Gasoline starts to oxidize and go bad once it is exposed to the outside air. How long it will take for you to notice drivability issues largely depends on the conditions in which it was stored, but it could be between three months and two years.
Gas can also go bad if it is contaminated, for example, by water ingress into the car’s fuel tank. On rare occasions, you might even accidentally pump contaminated gas straight from a gas station.
But how can you determine if bad gas has been put in your car? If you’ve got bad gas in your car, you might notice symptoms like reduced engine performance, knocking or pinging sounds from the engine, discolored or cloudy gas, debris in the gas, a check engine light on your dashboard, or even some funky smells coming from your car’s exhaust.
Yep, you guessed it—bad gas can cause significant damage to your engine over time.
Burning contaminated fuel in your engine can cause deposits on your engine’s valves and cylinders. This leads to poor engine performance, decreased fuel efficiency, and potential engine damage.
Now, if you suspect you’ve got bad gas in your car, you’ll need to drain the fuel tank. There are a few ways to do this. If you’re lucky, it’ll be pretty simple – you can use additives, siphon the contaminated gas out with a hose, or simply open a drain plug. If you’re not so fortunate, you might have to drop the fuel tank, which can be time-consuming and costly.
In this article, we’ll explain the main symptoms of bad gasoline, how to avoid it, and how to safely get it out of your vehicle so you can resume driving normally.
Top Reasons Why Gas in Your Car’s Tank Goes Bad
Bad gas refers to gasoline that has become contaminated or degraded, affecting its quality and performance.
There are a few reasons for gasoline to go bad in your car. Some relate to the properties of the gas itself, while others are caused by foreign contaminants finding their way into the fuel.
1. It oxidizes over time
The main reason for gas to go bad is simply time. The moment it is exposed to outside air, the clock starts ticking as it begins to oxidize. Oxidized gas changes from a clear color to a darker one, emits a sour smell and starts to gunk up your fuel system.
While you have no reason to be concerned about the gas that has been sitting for one or two months, if your car hasn’t been driven for six months or more, it can become a problem.
2. It is highly volatile
Gasoline is volatile, which means it evaporates easily. Some components are more volatile than others and can evaporate over time, leaving behind poorer-quality fuel in liquid form to be picked up by the fuel pump. This will lead to reduced performance in several ways.
3. It can be contaminated
Gas can also go bad if it is contaminated, for example, by water. The gasoline sold in most places today is actually E5 or E10, meaning it contains about 5 to 10 percent ethanol. Ethanol has some great advantages, but when it comes to gas contamination, it has one big issue: it is hydrophilic, which means it attracts water.
So, as your gas sits in the tank for too long, it will absorb water vapors and condensation, which will greatly affect its performance. Water from the outside can also find its way into your fuel system via a loose gas cap or a cracked fuel line. You could also pump in contaminated gas directly from the gas station itself, although that doesn’t happen too often.
6 Signs That You Have Bad Gas in Your Car
The thing about bad gasoline is that it doesn’t burn as it should. So, you will experience issues related to unstable combustion, unburned fuel in the combustion chamber, or a lean mixture caused by gunk in the fuel system clogging the fuel lines, fuel pump, or injectors. Here’s a list of some of the drivability issues you may experience if you have bad gas in your tank:
1. Difficulty starting the car
You may first notice something is wrong as soon as you turn the key. Starting is perhaps the most difficult time for the engine, as it requires a perfect combination of air, fuel, and spark to work. If one of the elements in this equation is off, it will have difficulty getting things going, As a result, the engine may crank, but fail to start. Having bad fuel in the tank will make starting considerably harder, or your engine might be unable to start at all.
2. Engine misfires
Bad gas doesn’t burn efficiently, and depending on the quality of the fuel that gets in the combustion chamber at any given time, it may be unable to light up at all. This will lead to unburned fuel that can foul spark plugs and damage the catalytic converter.
3. Acceleration issues
If the fuel is not up to spec, the engine will have to work harder to produce the same amount of power. That means you’ll need more throttle input to maintain the same speed on the highway, for example. You will struggle to go up steep inclines and may need to use lower gear.
When you have bad gas in your tank, you may also notice that acceleration is irregular, meaning you don’t really know what to expect when you press the gas pedal. Sometimes you’ll get a lurch of acceleration, while other times, you may get no response at all.
4. Engine cutting out
The engine might cut off completely if the fuel is bad enough that stable combustion cannot be sustained. Gunk in the fuel system might also clog the injectors, fuel pump, or fuel lines, so the engine won’t have enough fuel pressure to operate. If your engine is stalling randomly, that is a severe drivability issue that can often be attributed to bad gas.
