What happens if you put diesel in a petrol car? Effects and remedies

If you own a car or have a fair knowledge about automobiles, you may be well aware that mainly there are two types of car engines that use two types of fuel. Yes, we are talking about petrol and diesel engine cars. Misfuelling is one of the most common incidents that may happen due to human error while filling fuel in cars. So, what happens if you put diesel in a petrol car, or use petrol in a diesel engine? Read on as we answer this question in this article.

What happens if you put diesel in a petrol car or vice-versa?

Misfuelling is one of the most common mistakes at the refuelling stations. It may be due to lack of knowledge, misunderstanding or mistake. The reason may be anything, but the consequences are unavoidable once there is wrong fuel in the vehicle.

Both petrol and diesel fuels have different properties. Hence, the working mechanism of a petrol and diesel engine is different. For instance, the petrol engine uses spark plugs to ignite the air-fuel mixture, while the diesel engine uses compression to ignite the fuel. So whether it's petrol in a diesel car or diesel in a petrol car, it will end up damaging the engine's internal components. The following sections will explain the consequences of misfuelling a vehicle.

What are the consequences of misfuelling?

Whether you end up filling petrol in a diesel car or the other way around, the consequences are inevitable. However, the magnitude of implications may differ as per the engine type  — petrol or diesel. Below we explain both the scenarios and their consequences.

1. Diesel in a petrol car

In reality, it’s difficult to end up with diesel in a petrol car. That’s because the diesel fuel dispenser nozzle generally does not fit into the petrol car’s fuel filler nozzle. However, if you somehow fill diesel in a petrol car, it may cause damage to the engine’s internal components and fuel lines.

Petrol is a thin fuel and diesel is thick. Hence, the petrol engine is not designed to cope with the properties of diesel. If you crank a petrol engine with diesel in the fuel tank, the fuel filter may clog up because it’s not designed to handle the greasy property of diesel. 

Next, if the fuel somehow moves any further, it can clog up the fuel injectors. The spark plugs may also fail due to the soot build-up due to the mixture of petrol and diesel.

Symptoms of diesel mixed with petrol

• The fuel may not reach the engine due to the clogged up fuel filter. 

• It results in an engine misfire, and it begins to stutter and stall.

• You may also see a lot of white smoke coming out from the exhaust pipe.

• Ultimately, the engine will lose all its power, and you will not be able to fire it up.

2. Petrol in a diesel car

The probability of filling petrol in a diesel car is high as the petrol dispenser nozzle is smaller, and it can easily fit into a diesel fuel tank nozzle. It is also an unfortunate scenario as petrol in a diesel engine can cause extensive damage to the internal components.

In a diesel engine, the compression ignites the fuel. Hence, the fuel enters the internal parts of the engine. It also acts as a lubricant. So, if petrol enters the engine, the drying properties of the fuel may damage the components. Mainly, the fuel injection pump, which relies on the lubricating property of diesel, may not function properly.

The worst part is, it's not easy to know the presence of petrol in a diesel car. And by the time you realise it may cause significant damage to the engine.

Symptoms of petrol mixed with diesel

• As mentioned above, it’s tough to determine the presence of petrol in a diesel car.

• It is likely that you realise the mistake when you see black fumes coming out of the exhaust pipe.

• The unburnt fuel causes the black smoke.

• The engine will then stutter and will lose power.

• After that, you will not be able to start the engine.

What to do if your car is filled with the wrong fuel?

Misfuelling is an unfortunate incident, but it is better to know what to do and not to do when it happens. Knowledge of proper remedial measures can help limit the damage. So, in case you end up with the wrong fuel in your car, you can follow the steps mentioned in the below scenarios.

1. Diesel in a petrol car

While filling up fuel 

• If you realise the mistake while filling diesel in a petrol car, there’s not much to worry about. 

• Because if you fill diesel for less than 5% of the total tank capacity, all you need to do is fill the rest of the tank with petrol. 

• Since diesel easily mixes with petrol, you can continue driving, and there should not be any problem.

• However, to avoid fuel contamination, it’s better to drain the diesel out of the tank and flush it with petrol. But then it requires you to tow the vehicle to a service station.

