Quick Answer
Gas can generally sit in a gas tank for up to 3 to 6 months before it starts to go bad and cause problems in your engine. After 6 months, gas begins to oxidize and turn into a substance called varnish, which can clog fuel lines, fuel injectors, and carburetors. Using fuel stabilizers and keeping your gas tank full can help extend the life of gas.
How Does Gas Go Bad in a Gas Tank?
There are a few key ways that gas can go bad when sitting in a gas tank over time:
Evaporation
The lighter elements in gasoline like butane and pentane can evaporate over time when gas sits in a tank. This can cause the gasoline to become denser and not burn as cleanly or efficiently in the engine.
Oxidation
Gasoline is chemically unstable and over time it reacts with oxygen to form oxidation byproducts. The most troubling byproduct is a thick, gummy substance called varnish or lacquer. Varnish can clog carburetors, fuel injectors, and fuel lines.
Moisture Contamination
Condensation can build up inside gas tanks, allowing water to mix with the gasoline. The water can cause corrosion in the fuel system and allow bacteria and fungi to grow. These microbes produce slippery biofilms that can clog filters and small passages in the fuel system.
Dirt and Debris
Dust and dirt can find its way into gas tanks over time. This can quickly clog fuel filters and lead to problems.
How Long Does Gas Last in a Gas Tank?
Under ideal conditions, gasoline has a shelf life of 3 to 6 months when stored in a sealed container. However, gas will usually last longer when left in a gas tank, depending on these factors:
Tank Fill Level
Keeping your gas tank full is one of the best ways to prolong the life of gas. A full tank has less air space, which minimizes evaporation and oxidation. The opposite is true for partially filled tanks.
Additives
Using a fuel stabilizer additive helps gasoline stay fresh longer by up to 12 months. Fuel stabilizers inhibit oxidation and prevent varnish deposits from forming.
Temperature Fluctuations
Temperature swings create condensation inside tanks, leading to moisture problems. Gasoline stored in areas with moderate, consistent temperatures will last longer.
Tank Condition
Old tanks with rust and sediment will accelerate contamination and decomposition of gasoline. A clean, well-maintained tank will help gasoline last longer.
Age of Gasoline
The older the gasoline, the more degradation has occurred. Using fresh gasoline and avoiding storage beyond 6 months is best.
Signs Your Gas Has Gone Bad
Here are some signs that the gasoline in your tank has deteriorated and gone bad:
Difficult Starting
As gasoline oxidizes and creates deposits, it becomes less volatile. This causes hard starting or the need to crank the engine over several times before it starts.
Poor Performance
Bad gasoline burns less efficiently, so your engine may lack power or even stall during acceleration. Engine sputtering is another clue something is wrong.
Rough Idling
Contaminants in old gasoline can cause erratic idling, surging, or stalling as you come to a stop sign or light.
Fuel Filter Clogging
Sediment from oxidized gas collects in fuel filters, causing them to clog more quickly. This restricts fuel flow to the engine.
Corrosion
Moisture causes corrosion in steel fuel lines, aluminum carburetor components, and even fiberglass tanks. Look for signs of this.
Fuel Injector Problems
Varnish deposits coat and clog tiny fuel injector holes, reducing fuel atomization. This further worsens combustion efficiency.
Rotten Egg Odor
Gasoline containing sulfur compounds gives off a rotten egg smell as it ages and decomposes. This means it’s well past its point of usability.
How to Stabilize Gasoline in Your Tank
Adding gasoline stabilizers is the best way to keep gas fresh for longer when stored in a gas tank:
Use Fuel Stabilizer Additives
Fuel stabilizer additives like Sta-Bil are formulated to prevent oxidation, guard against moisture, and combat bacteria growth. Using stabilizers can keep gas fresh for up to 1 year.
Avoid Partially Filling Your Tank
Try not to let your tank drop below half full. This minimizes air space where gasoline evaporation and oxidation occur.
