idling. The engine runs unevenly or stalls at idle even after it starts.
- Carburetor gumming. Varnish deposits from oxidized fuel clog small passages, affecting fuel delivery.
- Clogged injectors. On fuel-injected models, the same varnish buildup restricts injector flow, causing misfires or reduced performance.
If you notice any of these after storage, drain and replace the fuel before attempting more invasive repairs, since a fuel-related cause is far more common, and far cheaper to fix, than a mechanical one.
Storage Environment and Ventilation
A full fuel tank stored in an enclosed space is a source of hydrocarbon vapor. Personal watercraft fuel tanks have venting systems that allow vapors to escape, and in a sealed or poorly ventilated space, those vapors accumulate. NFPA 303, the fire protection standard for watercraft storage environments, establishes adequate ventilation as a safety requirement, not a preference. Most storage facility policies reflect this standard.
Practical steps for owners:
- Never store a fuel-filled jet ski in a space with no air circulation, since accumulated vapor has nowhere to go and the risk compounds the longer the ski sits.
- Keep the storage area away from open flames, pilot lights, and electrical panels. Any of these can serve as an ignition source if vapor concentration builds up.
- Passive ventilation through vents or windows is the minimum standard. A space with even basic airflow is meaningfully safer than one sealed completely.
- Active airflow is better. A fan or mechanical ventilation system clears vapor faster than passive venting alone, which matters most in smaller or tightly sealed spaces.
Covered vs. Indoor Storage for Fuel-Filled Watercraft
Where you store a fuel-filled jet ski affects vapor accumulation risk as much as it affects UV and weather exposure. Here’s how the main options compare:
| Sealed Residential Garage | Covered/Semi-Enclosed Storage | Indoor Facility | |
| Vapor accumulation risk | Highest, no natural airflow | Lower, open sides allow natural airflow | Managed by professional ventilation systems |
| UV/weather protection | Full, but incidental | Yes, purpose-built | Yes, purpose-built |
| Who manages ventilation | You | Built into the design | The facility |
| Code compliance | Owner’s responsibility | Generally compliant by design | Meets code requirements |
A covered outdoor or semi-enclosed facility with natural airflow presents lower vapor accumulation risk than a sealed residential garage. Covered storage options allow airflow while protecting the hull and engine from UV exposure and weather. Indoor facilities with professional ventilation systems meet code requirements and remove the need for owners to manage airflow themselves.
Final Thoughts on Storing a Jet Ski With Fuel
Once the fuel system is treated and the tank is topped off, where the jet ski spends the off-season matters as much as how it was prepped. RecNation boat and jet ski storage facilities offer covered and enclosed storage options designed for personal watercraft, with the ventilation, security, and accessibility that keep a jet ski ready to launch when the season opens back up.
Frequently Asked Questions
How long can gas sit in a jet ski before it goes bad?
Untreated E10 gasoline can begin to degrade in as little as 30 days. Phase separation, where ethanol absorbs moisture and separates from the gasoline, can occur within that window in humid conditions. With a marine-grade fuel stabilizer added at the correct dose, treated fuel can remain usable for 12 to 24 months depending on the product and storage conditions. Draining the system is generally recommended for storage beyond one year.
What is the best way to store a jet ski for the winter?
Fill the fuel tank, add a marine-grade ethanol-compatible fuel stabilizer, and run the engine for five to ten minutes so the treated fuel circulates through the entire fuel system. Remove and store the battery separately on a maintenance charger, flush the engine with fresh water if it was used in saltwater, lubricate moving parts, and cover the jet ski with a fitted watercraft cover. Storage in a covered or enclosed facility with adequate ventilation will protect the hull and engine better than open outdoor storage.
Should I winterize my boat with a full tank of gas?
Yes. The same logic that applies to jet skis applies to boats. A full tank reduces interior air space where condensation can form, and a marine-grade fuel stabilizer prevents ethanol-blended fuel from phase-separating during storage. Always run the engine briefly after adding stabilizer so the treated fuel reaches the fuel lines and carburetor or injectors, not just the tank.
Is it OK to leave a jet ski outside during the off-season?
Outdoor storage is possible but introduces more risk than covered or enclosed storage. UV exposure degrades the hull and seat material over time, and temperature fluctuations accelerate condensation inside the fuel tank. If outdoor storage is the only option, use a high-quality fitted cover rated for UV and weather resistance, and place the jet ski on a trailer or stand to allow airflow beneath the hull.
How many miles can a jet ski go on a full tank of gas?
Range varies significantly by model and riding style. Consumer jet ski fuel tanks range from roughly 5 gallons on compact entry-level models up to 20+ gallons on full-size performance models, and typically achieve 50 to 100 miles per tank at moderate throttle. High-performance models ridden aggressively will reach the lower end of that range. Check your specific model’s owner’s manual for the most accurate estimate.
Do I need fuel stabilizer if I’m only storing my jet ski for a month or two?
For storage under 30 days, stabilizer is optional but not harmful. For storage of 30 to 90 days, a stabilizer is recommended because E10 fuel can begin to degrade within that window, especially in warm or humid environments. A bottle of marine stabilizer costs a few dollars. A varnished carburetor or clogged injector can cost hundreds to repair. Adding stabilizer for any storage period longer than a few weeks is worth it.
