Rajan Wilcox
Putting unleaded fuel in a diesel car is a common mistake, with someone in the UK making this mistake every three to four minutes. While older diesel cars may be more forgiving, newer diesel engines may not start or be damaged by unleaded fuel. This is because unleaded fuel acts as a solvent, rather than a lubricant, and can ruin a vehicle's fuel pump and injectors. If you realise your mistake at the pump, it is commonly agreed that mixing up to 5% petrol into diesel fuel is safe. However, if you have driven away from the pump, you may be faced with costly repairs.
Can you put unleaded fuel in a diesel car?
| Characteristics | Values |
|---|---|
| Possibility | Yes, but it is not advisable. |
| Compatibility | Unleaded fuel is not compatible with diesel engines. |
| Consequences | Engine damage, fuel pump damage, injector damage, sensor damage, engine breakdown, costly repairs |
| Prevention | Fuel nozzle size and shape differ for diesel and unleaded, fuel cap stickers |
| Action after misfuelling | Do not start the engine. Get the car towed to a mechanic or dealership for repairs. |
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What You'll Learn
What to do if you put unleaded fuel in a diesel car
Putting unleaded fuel in a diesel car is a mistake that can cause significant damage to the vehicle. If you have put unleaded fuel in a diesel car, the first thing you should do is not start the engine. This is crucial as turning on the engine could cause the unleaded fuel to start doing damage.
If you have not started the engine, you can get away with towing the car to a mechanic or dealership for assessment and repair. This is the recommended course of action. A mechanic may be able to help you evacuate the fuel system by disconnecting the line and commanding the fuel pump to run continuously by jumping its relay. They will then fully purge the system (including the tank), and replace filters.
If you have only put a small amount of unleaded fuel in your diesel car, you may be able to get away with filling up the rest of your tank with diesel fuel and driving your car as normal. However, this is not recommended and you should do this only if you are in a pinch.
If your vehicle is older, it may be more forgiving and may not suffer any serious damage from a small amount of unleaded fuel. In this case, you may be able to drain the tank and cycle/flush clean diesel through the system, along with adding new filters.
It is important to act quickly and not to drive the vehicle until the unleaded fuel has been removed, as this could cause expensive damage to your vehicle's fuel pump and injectors. Unleaded fuel can also damage sensors that record critical engine systems, causing them to overheat or become contaminated with soot.
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Differences between diesel and unleaded gasoline
Unleaded gasoline, also known as gas or petrol, is the most popular automotive fuel globally. It is made of shorter-chain molecules and contains less energy per litre than diesel. In petrol engines, an electric spark is used to ignite a compressed mixture of petrol and air.
Diesel, on the other hand, is made of longer-chain molecules and contains more energy per litre. Diesel engines do not require spark plugs as they rely on the spontaneous ignition of a compressed air/fuel mixture. The air is compressed first, and then the fuel is injected. Due to the higher compression, diesel engines are more efficient than petrol engines.
Another difference is the type of vehicles that use these fuels. Diesel-powered cars and trucks get about 30% better mileage than those powered by unleaded gasoline. However, diesel vehicles produce more emissions than any other vehicle on the road.
In terms of cost, unleaded fuel is generally cheaper than diesel. However, diesel engines provide enhanced torque, resulting in better acceleration and fewer refuelling stops.
Physically, diesel fuel nozzles are differently sized compared to standard unleaded gasoline nozzles, and a special plate fitted under the gas cap on designated unleaded vehicles prevents the insertion of diesel nozzles.
Finally, it is important to note that unleaded fuel should not be used in a diesel engine as it can cause significant damage to the engine, fuel pump, and injectors. While older diesel engines may be more forgiving, newer engines are likely to be severely damaged by the use of unleaded fuel.
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How to prevent putting unleaded fuel in a diesel car
Putting unleaded fuel in a diesel car can cause costly problems and even lead to expensive repair bills. While older diesel engines can be more forgiving, newer diesel engines will not start on unleaded fuel. Unleaded fuel in a diesel car can wreck the injectors, damage the fuel pump, and ruin sensors that record critical engine systems.
- Pay attention to the type of fuel your car needs. Be sure that you know what type of fuel your car requires and pay attention when filling it up.
- Check the fuel filler cap for warnings. There may be stickers on the cap or surround indicating what fuel type your car requires.