Fill Up with Fresh Gas
Before storing a vehicle, fill it with fresh gasoline. This minimizes air space and puts fresh gas into the tank rather than old deteriorated gas.
Drive Periodically
If storing a vehicle for the winter, drive it periodically to circulate the gas. This prevents moisture condensation and moves sediment around.
Monitor Fuel Filter
Change fuel filters more frequently in stored vehicles. As gasoline oxidizes, more particles clog the filter.
Using Old Gas in Your Car
It’s best to avoid using gasoline that has been stored for longer than 6 months. Here’s what happens if you do:
Hard Starting
Your engine will crank over slower and have difficulty firing up due to the poor volatility of old gas. Repeated cranking could drain your battery.
Rough Running
Contaminants and varnish deposits disrupt smooth combustion. Your engine may run rough or unevenly until the old gas clears the system.
Lack of Power
Oxidized gasoline has less energy content, so your vehicle will lack power and acceleration. Driving uphill or towing will be more difficult.
Stalling or Hesitation
Old gasoline can cause random engine stalling, surging, or hesitation as you drive. This creates dangerous situations when pulling out into traffic.
Misfiring
Poor atomization of bad gas leads to incomplete combustion and engine misfiring. This produces a noticeable jerking sensation.
Carburetor or Fuel Injector Damage
Varnish deposits may permanently clog fuel system components like injectors. This can necessitate expensive repairs.
Is it Possible to Restore Bad Gas?
While you can’t turn back time on gasoline that has oxidized and decomposed, you can try to improve old gas:
Use Fuel Injector Cleaner
An injector cleaner containing polyetheramine can help dissolve some varnish deposits and improve injector spray patterns.
Replace the Fuel Filter
Installing a new fuel filter can catch contaminants and sediments from bad gas as it circulates through the fuel system.
Drain and Refill the Tank
Siphoning out all old gas and refilling with fresh gasoline is the best solution, although not very practical or economical.
Add Fresh Gas
Topping off the tank with fresh gas dilutes some of the old gas. This marginally improves the fuel quality and stability.
Octane Booster
Pouring in an octane booster marginally increases the energy content of old gasoline. It may help performance slightly.
Isopropanol Treatment
Adding 15% isopropanol alcohol can deter moisture absorption in gasoline. But this doesn’t reverse oxidation.
Overall, it’s very difficult to fully restore gasoline that has sat for too long and begun to deteriorate. Replacing the old gas with fresh fuel is the only surefire solution.
Safely Storing Gasoline for Emergency Use
If you have gas stored for emergency generators or other backup equipment, follow these tips:
Use Approved Gas Storage Containers
Store gasoline in portable containers designed for fuel, not random containers like milk jugs which can degrade.
Fill the Container Completely
Leave only about 2% ullage space at the top for expansion. This minimizes air for oxidation.
Add Fuel Stabilizer
Use stabilizers to keep stored gasoline fresh for up to 1 year. Follow treatment ratios on the label.
Store Containers Properly
Keep containers off concrete floors, away from heat sources, and at moderate temperatures between 40°F-85°F.
Inspect Periodically
Check storage containers for leaks, moisture, and other signs of deterioration every 1-2 months.
Cycle Out Old Gas
Use stored gasoline within 1 year and refill with fresh gas to maintain quality. Keeping detailed logs helps.
Proper storage procedures ensure your stored emergency gasoline stays viable as long as possible. Just remember to always use approved storage containers.
Conclusion
Leaving gasoline sitting unused for too long leads to oxidation, moisture contamination, and varnish deposits which degrade fuel quality. While gas can last 3-6 months in a tank, it’s best to use stored gasoline within 6 months to prevent performance problems. Keeping your tank full, using fuel stabilizers, monitoring your fuel filter, and exercising good storage practices can help extend the life of gasoline. But ultimately, the only permanent solution is to drain old gasoline and refill your tank with fresh fuel. Follow these gas storage tips, and your car or equipment will run smoothly.
