How do I know if gas is bad?

Gas that has gone bad or is contaminated can cause major issues for your car’s engine and performance. Here are some quick answers on how to tell if the gas you are using might be bad:

  • Check the smell – Fresh gas should have little to no smell. Rancid or strong odors likely mean contamination.
  • Look at the color – Good gas is clear/light yellow. If it’s dark or cloudy, that’s a red flag.
  • Pay attention while pumping – If the flow is slow or keeps stopping, could be sediment buildup.
  • Monitor your engine – Problems like hard starting or stalling could indicate fuel issues.

Knowing the signs of bad gas can help you identify potential problems and deal with them promptly before major damage occurs.

How Does Gas Go Bad?

There are a few key ways that gas can go bad or become contaminated over time when stored in a gas tank, container, or your vehicle’s fuel system:


As gas sits, the more volatile components can evaporate. This leaves behind less volatile elements that can cause issues, like:

  • Varnish/gum deposits
  • Sediment
  • Thickening

Even a small amount of evaporation can increase the octane rating of the gas and affect performance.


Contact with oxygen can cause the gas to oxidize. This forms gums, varnishes, and insoluble sediments. Oxidized gas has decreased volatility and combustibility.

Microbial Contamination

Microbes like bacteria and fungi may breed in gas tanks/storage and produce byproducts that clog fuel systems and damage engines. Higher sulfur fuels are more prone to “microbial contamination.”

Mixing with Water or Other Fluids

Water and other liquids like diesel or oil can mix with gas due to condensation or leaks in storage tanks. This contamination reduces combustion ability and can corrode fuel lines/injectors.

Chemical Changes Over Time

Compounds in fuel may degrade over months of storage. Contaminants can also leach into gas from storage containers. These changes can impact viscosity, vapor pressure, octane level, volatility, and other properties.

Signs Your Gas May Be Bad

Watch out for the following indicators that the gas you are using may have gone bad:


  • Cloudy or Dark Color – Good gas is typically light yellow or clear. Over time it can turn brown or black as oxidation occurs.
  • Particle Contamination – Sediment or water contamination will make the gas appear cloudy or dirty.
  • Oily Sheen – Gas mixed with other fluids like oil or diesel may leave an oily film on the surface.


  • Rotten Eggs Smell – The additive mercaptan added to natural gas and propane gives it a distinct sulfur/rotten egg odor when gas goes bad.
  • Gasoline Odor – Rancid or strong gas fumes signal contamination and chemical breakdown.

Performance Issues

  • Hard Starting – Fuel that doesn’t ignite properly makes it difficult to start the engine.
  • Engine Stalling/Misfires – Contaminants and loss of combustibility cause stalls, especially under high load.
  • Power Loss – Reduced volatility and lower energy content gives less engine power.
  • Knocking Sounds – Bad gas can cause premature combustion (engine knock) which makes loud pinging or knocking noises.

Fuel Pump and Injector Problems

  • Trouble Pumping – Sediment/gum clogs pumps and fuel filters causing low flow.
  • Leaking – Corrosion from water/contaminants causes leaks and damage in fuel system components.
  • Flow Interruption – Lack of lubricity in contaminated fuels damages injector pumps and causes intermittent fuel delivery issues.

Engine Damage

Prolonged use of contaminated fuel can lead to:

  • Corroded tanks/lines
  • Gunked up injectors
  • Ruined pistons/cylinders
  • Scored valve seats
  • Clogged catalytic converter

How Long Does Gas Last in a Car’s Tank?

In general, gas lasts 1-3 months in your car’s gas tank before going bad, but many factors impact shelf life:

Factor Impact on Fuel Shelf Life
Tank Level Higher tank level = longer lasting. Less airspace = less oxidation.
Fuel Additives Additives like stabilizers help preserve fuel quality longer.
Temperature Heat speeds evaporation and oxidation. Keep tank/garage cool.
Gas Composition More complex fuel molecules break down faster.
Age of Your Car Older tanks/lines allow more water condensation and leaks.

Tips to extend gas life in your car:

  • Fill up more frequently to keep tank full
  • Drive regularly to cycle fresh gas through the tank
  • Add fuel stabilizer each fill-up
  • Keep tank cool and protect from sunlight/moisture

Does Gas Go Bad in Storage Containers?

Stored gas can go bad after just a few months. The small airspace of jerry cans accelerates fuel deterioration.

