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    What happens if you put petrol in a diesel car?

    Not all fuel types are created equal. Knowing what fuel your car uses can be the difference between cruising down the motorway and staying stuck at the curb. 

    Aside from decreased fuel efficiency, putting petrol in a diesel car can result in misfiring, engine damage, and potential long-term damage to your vehicle’s fuel system. This common mistake can rack up substantial repair bills and impact the value of your car if and when you decide to sell your vehicle.  

    Made a mistake at the petrol station? It’s vital to address improper fueling issues promptly. Read on for more on what to do if you accidentally filled your diesel tank with petrol, as well as tips on how to prevent this mishap in the first place. 

    Petrol vs. diesel engines

    Petrol and diesel engines have fundamentally different combustion processes. Petrol engines rely on spark ignition, while diesel engines use compression to ignite. This distinction affects the car’s efficiency, power delivery, and fuel economy. 

    Diesel engines generally have higher torque, making them suitable for heavy-duty and long distance driving. This accounts for most vans and Heavy Goods Vehicles (HGVs) being diesel engines. These engines’ construction and functioning vary significantly from their petrol counterparts. 

    To function properly, cars should only use their prescribed fuel type, as noted by the manufacturer. Failure to use diesel for a diesel engine, or petrol for a petrol engine, can cause serious issues that impact the functionality and the condition of your car, and your resulting price if you choose to sell. 

    Misfueling a diesel engine

    fuel being put into a vehicle
    Filling your diesel engine with regular petrol can severely damage your vehicle’s engine.

    Misfueling, or inadvertently putting the wrong fuel into your vehicle, poses serious consequences to the health and performance of your vehicle. This is especially true when petrol is mistakenly added to a diesel car. 

    Petrol and diesel fuels combust differently, so using the wrong fuel type can disrupt engine function. Introducing petrol into a diesel engine causes an immediate reaction between the two fuel types. Petrol lacks the necessary lubricating qualities for diesel engines, leading to increased wear on critical components. This disrupts the engine’s smooth operation, affecting performance and potentially causing immediate engine damage, as well as a complete breakdown of engine functionality. 

    Mixing petrol with diesel in a vehicle can result in lasting harm to the engine. Beyond immediate operational issues, prolonged use of incorrect fuel may lead to irreversible damage, affecting crucial components such as injectors, fuel pumps, and the entire fuel system. 

    Damage from misfueling may require extensive and costly repairs. In addition to inconveniencing drivers, residual damage from using the wrong fuel can lower the vehicle’s overall value.

    How much does misfueling devalue a car?

    A misfueling incident typically won’t devalue your car directly. Rather, the engine and internal damage caused by misfueling can seriously impact the vehicle’s value should you choose to sell.

    Most modern vehicles have safeguards to mitigate damage from misfueling, but it’s essential to address the issue promptly to prevent long-term issues. This includes speaking with a qualified mechanic to drain the incorrect fuel from your fuel tank as soon as you realise you’ve filled with the wrong fuel. 

    As long as the problem is resolved and your car is noted as being in good working condition by a professional, an instance of misfueling shouldn’t significantly affect its value. 

    However, driving with the incorrect fuel, or enduring multiple misfueling instances, can degrade your engine system and require substantial repairs. Given the hefty price of fixing engine issues, consumers may not be willing to pay top dollar for used cars with this sort of damage. 

    Symptoms of petrol in a diesel car

    car engine
    Misfueling happens, but it’s essential to act quickly to reduce damage to your vehicle.

    ❌ Engine misfires and reduced performance

    Experiencing engine misfires or noticeable decreases in your diesel vehicle’s performance are common symptoms of misfueling. This occurs due to the conflicting combustion properties of the two fuels in your car’s system, disrupting the engine’s usual operational efficiency when running solely on diesel. 

    Drivers may sense uneven power delivery, hesitation during acceleration, or an overall decrease in the engine’s responsiveness. Seek professional support immediately to prevent prolonged damage and ensure your engine’s future performance.

    ❌ Difficulty starting the engine or stalling

    Difficulties in starting the engine or, in more severe cases, unexpected stalling shortly after ignition can also be signs of misfueling. 

    The clash between petrol and diesel properties can compromise your vehicle’s ignition system, leading to prolonged cranking and issues starting your engine smoothly. Inconsistent fuel combustion can lead to stalling and impact engine functionality.

    ❌ Strange noises or vibrations from the engine

    When petrol is mistakenly introduced into a diesel engine, it can lead to peculiar noises and vibrations. 

    Knocking or pinging, accompanied by unexpected vibrations, indicates internal stress and potential engine damage.

    Immediate steps to take after misfueling

    1. Turn off engine – Do not run your engine if you’ve added petrol to the diesel car! This can exacerbate the issue by initiating the improper combustion process and causing immediate – and substantial – engine damage.

    2. Call for professional assistance – Once the engine is turned off, seek help from automotive experts. Professional assistance ensures a proper diagnosis of the issue and helps determine the most effective course of action to rectify the misfueling error.

    3. Tow the car – Arrange for your car to be towed to a secure location. Towing prevents further damage that might occur if the vehicle is driven with the incorrect fuel mixture. A qualified towing service can bring your car to a garage or service centre for professional repairs.

