Symptoms of Bad Gas in a Car or Truck

By | August 6, 2023

Symptoms of Bad Gas in a Car or Truck 1

If you have a car or truck and suspect it may have bad gas the symptoms can vary depending on how old or bad the gas is.

Gas must be combustible for the engine to fire and run, and the less combustible it becomes, the worse the engine will run, if at all.

Bad gas will have the same symptoms as any fuel-related issue, such as a bad fuel pump, clogged fuel filter, or clogged injectors.

What Causes Fuel to go Bad?

Gas Fuel
Old gas that has been sitting for a long period of time will go bad and become non-combustible.

If water or other contaminants get inside the gas, that can also cause issues.

For example, if the gas tank has corrosion, rust can clog up the fuel filter and lines.
How To Replace a Fuel Tank Sending Unit on a 1978 Dodge Truck Out 2
If water has been mixed with the gas, it will not be usable, and the engine will run very badly if at all.

A vehicle that has been in storage for longer than 6-months may also have issues as gas has a 5-6 month shelf life.

How Long Can Gas Sit Before It Goes Bad?

How Long Can Gas Sit Before It Goes Bad
Adding ethanol to gas became mandatory with the Energy Policy Act of 2005, which requires oil companies to add ethanol to their gasoline.

Ethanol has a shorter life span than a gas-only mixture at 2-3 months.

Modern gas loses combustibility after roughly 5-6 months.

Modern gas mixtures in the USA are 10 percent ethanol and 90 percent gas.

Keep in mind gas is stored before being delivered to a gas station, so the age can vary. Gas is often 1-2 months old when delivered to a gas station.

A chemical process happens to gas as it ages, which makes it non-combustible to an engine.

As gas ages, it will turn into varnish, which is a much thicker substance.

How To Tell If Gas is Bad?

Bad Fuel

  • Very Dark Color
  • Worse Smell than Usual
  • More Thick than Usual

If the gas has been sitting for 5-6 months or longer, it very likely is bad or going bad.

As gas ages, it becomes dark like whisky or Coke-cola and can smell much worse than usual.

Gas also thickens and, as it ages and becomes varnish.

Symptoms of Bad Gas in a Car or Truck

  • Engine Does Not Start
  • Engine Does No Start
    Bad gas can cause the gas to become non-combustible or clog up components such as the fuel filter.

    When the gas is very old, the engine will not start.

    The fuel either will not ignite or is not reaching the engine.

  • Engine Starts But Run Rough
  • Engine Starts But Run Rough
    If the gas is still partially good, the engine may start and run rough.

    It is best not to drive the vehicle but to drain the gas out.

    While it may be possible to let the engine run and use up the partially bad gas, it may still clog the fuel filter and possibly the fuel line and injectors.

  • Engine Stalls Out
  • Engine Stalls Out
    The engine may start and then stall out since the gas does not have enough combustibility to keep it running.

    Sometimes, the engine will run good but suddenly start stalling out and shutting down.

  • Hard to Keep Engine Running When Idling
  • Hard to Keep Engine Running When Idling
    Depending on how old the gas is, the vehicle may start and run, but it is hard to keep it idling.

    When this happens, often pressing on the gas pedal to give the engine more gas can keep it idling.

    If the car or truck is put into great often, the engine will shut down.

  • Hard to Accelerate
  • Hard to Accelerate
    The engine may start and run but not have power like it once did.

    When the gas pedal is pressed, there is a lack of performance, and it takes a while to get going.

What is the Best Method for Gas When Placing a Vehicle In Storage?

  • Storage for Less Than a Year Use a Gas Stabilizer
  • Storage for More Than a Year Drain the Gas Tank
  • If Possible Use Non Ethanol Fuel

If the car or truck is to be stored for less than a year, a gas stabilizer will often work well.
Gas stabilizer
If the car or truck is to be stored for more than a year, it is usually best to drain the gas tank.

It is always best to try and find Non-Ethanol fuel, sometimes called recreational fuel, since it lasts much longer without the ethanol added.

Non-ethanol fuel costs more but can double the life span of the gas.

A gas stabilizer can be used, which can extend the life of fuel.

Some brands claim it will extend the life of gas up to 2 years.

Personally, if I had to store a vehicle for longer than a year, I would drain the gas tank.

Some vehicles have drain plugs at the bottom of the tank, but most do not.

Often, the vehicle can be parked at the storage location and left running until the gas runs out.

What To Do If Gas Is In a Vehicle for a Long While?

  • Use a Gas Stabilizer
  • Drain and Flush the Gas Tank
  • Replace Fuel Tank Sending Unit
  • Replace Fuel Pump
  • Replace Fuel Filter
  • Flush Fuel Lines

If the car or truck is to be stored for more than a year, it is usually best to drain the gas tank.

If a vehicle has been sitting for a long while, it is best to drain and flush the gas tank.
Gas Tank
If it has a metal gas tank, check for rust, as they often can rust up and cause issues.

A rusted gas tank will need to be replaced as, in my experience, it will come back and clog up the gas lines.

It is also best to replace the fuel tank sending unit and fuel pump, which often is the same component.

Corrosion and rust cause them to have reliability issues when they sit in old gas.

If there is an inline fuel filter, it will definitely need to be replaced.

Not all vehicles will have an inline fuel filter, but if there is, buy an extra and swap it out again after the vehicle is driven 100-200 miles.

Besides getting rid of all the old gas, the main goal is to be sure the fuel injectors keep clean, or there will be more issues.

Adding good fuel to bad fuel usually does not work as the gas is very old.

Gas stabilizers also often don’t work well with very old gas.

Often, the results can vary depending on the engine. An engine from the 1970s will have different results from an engine built in the 2020s.

How Do You Know if a Car or Truck Has Bad Fuel?

When a car or truck has bad gas, the engine will run very badly or not run at all.

The same symptoms will occur as when there is a fuel delivery issue, such as a clogged fuel filter.

If the gas has been sitting for longer than 6-months, it likely has gone bad as modern gas has a shelf life of roughly 2-3 months.

Remove the gas cap and smell the gas since Bad gas also smells much worse than new gas.

Category: General Automotive Repair

About Bob Thomas

Bob Thomas is an automotive mechanic with 30 plus years experience. He is a contributor to several publications and has written many articles on automotive maintenance and repairs. He also has created many auto repair videos on YouTube.

2 thoughts on “Symptoms of Bad Gas in a Car or Truck

  1. Mike Pettengill

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