Factors impacting how long gas lasts in storage:

Factor Effect on Gas Storage Life
Container Type Porous plastic/metal allows more air contact and evaporation vs sealed metal cans.
Oxygen Absorbers Oxygen absorber packets help remove oxygen and extend shelf life.
Fill Level More ullage (airspace) allows more fuel evaporation and degradation.
Opening Cans Each opening exposes gas to more oxygen and moisture.
Temperature Heat accelerates fuel oxidation, evaporation, and microbe growth.

Tips for storing gas properly:

  • Use sealed metal containers
  • Fill containers as fully as possible
  • Use fuel stabilizers and antioxidants
  • Store in a cool, dark place
  • Avoid opening container repeatedly

Rotate stored gas into your vehicle every 3-6 months.

How to Tell if Gas is Bad When Pumping

Be alert for issues when pumping gas that could signal potential fuel quality problems:

1. Slow/Stopping Flow

  • Sediment or varnish buildup can clog filters and slow fuel flow from the pump.
  • May have to keep stopping and restarting the pump as flow sputters.
  • Lower flow can indicate microbial contamination in tanks.

2. Pump Keeps Clicking Off

  • Problems like debris-clogged intake valves, failing pump motors, or vapor blockages can repeatedly stop the pump before tank is full.
  • Fuel flow issues will make the pump shut off prematurely.

3. Leaks Around the Pump

  • Worn or damaged pump hoses and seals can leak fuel during dispensing.
  • Corroded underground tanks may have cracks allowing moisture intrusion.

4. Sputtering Nozzle

  • Contaminated fuel with particles can disrupt steady fuel flow from the pump nozzle.
  • May splatter or have surging/interrupted flow.

5. Strong Odors

  • Aromatic hydrocarbons added to gas for octane can produce toxic fumes if fuel mixture is off.
  • Stale or rotten smells signal oxidized/contaminated old gas.

If you notice any off signs when pumping gas, the station may have a fuel quality issue. Avoid using that pump or station until they resolve the problem.

How to Treat Bad Gas in a Car

If you have a strong suspicion your car has bad gas, here are ways to treat it:

1. Add Fuel Injector Cleaner

  • Using a good fuel injector cleaner like Techron or Red Line can help clean deposits.
  • Run through full tank immediately to treat whole system.
  • Do NOT just add to a partially full tank.

2. Drain the Tank

  • Draining the entire fuel tank is the only way to truly remove all bad gas.
  • This can be challenging – may need a mechanic’s help.
  • Drop the tank and pump out all fuel, flush with fresh gas.

3. Dilute the Fuel

  • Add several gallons of fresh high-quality gas to dilute the ratio of bad fuel.
  • The more you can add, the more it will minimize issues.
  • Use a fuel stabilizer like Sta-Bil to help treat.

4. Use Fuel Conditioner

  • Fuel conditioners like Heet improve fuel quality, boost volatility, disperse water, and prevent corrosion.
  • Add regularly when you suspect contaminated gas.
  • Helps counteract some problems until you can drain tank.

5. Replace the Fuel Filter

  • A clogged fuel filter can compound problems of bad gas in the system.
  • Replacing it gets rid of built-up gunk and ensures good flow.
  • Cheap maintenance that improves performance with bad fuel.

Treating sooner rather than later will minimize any damage from contaminated gasoline in your vehicle.

Signs of Bad Gas Health Effects

Gasoline exposure can irritate the eyes, skin, and lungs. Signs of health effects include:

  • Eye redness, burning, or swelling
  • Skin redness, drying, or chapping
  • Cough, sore throat, or runny nose
  • Headaches, dizziness, or nausea
  • Breathing difficulty

Symptoms may result from:

  • Raw fuel exposure – Liquid gasoline or concentrated fumes can cause chemical burns and intoxication.
  • Combustion byproducts – Carbon monoxide, sulfur oxides, nitrogen oxides, and other emissions are hazardous.
  • Fuel system leaks – Evaporated vapors contain toxic chemicals like benzene, toluene, and xylene.
  • Microbial contamination – Microbe waste products include acids, alcohols, and neurotoxic ether.

Seek fresh air and medical attention if you experience bad gas exposure effects. Prevent contact when fueling and avoid breathing fumes.


Bad or contaminated gas can create serious drivability and engine issues if used in your vehicle. Knowing how to recognize potential warning signs – like smell, appearance, pump behavior, and vehicle performance – allows you to identify problematic gas and deal with it promptly. Implementing preventive measures such as draining tanks before storage, using fuel stabilizers, not storing gas long term, and regularly cycling fresh fuel through your car will help minimize the chances you end up with gas that has gone bad. Be vigilant when buying gas and watch for any indicators of fuel quality issues. With care and caution, you can keep your car running smoothly on high quality fuel.

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