5. Increased fuel consumption
If you see that your car isn’t getting as many miles per gallon as it used to, it might mean you have bad gas. When the internal combustion engine is subjected to increased labor, it consequently exhibits heightened fuel consumption. Also, a lot of fuel goes unburned and exits the engine without producing any positive work. The culmination of these factors manifests in a marked reduction in overall fuel economy.
6. Check engine light turning on in the dash
If your car’s check engine light turns on, it might indicate various problems, including poor-quality gas. Bad fuel can lead to misfires, as previously mentioned. In vehicles equipped with fuel injection, the Engine Control Module (ECM) closely monitors combustion and detects any issues. This triggers the check engine light on the dashboard, alerting you to the problem.
5 Expert Tips to Help You Avoid Contaminated Gas and Extend Your Car’s Life!
Just because gas oxidizes naturally over time doesn’t mean there’s nothing you can do to keep it fresh. Also, while getting bad gasoline from the pump is rare, there are a couple of tricks you can employ to make sure you always get the best gas for your money.
1. Don’t let gas sit in the tank for too long
The best way to avoid having bad gas in your car is not to let it sit undriven for too long. If you are going to store the car for a prolonged period of time, use up as much of the fuel as possible before locking it away.
There are several benefits to keeping your car active other than just having fresh fuel in the tank, so if it has been more than a couple of months since you last topped off, go for a drive and try to burn off some of that unused fuel whenever possible.
If you know you won’t be driving much in the near future, avoid filling up with gas to the brim. Instead, put in just enough fuel for the next month or two.
2. Use fuel stabilizers
If you’re not going to drive the car for a while, you can also pour in some fuel stabilizers into your tank before storing the car. These are simple products that you add through the fuel filler neck and will preserve the properties of gasoline for a much longer period of time before it starts to go bad.
If you’re storing a car for the winter and the temperatures drop into the negatives where you live, also consider using a water remover and fuel anti-freeze product.
3. Fill up at a reputable gas station – and keep the receipt
It’s also important to fill up at good, reputable gas stations, to avoid contaminated gas straight out of the pump. Make sure you keep the receipts at least until you fill the tank up again.
That way, you can prove you are topped up there and safeguard yourself if it is ever found that the station has sold contaminated gas. You may be able to get a reimbursement for your gas and any repair costs you incur in this way.
4. Don’t fill up while the pump is being refueled
Some people claim it’s best to avoid topping up with gas if the tanker delivers or just delivers fuel to the service station. This might sound paradoxical, as you’d be getting the freshest possible gas, right? Well yes, but there is a bigger problem.
Sediments often settle down at the bottom of underground fuel storage tanks. The pickup for the hoses you fill up with at situated higher up so that you don’t put those on your car. The problem is when the fuel truck fills up these tanks and stirs the fuel inside, causing the sediments to rise to a level where you can pump them into your tank.
5. Buy high-quality fuel
While in the pump, make sure you fill up with gas that meets your engine’s minimum octane rating. While gas with a lower octane rating doesn’t count as “bad” fuel, it might cause your car to run poorly due to detonation, or the system could pull the ignition timing to compensate, increasing fuel consumption while decreasing power output.
Remember that some engines might benefit from using an octane rating above the recommended minimum, while others won’t. It all depends on how sophisticated the engine control systems are and whether the ECU can advance the timing to make use of the higher detonation resistance.
That said, higher-octane fuel often has more and better additives, making it a higher quality fuel overall that all cars can benefit from. Take a look at the fuel information at the pump to determine if that is the case at your service station.
How to Remove Bad or Old Gas from Your Car’s Tank
So you have concluded you have bad or contaminated gas in your fuel tank. What now? There are a few ways to get it out. These range from relatively simple and cheap methods, like siphoning or opening a drain plug, too much more time-consuming and potentially expensive methods, like dropping the fuel tank altogether.
Before you start, remember that gasoline and its vapors are highly flammable, so avoid spillage and keep ignition sources away during the process.
1. Open the drain plug
Unfortunately, unlike an oil plug, modern cars don’t have a simple plug you can unscrew to let gas drain out of the tank. While this is a good theft deterrent and one fewer source of leaks, it makes your life harder if you want to drain the tank. Old cars from the 80s came with metal fuel tanks that might have a drain plug, so if you own one, you are in luck.
Raise the car high enough that you can fit your container under it. Place it under the drain plug and use a funnel to make sure there’s no spillage. Open the drain plug using a wrench and socket of the correct size, then let the tank empty its contents into your receptacle. Be careful as gasoline might initially gush out at high pressure, depending on the fuel level in the tank. Replace the drain plug once you’re done draining the tank.
2. Isopropyl alcohol or HEET additives
If your gas is bad because it has water in it, then you can try some water-removing additives. The more common ones are Isopropanol (or Isopropyl alcohol) and HEET. The latter is essentially pure methanol, and both are 100% water soluble.
