What Happens If Diesel Fuel Is Put in a Gasoline Car?

Putting the wrong fuel into a car is one of the most common fueling mistakes, and accidentally putting diesel fuel into a gasoline car can create confusion about what to do next. The good news is that putting diesel into a gasoline vehicle is generally less likely to cause immediate catastrophic engine damage than putting gasoline into a diesel engine. However, continuing to drive the vehicle can still cause serious running problems and damage to components of the fuel and emissions systems.

The severity of the problem depends on how much diesel was added, how much gasoline was already in the tank, and whether the engine was started after the mistake.

Why Is Diesel Fuel a Problem in a Gasoline Engine?

Gasoline and diesel fuel are both petroleum-based fuels, but they are designed to work in very different combustion systems.

A gasoline engine normally uses a spark plug to ignite a properly mixed gasoline-air mixture. Gasoline is relatively volatile and is designed to vaporize easily so that the fuel can mix with air and burn efficiently.

Diesel fuel is heavier, less volatile, and formulated for compression-ignition engines. A diesel engine injects fuel into highly compressed, hot air, allowing the diesel fuel to ignite without a conventional spark ignition system.

When diesel enters a gasoline engine, the engine is receiving a fuel that does not behave as it should in a spark-ignition combustion system. Diesel may not vaporize properly, the air-fuel mixture can become unsuitable for combustion, and the engine may begin running poorly or fail to run altogether.

What Happens If You Put Diesel in a Gasoline Car?

The symptoms can vary considerably depending on the amount of diesel that entered the tank.

If only a small amount of diesel was accidentally added to a tank that already contains a large quantity of gasoline, the vehicle may initially appear to operate normally. However, as the contaminated fuel reaches the engine, symptoms can begin to appear.

Typical symptoms include:

Difficulty starting

The engine may crank normally but take longer than usual to start. If the diesel concentration is high enough, the engine may crank without starting at all.

Rough running

The engine may shake, hesitate, or run unevenly because the fuel is not burning correctly.

Misfires

Diesel can interfere with normal combustion and cause one or more cylinders to misfire. The check engine light may illuminate as the engine computer detects abnormal combustion.

Poor acceleration

The engine may feel weak and respond poorly when the accelerator is pressed. The vehicle may struggle to increase speed, particularly under load.

Excessive smoke

Because diesel does not burn in the same way as gasoline, contaminated combustion can produce unusual exhaust smoke. Depending on the engine and the amount of diesel present, the exhaust may become darker or have a noticeably different smell.

Engine stalling

A vehicle that initially starts may begin running worse as more contaminated fuel reaches the injectors. Eventually, the engine may stall.

Unusual exhaust smell

The exhaust can have a heavier, oily, or distinctly different odor because diesel fuel is not being burned as intended.

What If You Have Not Started the Engine?

This is the best possible situation.

If you realize that diesel has been put into a gasoline car before starting the engine, do not start it.

Do not even turn the ignition on unnecessarily if doing so can activate the fuel pump and circulate the contaminated fuel through the fuel system.

The safest approach is to have the fuel tank drained and the contaminated fuel removed. Depending on the vehicle, the fuel system may then need to be flushed with the correct gasoline before the engine is started.

In this situation, the mistake is usually much easier and cheaper to correct because contaminated fuel has not yet been circulated through the entire fuel system.

What If You Started the Engine?

The situation becomes more complicated if the engine has already been started.

Once the engine starts, the fuel pump can move the contaminated mixture from the tank toward the engine. Diesel can then reach the fuel rail, injectors, combustion chambers, and other components.

If the engine starts and immediately runs badly, stalls, or begins misfiring, it is best to switch it off as soon as it is safe to do so.

Do not keep driving in an attempt to "use up" the diesel.

This is particularly important when a significant quantity of diesel has been added. Continuing to operate the engine allows more contaminated fuel to pass through the system and increases the possibility of fouling spark plugs, contaminating engine oil through severe misfire, damaging the catalytic converter, or causing other drivability problems.