After filling up fuel 

• In case you realise the mistake after filling diesel for more than 5% of the tank capacity, the situation can be controlled. 

• All you have to do is not start the engine and call for a tow service.

• Tow the vehicle to the nearest service centre and drain the fuel tank. Once done, flush the tank and fill it up with petrol, and you are good to go.

After cranking the engine

• If you have started the engine after filling diesel in a petrol car, turn off the engine immediately. 

• Tow the vehicle to a service centre, drain the fuel and flush the tank. 

• Since you also cranked the engine, the fuel lines need a thorough flush with petrol. You may also need to clean the spark plugs. 

• After that, you can refuel with petrol and drive without any problems.

After driving the car for some time

• It is the worst out of all the mentioned scenarios. The first thing you should do is immediately stop the vehicle and turn off the engine. 

• Call roadside assistance/tow service and transport the car to the nearest service station.

• Get the fuel tank and fuel lines cleaned and flushed with petrol to avoid fuel contamination. 

• Additionally, you may also change the fuel filter to prevent further complications. After that, you can refuel with petrol and drive without any worries.

2. Petrol in a diesel car

The general remedial measures remain the same as the above scenario. If you realise that your diesel car has petrol in the tank, immediately tow the vehicle to the service station. Get the fuel tank and fuel lines cleaned and flushed before refuelling diesel. However, there are two scenarios you should be extremely careful about. Below are more details on the same.

If you start the engine

• Even if you turn the ignition key on, the fuel pump may send fuel through the lines. 

• That means the tank and fuel lines need a thorough clean and flush. 

• If you start the engine, the procedure gets a bit complicated as the wrong fuel may have damaged the engine's internal components. 

• Hence, the engine needs a thorough inspection.

If you drive the car for some time

• It's the worst-case scenario. If you have driven the diesel car with petrol fuel, the petrol may have entered the nook and corner of the engine. 

• Immediately switch off the engine and call for roadside assistance/tow service.

• Tow the vehicle to the nearest service centre and get help from an expert mechanic because it requires more than cleaning and flushing the fuel tank and fuel lines. 

• The amount of work and repair bill depends on the extent of damage to the engine components.

General remedial instructions for wrong fuelling

Here are some of the general remedial steps for wrong fuelling.

• Never start the engine if you realise that there is wrong fuel inside the tank. It will save you a lot of money.

• Next, push the vehicle to a safe location within the premises of the fuel station.

• Call the roadside assistance service and tow the vehicle to the authorised service centre.

• If you drive an automatic car, turning on the ignition key to shift to ‘Neutral’ can trigger the fuel pump to push fuel to the system. So, be aware of that fact.

Tips to avoid misfuelling

You can avoid the hassle of dealing with misfuelling if you are a bit cautious. Below are some tips you can consider to prevent filling the wrong fuel in your car.

• Have a fuel sticker: One of the most basic and simple solutions to avoid misfuelling is to have a fuel sticker on the vehicle's fuel filler cap. The sticker will indicate whether it's a petrol or diesel car.

• Check before entering the fuel station: Always check before parking your car in a fuel filling station. Make sure it's the correct fuel dispenser. Slight negligence can cost you a lot.

• Get down from the car while refuelling: It's better to get down from the vehicle and inform the fuel station attendant about which fuel to fill. It will avoid any confusion, and you will see which fuel is going inside the tank.

Frequently asked questions

Below are some of the most commonly asked questions about petrol in a diesel car and vice-versa.

How much diesel fuel is safe in a petrol car?

If you have filled diesel for less than 5% of the tank’s capacity, then it’s safe. You can fill the rest of the tank with petrol and continue driving. However, it’s better that you take the help of an expert mechanic before filling up the tank with petrol.

What happens if I mix petrol and diesel in a car?

The consequences of mixing petrol and diesel vary depending on whether it’s a petrol or diesel car. If it’s a petrol car, the effects may not be serious. However, if it’s a diesel car, the extent of the damage may be significant if you have driven the vehicle after mixing the fuel.

How long can a diesel engine run on petrol?

First of all, you shouldn’t be running a diesel engine on petrol. However, if you do so by mistake, the car engine will run for a few minutes and then lose power. The contaminated fuel may cause severe damage to the internals of the engine.

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