The idea behind these products is that they absorb water and run it harmlessly through the system, where it can be burned normally. As an added advantage, HEET also acts as an antifreeze, so it should prevent the water in your fuel from freezing when the temperature drops below zero degrees Celsius. That makes it a good additive to add to your gas along with fuel stabilizers if you are going to store away a vehicle for the winter months.
3. Siphon it out
One of the easiest ways to remove fuel from a car – as any petty thief worth their salt will know – is by siphoning it. We’re not going into the full physics of how siphoning works here, but essentially it is used to elevate a liquid from a container upwards through a tube and then down into another container at a lower level from the first. Once the fluid flow has started, it continues to drain only through the force of gravity, without the use of a pump.
This is a straightforward procedure that, in its most basic form, only requires a length of garden hose. Here’s the step-by-step guide.
Remember that most modern cars are equipped with anti-siphoning devices or other measures that make siphoning harder or even impossible. If that’s the case with your vehicle, you’ll need to try a different method.
4. Disconnect the fuel filter and run the fuel pump
If you are in a pinch, this is another way to get water or bad gas out of your tank, but it’s a bit more convoluted process. The idea is to use the vehicle’s own fuel pump to push out the bad gas. Locate your fuel filter and disconnect the hose upstream of the filter (on the fuel pump side). Place the end of the hose in a clear container and activate the fuel pump until all the water or bad gas is drained out.
You can activate the fuel pump in your car in a couple of ways:
Turn the ignition to the “on” position until the lights turn on in the dash but before you start the car.
This will cause the fuel pump to “prime” for a few seconds so the engine has enough fuel pressure to start the car. It will turn off after around five seconds or so, at which point you will have to turn the ignition off and on again to restart the pump. Repeat this process until all bad gas is pumped out of the system.
Bypass the fuel pump relay
Water is denser than fuel, so it should deposit itself on the bottom of the tank, where the fuel pump pickup is located. That means this method should pump out the water in your tank first. Having a clear container will allow you to see when you stop pumping out water and start to pump out actual gas.
If your goal is simply to pump out water in your tank, you can stop at this point. Otherwise, you can continue pumping until all the bad gas is out of the tank and into your container. Keep in mind you will need very large or multiple containers if your tank has a lot of gas or water in it.
This process has a few disadvantages. The first of which is that you are pushing bad gas and/or water through your fuel pump, potentially damaging or clogging it. The second is that running the tank dry will overheat the pump, so you must be careful to stop the process before the tank is completely empty. You don’t want to replace your fuel pump on top of all the other expenses you might incur due to your bad gas problem.
5. Drop the fuel tank
If all else fails, you might have to drop the fuel tank to remove the bad gas from it. This complex job can’t be explained in a paragraph or two. If you feel confident enough to do this on your own, refer to your car’s repair manual. Otherwise, this undertaking is best left to a professional mechanic in a well-equipped workshop.
Will bad gas ruin your engine?
Generally, bad gas won’t damage your engine permanently, but it will cause it to run badly and misfire, which can damage the catalytic converter or foul your spark plugs. If you are unlucky, deposits can block a fuel line and cause the mixture to lean. If this happens during hard acceleration, especially with a boosted car, it could result in knock and catastrophic engine failure.
What does bad gasoline look like?
Good gasoline has a clear, off-yellow color. Bad gas will be much darker and look more like apple cider. It also has a sour smell.
Will adding new gas to old gas help?
Adding fresh, good gas to a tank with bad gas can help in certain situations, depending on how bad the gas is in your tank. However, to avoid wasting money on good gas in vain, it’s better to drain out the bad gas before pumping in some good, high-quality premium fuel.
How long does it take for gasoline to go bad?
Gas doesn’t go bad immediately at one point in time; instead, it worsens gradually as time passes. As such, it’s not possible to determine the exact moment when “good” gas becomes “bad” gas. However, you could start seeing drivability issues as soon as three months after the car has been driven or as long as two years or more if you have used fuel stabilizers. It all depends on the environment and the conditions in which it was stored.
Will the check engine light come on with bad gas?
Yes, the check engine light will likely come on if you attempt to drive with bad gas. That’s because bad gas doesn’t burn as it should, which can cause the engine to misfire. The engine control unit will generally be alerted to this and trigger a CEL to inform you of something wrong.
Victor is a Mechanical Engineer BSc and certified Automotive Mechatronics technician from Portugal. He worked as an intern mechanic for Volvo and has been fixing his own vehicles for over 10 years. Writing and cars are his passion, so now he combines the two by creating content around the automotive industry. He specializes in automotive technology and maintenance.