Can Diesel Damage the Gasoline Injectors?

Diesel fuel can cause problems for gasoline fuel injectors because gasoline and diesel have different physical and combustion characteristics.

The injectors themselves are not necessarily destroyed simply because diesel entered the fuel system. However, contaminated fuel can interfere with correct fuel atomization and combustion.

If the engine runs with severe misfires, deposits and incomplete combustion products can also accumulate in the combustion and exhaust systems.

The amount of diesel matters significantly. A small amount diluted into a nearly full gasoline tank is very different from filling most of the tank with diesel.

Can Diesel Damage the Spark Plugs?

It can cause spark plugs to become fouled.

Gasoline engines depend on the spark plugs to ignite the air-fuel mixture. Diesel does not vaporize and burn in the same manner as gasoline, so contaminated combustion can leave deposits on the spark plug electrodes.

A fouled spark plug may produce:

• Hard starting

• Rough idle

• Misfires

• Poor acceleration

• Reduced engine performance

• Increased fuel consumption

In many cases, the spark plugs can recover after the contaminated fuel has been removed and the engine is supplied with clean gasoline. If they remain heavily fouled, however, cleaning or replacement may be necessary.

Can Putting Diesel in a Gasoline Car Damage the Catalytic Converter?

This is one of the more important potential consequences of continuing to drive with contaminated fuel.

A gasoline engine that is misfiring can send unburned fuel into the exhaust system. The catalytic converter is designed to operate within a specific temperature range and with properly controlled exhaust gases.

Continuous or severe misfires can cause the catalytic converter to overheat. If contaminated combustion continues for an extended period, the catalyst can potentially become damaged.

For this reason, a flashing check engine light should not be ignored after putting diesel into a gasoline vehicle.

A flashing check engine light commonly indicates a severe misfire that can potentially damage the catalytic converter.

Can Diesel Damage the Fuel Pump?

Simply putting diesel into a gasoline vehicle does not automatically mean the fuel pump will be destroyed.

The bigger concern is that the wrong fuel is being circulated through the fuel system and delivered to the engine.

Modern gasoline vehicles can have relatively complex fuel systems, including an in-tank fuel pump, fuel lines, a fuel rail, injectors, pressure sensors, and various control components.

The exact consequences depend on the vehicle's fuel system design and the concentration of diesel in the tank.

What If Only a Small Amount of Diesel Was Added?

This is a common question.

Suppose a gasoline car already has a large amount of gasoline in the tank and someone accidentally adds a small quantity of diesel. The resulting mixture may still allow the engine to operate.

However, that does not necessarily mean it is safe to continue driving.

The smaller the amount of diesel compared with the amount of gasoline, the lower the concentration of contamination. But there is no universal amount that can be considered completely safe for every gasoline vehicle.

Modern engines have different fuel systems, injection strategies, emission controls, and engine management calibrations.

If the mistake is discovered before driving, draining the tank remains the safest solution.

What If the Tank Was Filled Completely With Diesel?

If the gasoline tank was nearly empty and it was filled with diesel, the situation is much more serious than accidentally adding a small amount of diesel to a full gasoline tank.

The engine may:

• Refuse to start

• Start and immediately stall

• Run extremely rough

• Misfire heavily

• Produce excessive smoke

• Lose significant power

• Trigger the check engine light

The correct response is to stop using the vehicle and arrange for the tank and fuel system to be drained.

Trying to dilute a nearly full diesel tank by simply adding gasoline is generally not the preferred solution, especially with a modern gasoline vehicle.

What Should You Do Immediately?

If diesel has been put into a gasoline car, the most important thing is to avoid making the mistake worse.

If the engine has not been started:

Do not start it. Arrange for the contaminated fuel to be removed.

If the engine has been started but the mistake was noticed quickly:

Stop the engine as soon as it is safe to do so and avoid further driving.

