What Happens if You Put the Wrong Fuel in Your Car?

Whether it was curiosity, or necessity led you to think of it, you’ve certainly questioned yourself before what happens after pumping the wrong fuel in your car. Is it still safe to drive your car after the deed is done?

What happens if you put the wrong fuel in your car depends on how much of it you’ve put, which kind, the fuel your engine runs on, and the ratio between the right and the wrong fuel.

Why Would I Put the Wrong Fuel in My Car?

Most of the time, misfuelling happens by accident.

Different gas stations located around the same area may have different nozzles or label colors for the same fuel, or the person refueling may be unfamiliar with the car.

No matter what’s the reason behind it, frequent human distraction makes unintentional misfuelling plausible.  

There really isn’t any good reason to put the wrong fuel in a car, but the truth is that misfuelling is a fairly common occurrence.

What Classifies as Putting the Wrong Fuel? 

Putting the wrong fuel doesn’t always translate into pumping diesel into a gasoline tank, or vice versa.

The same kind of fuel can actually have different properties, and this is the case with gasoline.

There are different octane grades recommended for different engines, therefore, not every type of gasoline is best suited for every gasoline engine.

Octanes in a Nutshell

what happens if you put the wrong fuel in your car

Octane grades are a way to measure how much compression a fuel withstands before igniting.

The higher the octane grade, the less likely the fuel is going to explode unexpectedly at higher pressures and damage the engine.

Premium fuel contains higher octane grades and, on paper, it’s better for the engine.

In reality, unless you’re driving a car with a high-performing engine that’s very sensitive, it’s not mandatory to use premium fuel.

The Most Used Octane Grades Across the U.S

Regular Generally 87-88 (lower octane grade)

Midgrade 89-90 (Middle range octane grade)

Premium Between 91 and 94 (Higher octane grade)

Higher octane grades have a positive effect on high-performing vehicles, allowing better ignition timing, controlled explosions, and greenhouse gas reductions.

Which Octane Grade is Better for My Vehicle?

Of course, the fuel instructed in the owner’s manual should always be your first choice.

However, if it’s not available, it’s better to refuel the tank with a high octane grade fuel.

This is to prevent “engine knocking”, resulting from filling the tank with lower than recommended octane grade fuel.

What is Engine Knocking?

When fuel mixes with air inside the engine and the octane grade is lower than optimal, the mixture can combust too early, producing an audible “knock” sound.

This combustion can potentially result in a damaged engine if one keeps driving the car under these conditions long enough.

It’s not wise to put a lower than recommended octane grade fuel in your tank unless the owner’s manual specifically allows this practice.

What Happens if I Put the Wrong Fuel in My Car?

Misfuelling often happens by pumping gasoline into a diesel car, or vice versa.

Other types of misfuelling are not as common as both of these examples.

To better understand what happens if you put the wrong fuel in your car, it’s good to know what makes fuels different from each other.

Both gasoline and diesel come from crude oil but have different physical properties.

Gasoline is thinner than diesel and has a stronger smell. It plays its part in a car as just fuel.

Diesel on the other hand is thicker, doesn’t smell as much as gasoline and it’s not just fuel, it also acts as a lubricant for the fueling system and engine alike.

Gasoline engines use spark plugs to ignite the fuel whereas diesel engines ignite diesel with heated compressed air. 

Due to differences in engine architectures and fuel properties, engines running on any of these fuels are designed to only take in their specific type of fuel.

Here’s what happens if you put the wrong fuel in your car:

Diesel in a Gasoline Engine

what happens if you put the wrong fuel in your car

The fuel pump will have a hard time moving Diesel through the fuel system because it’s thicker than Gasoline.

Any amount of diesel that’s able to pass through the fuel system and make its way to the engine will clog up the fuel injectors and sooner or later, make them inoperable.

Depending on the right to wrong fuel ratio, a gasoline engine may “apparently” run on diesel for a while before burning through the gasoline still left in the tank.

Once the gasoline vanishes and the fuel system is overloaded by diesel, sooner or later, the engine will stop the activity.

As far as damage goes, diesel is heavier and less flammable than gasoline, so pouring diesel in a gasoline engine is not as big of a threat as the other way around.

If the mistake is promptly noticed and the quantity is low enough, both the fuel system and the engine will be up and running after having fully drained the wrong fuel.

Gasoline in a Diesel Engine

what happens if you put the wrong fuel in your car

Unlike diesel, gasoline will not ignite with hot compressed air, it needs a spark, thus the need for spark plugs in gasoline cars.

Diesel cars don’t have spark plugs so there must be a high diesel-to-gasoline ratio inside the tank for the gasoline to ignite in the first place.

If you only refill when running low on fuel, it’s likely that your car will be alright. However, when mixing gasoline with high levels of diesel things can go sideways very fast.

Gasoline running through a diesel fuel injection system will not only contaminate it but at the same time, decrease its components lubrication properties.

Under the lack of lubrication, metal components in the engine and fuel system will start rubbing together and breaking into smaller pieces.

While not immediately noticeable, these small pieces will be running freely through the system until they cause permanent damage.

Gasoline in a diesel engine will also cause unpredictable detonation the engine is not prepared to handle, resulting in vital components being destroyed.

Since gasoline has a hard time igniting inside a diesel engine, large quantities of unburned fuel will come out of the exhaust in the form of black smoke.

Not only does this pollute the air, but it’s also an indicator of an impending system clog.

At the end of the day, these are extreme scenarios. Misfuelling is very common and shouldn’t be a serious problem unless it’s ignored for a long period of time.

What to do After the Wrong Fuel is Inside the Tank

what happens if you put the wrong fuel in your car

The outcome of what happens if you put the wrong fuel in your car is dependent on a few key points:

  • Quantity
  •  How long until you notice the mistake
  •  How quick you take action to dispose of the wrong fuel
  • Whether you consult a mechanic afterward or just let it go

You can possibly save yourself thousands of dollars in repairs just by catching the mistake in a timely fashion.

Once you put the wrong fuel there’s no going back, here’s what you should do immediately after:

  • Do not start the engine. If you’re lucky enough to catch the mistake at the gas station, make sure your engine stays turned off, this will prevent the wrong fuel from leaving the tank and reaching the fuel system.
  • Move the car away. Make room for other customers and move your car out of the way while in neutral gear. Ask the gas station staff for help if you don’t have passengers riding with you.
  • Call your auto insurance provider. Make sure to clearly explain what happened, chances are that your insurance company will have on-the-road services available to help you.
  • Call a specialist misfuelling service. If your insurance doesn’t cover fuel drainage, proceed to call a specialized service.

What if The Mistake Isn’t Immediately Noticed at the Gas Station?

The longer you ride your car with the wrong fuel, the higher the chances of causing irreversible damage to your engine and fuel system will be.

If you act as soon as you notice the mistake, permanent damage can be avoided even if you’ve pushed your luck a little

It’s important to seek professional help if you drove more than you should with the wrong fuel cycling through the system.

Oftentimes, the true extent of damage is easily overlooked until it builds up to a point where all hell suddenly breaks loose.

The Bottom Line

It’s not muchly predictable what happens if you put the wrong fuel in your car. What’s known is that the longer you drive your car under these conditions, the more costly the damage will be.

If you take action as soon as the mistake is noticed, chances are that your car will be completely fine after just draining the fuel system.

Nevertheless, it’s still a good idea to have your car checked by a professional to know the true extent of the damage.

Remember, you could potentially cost yourself several thousand bucks just by being careless after misfuelling.

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Make sure to read our article on whether you should buy a car with over 100k miles or not.


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