- Be aware of the size of the fuel nozzle. Diesel fuel nozzles are differently sized compared to standard unleaded gasoline nozzles. Diesel nozzles are larger and will not fit into the tank opening of most gasoline vehicles manufactured in the last 25 years.
- Take advantage of fuel cap systems that prevent misfuelling. Some manufacturers, such as Ford, have used a special fuel cap system that won't allow misfuelling. These systems use an inhibitor that will only admit the correct fuel nozzle. You can also buy aftermarket devices that do something similar.
- Be aware of warning signs that your car may be misfuelled. These can include a decrease in fuel efficiency, a decrease in power, and an increase in exhaust smoke.
- If you suspect you have put unleaded fuel in your diesel car, do not start the engine. Have the car towed to a mechanic to assess the issue and perform any necessary repairs.
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What happens if you put diesel in an unleaded car
Putting diesel in an unleaded car is a mistake that is more common than one might think. This is because fuel pumps often house the diesel nozzle right next to the unleaded nozzle, and it is easy for drivers to grab the wrong one. However, diesel pumps are generally labelled in vibrant green to differentiate them from unleaded pumps. Additionally, diesel nozzles are differently-sized compared to standard unleaded nozzles, and a special plate fitted under the gas cap on designated unleaded vehicles makes the insertion of a diesel nozzle impossible.
If diesel is put into an unleaded car, the car will not start. This is because unleaded cars are equipped with a fuel injection system that uses fuel injectors to shoot gasoline into the cylinders of the engine. The fuel is then ignited by spark plugs, which turn the engine over. If diesel is used instead of unleaded gasoline, the fuel injectors will transport the diesel into the engine cylinders, but the spark plugs will not be able to ignite the fuel.
While diesel in an unleaded car will not cause any damage to the engine or any other system within the vehicle, the fuel tank will need to be drained and refilled with the proper fuel. This can be a tedious process and should only be done by someone familiar with routine automotive work. If the car is started with diesel in the tank, the diesel fuel could enter the fuel line and engine system, which will make the repair process far more complicated and costly.
In older cars, it was once possible to add small amounts of unleaded gasoline to diesel to thin it out and prevent gelling while driving in higher elevations. However, this is no longer recommended.
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Repairing a diesel car with unleaded fuel
Putting unleaded fuel in a diesel car is a mistake that can lead to serious engine damage. Unleaded fuel lacks the lubricity that diesel has, as it is thinner and more viscous. This means that it can ruin a vehicle's fuel pump and injectors, as well as damaging sensors that record critical engine systems.
If you realise your mistake at the pump, it is important not to start the engine. Mixing the two fuels is not dangerous, but driving with unleaded fuel in a diesel car can cause serious and expensive damage. If you spot the mistake early, you may be able to get away with mixing a small amount of unleaded fuel with diesel without serious consequences. However, if you have driven the car, further repairs may be necessary.
If you have put unleaded fuel in your diesel car, the first step is to call a repair shop or dealership. They will advise you to tow the car to them so that they can assess and repair the damage. If you are confident in your DIY abilities, you can try to evacuate the fuel system by disconnecting the line and commanding the fuel pump to run continuously by jumping its relay. Fully purge the system (including the tank), and replace the filters.
If you have driven the car, the repair shop will drain and clean the tank to remove the unleaded fuel and its residue. They will then flush the fuel line, service the fuel injector system, and check the fuel pump for damage. These additional repairs can be costly, with fuel injector servicing costing $250 and replacement costing $1,200 or more.
It is important to act quickly and not to turn on your vehicle if you have put unleaded fuel in a diesel car. While older diesel engines may be more forgiving, newer diesel engines are more likely to be damaged by unleaded fuel.
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Frequently asked questions
If you realise your mistake at the pump, you can mix up to 5% petrol into diesel fuel without causing any damage. If you have already driven away, you will need to call a breakdown service to get your car to a service centre where they can drain the tank.
Unleaded fuel in a diesel car can wreck the injectors and fuel pump. It can also damage sensors that record critical engine systems, by causing them to overheat or contaminating them with soot.
Diesel fuel nozzles are differently sized compared to standard unleaded gasoline nozzles, and there is a special plate fitted under the gas cap on designated unleaded vehicles, making the insertion of diesel impossible. You can also apply a sticker somewhere inside the filler door or on the filler cap itself to remind you what fuel to put in the tank.
