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9 Worst Things To Put In A Gas Tank (Explained By Mechanic)

9 Worst Things To Put In A Gas Tank (Explained By Mechanic)
Written By: Kris Jackson, ASE-Certified Mechanic
Kris Jackson has been a mechanic since 2010 after graduating from UTI. He’s worked with several master mechanics and holds several ASE Certifications. You can read more about Kris here.

There is a reason why the name “gas tank” is given to that component of the car… because it’s meant to only hold gasoline! When anything else besides gas is put into the tank by itself or mixed with gasoline, there is going to be trouble.

But there’s a big difference between accidentally adding the wrong octane and ending up with bleach in your tank. So what are the worst things that can end up in your gas tank? 

The worst thing to put in a gas tank is bleach because it’s highly corrosive and acidic, damaging fuel lines and seals. However, diesel, which miscombusts in gas engines, Coca-Cola forms corrosive sludge, and sand or dirt, which clogs systems, are also harmful.

That’s the quick answer, but we’ll take a closer look at 9 of the worst things that can end up in a fuel tank and why they cause so many problems. 

1. Diesel fuel 

Even though this is pretty difficult to achieve, it’s still possible. Fuel nozzles at the pump are designed to avoid making this mistake (at least for putting diesel fuel into your gasoline engine). The diesel fuel nozzle is bigger, green, and won’t fit inside the gasoline filler neck.

However, a gasoline fuel nozzle will easily fit inside a diesel filler neck since it is smaller.

So what happens when you put diesel fuel into your gas tank? Well, it depends on how much reaches your engine but usually, this is more forgiving than if you put gasoline into a diesel engine. Here are a few of the issues you might run into: 

Clogged fuel system

Since diesel fuel is much thicker and denser than gasoline, it tends to clog up the fuel system first before making it to the engine (which is a good thing). The way the system typically flows is gasoline starts from the fuel tank, is pumped out by the fuel pump, and then makes its way to the fuel filter before entering the engine.

Usually, the fuel pump has a tiny pre-filter on the suction side of it that would become ruined and if it makes it past that chances are your fuel filter will be next. If it continues beyond that and makes it to your engine then your fuel injectors will become clogged. Not the cheapest mistake to make and quickly becomes a headache!

Runs roughly

Diesel and gasoline engines combust differently. Diesel engines ignite from the heat generated by highly compressed air while gasoline engines ignite from your spark plugs setting the fuel on fire. Since they operate differently diesel fuel has a lower octane rating compared to gasoline. When used in a gasoline engine, diesel can lead to the fuel igniting incorrectly and running rough.

Fuel system damage

Diesel fuel can deteriorate the seals and gaskets in a gasoline engine’s fuel system due to its different chemical composition. This can lead to fuel leaks and costly repairs.

Correcting the problems caused by using diesel fuel in a gasoline engine can be expensive, as it often involves cleaning the fuel system, replacing damaged components, and addressing any engine damage.

2. Old Gasoline

Maybe you’ve had a gas can filled in your garage sitting for a long time or maybe you’ve somehow got your hands on older gasoline. Well, you might want to think twice before you use it because putting old gasoline in your gas tank is not a good idea

Just like putting diesel into your gas tank by accident, old gasoline has some similar effects on the fuel system and you can end up with:

  • Poor gas mileage
  • Stalling and running rough
  • Poor engine performance
  • Difficulty starting the engine
  • Clogged fuel system due to gum residue forming
  • Increased emissions and in worse case scenarios damage to your catalytic converter

A lot of times when we get classic cars into the shop you can immediately tell if the car has old gasoline just by the smell. It’s difficult to describe the smell in text but it is similar to a strong pungent varnish. Typically fuel that is a year or older might be best to properly dispose of instead of using.

3. Wrong octane fuel

This is an easy one to accidentally put in your gas tank whether it’s from a lack of knowledge, a mistake, or maybe because of fuel prices. Fortunately, this is one of the few that won’t be an immediate problem on your hands and will most likely only cause problems if used over an extended time. 

Knocking and pinging

If you use lower-octane fuel in an engine designed for higher-octane fuel, the engine may experience knocking and pinging. This is because the lower-octane fuel is more likely to ignite prematurely, causing a knocking sound. This can damage the engine over time if continually used.

Reduced engine performance

Lower-octane fuel can result in reduced engine power and overall performance. The engine’s computer may adjust settings to prevent knocking, such as retarding the ignition timing, which reduces power output.

Poor fuel efficiency

Lower-octane fuel can lead to poor fuel efficiency, as the engine may need to use more fuel to achieve the same power output due to the adjusted ignition timing.

4. Water

From here on out on this list are things that you would probably never find yourself putting in your gas tank. Although you wouldn’t, that doesn’t mean there aren’t people out there who might have plans to ruin your vehicle.

An easy, free, and readily available liquid that can find itself in your gas tank is water. While it will still suck if this is mixed in your gas tank there is a good chance it won’t destroy your engine. It might just ruin your week.