If the car has already been driven:

Stop driving if the engine begins misfiring, loses power, stalls, or the check engine light flashes. The vehicle may need to be transported to a workshop so that the contaminated fuel can be removed.

After draining the tank, the technician may inspect the fuel system and determine whether the fuel lines, fuel rail, injectors, spark plugs, and engine oil require additional attention.

Does the Fuel Tank Need to Be Drained?

In most cases, draining the contaminated fuel is the safest solution when a significant amount of diesel has entered a gasoline vehicle.

The exact procedure depends on the vehicle.

Some cars allow relatively straightforward fuel removal, while others require more extensive access to the fuel tank or fuel system. Modern vehicles can also have additional emissions and fuel-vapor components that need to be considered.

After the contaminated fuel is removed, fresh gasoline can be added and the vehicle can be checked for normal operation.

Should You Add More Gasoline to Dilute the Diesel?

Adding gasoline may seem like an easy solution, but it is not always appropriate.

If only a very small quantity of diesel accidentally enters a large amount of gasoline, dilution may reduce the concentration of diesel considerably. However, this does not mean that adding gasoline is always the correct repair.

If a large amount of diesel was added, simply filling the remaining space with gasoline still leaves a significant quantity of diesel in the tank.

For a modern gasoline vehicle, removing the contaminated fuel is generally the more reliable approach.

What Happens After the Wrong Fuel Is Removed?

Once the contaminated fuel has been drained, the vehicle should be checked before assuming that everything is back to normal.

Depending on how long the engine operated with diesel, a technician may check the spark plugs, fuel pressure, injectors, engine oil, and diagnostic trouble codes.

If the engine experienced repeated misfires, previously stored fault codes may remain in the engine control module even after the fuel problem has been corrected.

The engine may also require some time to return to normal operation after clean gasoline is introduced, particularly if the spark plugs were heavily contaminated.

Can You Drive the Car After Adding Gasoline?

If the contaminated fuel has been properly removed and the vehicle starts and runs normally on fresh gasoline, it can generally return to normal operation.

However, if the engine continues to misfire, hesitate, smoke excessively, or display a flashing check engine light, driving should be avoided until the cause has been diagnosed.

A vehicle that runs normally after the contaminated fuel is removed may not have suffered permanent damage. In many cases, the biggest mistake is not the accidental fueling itself but continuing to drive for a long distance afterward.

Diesel in a Gasoline Car vs. Gasoline in a Diesel Car

The two mistakes are not identical.

Diesel in a gasoline vehicle often causes poor combustion, misfires, hard starting, stalling, and fuel-system contamination. Gasoline in a diesel vehicle can be particularly serious because diesel fuel also provides important lubrication characteristics for components in certain high-pressure diesel fuel systems.

Modern diesel engines, especially common-rail systems, can be very sensitive to incorrect fuel.

For this reason, neither type of misfuelling should be treated as something that can simply be ignored.

Can the Engine Be Permanently Damaged?

Yes, permanent damage is possible, but it is not inevitable.

A small amount of diesel that is discovered before the vehicle is driven may result in little more than the cost of draining the tank.

On the other hand, continuing to drive a gasoline car with a heavily contaminated fuel mixture can lead to severe misfires, catalytic converter damage, spark plug fouling, poor lubrication of certain engine components due to fuel contamination of the oil, and other secondary problems.

The longer the vehicle is operated under these conditions, the greater the risk of additional damage.

The Most Important Rule

If diesel has accidentally been put into a gasoline car, do not keep driving just because the car still runs.

The safest response is determined primarily by whether the engine has been started and how much diesel entered the tank. If the mistake is discovered before starting the engine, the problem is usually much easier to resolve. If the engine has already been running, stopping it quickly can prevent a relatively simple fueling mistake from becoming an expensive repair.

In short, diesel in a gasoline car does not automatically mean that the engine is ruined. The most important factors are the amount of diesel, the amount of gasoline already present, and how long the engine was operated with the contaminated fuel. Acting quickly and removing the incorrect fuel is usually the best way to minimize the risk of damage.