Depending on how much gasoline is in the tank before water is added will determine what happens. Usually, it will hit your fuel filter and stop there causing your engine to stall. If by chance it makes it past the fuel filter and to your engine it will most likely run rough and even stall.

Checking to see how much water is in the gas tank is simple. All you need is a fuel sample.

Check this video out to see the procedure on how to check if water is in your gasoline.

5. Sugar

This is commonly thrown around the internet that when sugar is put into your gas tank it will destroy your engine. Fortunately, this is not the case. You may have noticed so far in this article that the fuel filter in the system does a pretty good job of filtering the gasoline before it makes it to the engine.

In the case of sugar when it enters your gas tank it will usually stay there and not dissolve. Since it doesn’t dissolve it will settle to the bottom and stay there. Even if it is disrupted in the tank, the fuel pump and/or fuel filter will clog up and not make it to the engine. Same with the fuel injectors if some sugar does manage to make it past the filters. 

It will still cost money to replace, clean, and fix components but not as much as destroying your engine. 

6. Coca-Cola

Depending on where you’re from you might call it pop or soda but regardless pretty much any soft drink that finds its way into your gas tank is not going to be good. Most of the other things on this list have been somewhat forgiving when looking at the big picture and well this isn’t much different.

A variable to consider is how much gasoline to Coca-Cola mixture there is. If it’s only a little bit you might get lucky to where it will just clog up your system before reaching your engine. If the mixture is leaning on the Coca-Cola side then you might create a very sludgy concoction.

The mixture of gasoline with a soft drink along with high temperatures creates a corrosive sludge that will be sure to eventually lead to some problems. 

Here’s a video of someone testing out what would happen when they put a two-liter of Coca-Cola inside the gas tank so you don’t have to do your own experiment! 

7. Urine 

You’d have to either be dealing with a psycho or really have had to upset someone for them to want to pee into your gas tank. Luckily urine has similar effects as water would have if mixed inside your gas tank. Although I imagine it’s more corrosive than plain water.

Not to keep repeating the same information but it all depends on how much the mixture is of gasoline to urine. Did this person drink an entire gallon of something or was it just a simple “duty calls” as they were walking next to your fuel filler neck?

There is a good chance it won’t destroy your engine. You might not even notice anything unless this person is treating your car as a toilet daily. I couldn’t even imagine catching someone doing this.

8. Bleach

If you’ve ever wondered what happens to a car when bleach is put into your gas tank then as you may have guessed it’s not great. Bleach mostly contains water inside its mixture however the bleach part of the mixture is highly corrosive and acidic. If left over time in large amounts it would wreck your fuel lines and seals.

Most likely though you will experience the same effect as if you’d just mixed the gasoline in your tank with water. If it goes unnoticed for some time your engine might begin to experience excessive sludge but that would have to be run on bleach for an extended amount of time to happen which is unlikely.

9. Sand, Dirt, Or Other Solids

This is something that might be readily available around your car for someone to put inside the gas tank. Although I think it would be easy to tell if your car makes it to the fuel pump that the filler neck is covered in junk.

The outcome would be similar to the consequences of putting sugar into the fuel tank. Most likely the chosen thing put into the gas tank would end up settling into the bottom of the tank and would immediately clog up the fuel pump. This would cause your engine to not even start or stall while driving. It might make it past your fuel pump but is unlikely to make it past your fuel filter…which is a good thing.

What Should You Do If You Think Something Bad Is In Your Gas Tank?

If you have a strong enough concern that there is something in your fuel tank other than gasoline, it’s best to just not drive or start the vehicle at all. The safest option you can do is to have the car towed to a repair shop or somehow get your hands on a fuel sample.

Some of today’s modern vehicles have gone away with a fuel filter that is external to the gas tank. They may put it inside the gas tank or even on the fuel pump itself, which is also inside the tank on certain cars.

It’s just not worth the risk – especially with fuel systems containing more complex components to fail such as a direct injected gasoline engine.

How Can You Prevent This?

Whether it’s someone who’s mischievous or maybe that ex trying to get back at you, you can’t always watch your car. However, here are some things you can do to prevent any issues with your fuel system:

  • Be aware and educate yourself. Determine the octane fuel you need from your car’s user manual and be observant of the fuel you’re selecting at the pump.
  • Invest in a locking fuel cap. It might be slightly inconvenient unlocking it each time, but it ensures no one tampers with your fuel. Many modern cars already come with a locking fuel door.
  • Be a good person or make new friends. It may sound odd, but if you avoid conflicts and disputes, people are less likely to harm you or your belongings.


Hopefully, this guide has given you a clearer understanding of what the worst things to put into your gas tank are and the potential outcomes.

While most items on this list won’t result in permanent engine damage, they could still be problematic. Using anything other than the recommended gasoline for your engine can lead to complications.

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