    4. Consult a mechanic to inspect damage – Performing a check-up on your vehicle after misfueling helps mitigate damage. A professional diagnostic test will identify specific issues within the individual engine components that resulted from misfueling. 

    5. Perform repairs or replacements if needed – Based on the examination results, prioritise draining and flushing the fuel system, as well as the repair of damaged parts. Address critical components first to restore the engine’s functionality and prevent further complications. If certain engine components are severely damaged and cannot be effectively repaired, consider their replacement with new, genuine parts to ensure optimal performance and longevity. 

    6. Conduct quality assurance – After repairs, conduct thorough testing to ensure the successful restoration of engine components. Monitor the engine’s performance, addressing any residual issues promptly to prevent future problems.

    Draining and flushing the fuel system

    After misfueling, it’s essential to remove the incorrect fuel from your gas tank and flush the system to prevent additional damage. This one-two process of removal and flushing should only be undertaken by those with significant automotive repair experience. Otherwise, consult a qualified mechanic.

    For those with DIY car repair experience, below is a cheat sheet to flushing your fuel system after misfueling: 

    ✅ Drain contaminated fuel

    1. Prepare car – Ensure the vehicle is in a safe and well-ventilated area. Put on appropriate safety gear and conduct an audit of the fuel system components. A typical diesel fuel system consists of the fuel tank, fuel lines, fuel pump, injectors, and combustion chamber.

    2. Locate drain plug – Identify the drain plug on the fuel tank. Refer to the vehicle’s manual for guidance if needed.

    3. Drain incorrect fuel – Place a container beneath the drain plug and carefully remove the plug so that the contaminated fuel can flow into the container. Exercise caution to prevent spills.

    4. Inspect and clean – Examine the drained fuel for any visible contaminants. Clean the drain plug before resealing it.

    5. Dispose of contaminated fuel – Safely dispose of contaminated fuel according to local regulations.

    ✅ Flushing with clean diesel fuel

    1. Disconnect battery – Don’t get electrocuted! Disconnect the vehicle’s battery to eliminate electrical risks during the flushing process.

    2. Refill fuel tank – Fill the fuel tank with fresh, clean diesel fuel to dilute any remaining contaminated fuel.

    3. Cycle the ignition – Turn your car’s ignition on and off several times without starting the engine. This helps circulate the clean fuel through the fuel system.

    4. Replace fuel filter – Consider replacing your vehicle’s fuel filter to ensure optimal filtration of clean fuel. 

    5. Inspect fuel system – Inspect the fuel system for leaks and, if all is secure, start the engine. Monitor for any irregularities in operation.

    6. Test drive – Take your vehicle for a short test drive to ensure proper performance and verify that the misfueling issue has been successfully addressed.

    Misfueling prevention

    ✅ Follow manufacturer recommendations

    Adhering to your vehicle manufacturer’s recommendations is essential in preventing misfueling. Manufacturers provide clear guidelines regarding the suitable fuel type for specific vehicle models. Consult your owner’s manual for precise information on fuel requirements and follow these instructions.

    ✅ Create good fueling habits and precautions

    Developing prudent fueling habits minimises the likelihood of misfueling incidents. 

    Establish a routine of checking the fuel type indicated on the gas pump before inserting the nozzle into your vehicle. Make sure the service station has proper lighting and you can clearly read the fuel type during this check.

    ✅ Ask for help

    If you’re unsure of what fuel your vehicle uses and do not have the car’s manual, ask a qualified mechanic or professional.

    FAQs

    How long will a diesel car drive with petrol?

    Driving a diesel car with petrol can cause immediate damage. Avoid driving and seek professional assistance promptly to prevent further harm to the engine.

    Does insurance cover putting the wrong fuel in my car?

    In many instances, insurance does not cover putting the wrong fuel. However, review your policy or contact your provider for specific details on coverage and potential exclusions.

    How long does it take to drain the wrong fuel?

    Draining the wrong fuel from your gas tank typically takes 1 to 2 hours. Quick action is crucial to minimise damage and ensure the vehicle’s proper functioning.

    How much of the wrong fuel can you put in your car?

    Even if you have only put in a small amount of wrong fuel, it is not advisable to drive that vehicle. Immediately consult a professional on the best next steps to avoid further damage.

    Can I drain the fuel tank myself?

    While possible, draining the fuel tank yourself is risky. Consult your vehicle manual and consider professional help for a thorough and safe removal of the wrong fuel, as well as to ensure that your vehicle does not incur additional damage during the removal process.

    How can I track the value of my car?

    graph of a BMWs value
    Tracking your car’s ongoing valuation is a smart move for your personal finances

    A history of incorrect fueling may reduce the price of your car when you decide to sell. However, without knowing what your car’s value is to begin with, it’s nearly impossible to know just how much of an impact improper fueling may have on your final price tag.

    All vehicles depreciate at varying rates, with no rule of averages accurately describing any one car’s changing value. Luckily, Motorway’s Car Value Tracker takes the guesswork away by providing free, reliable monthly price alerts for up to six vehicles at once. 

    Use the Car Value Tracker to follow changes to your car’s value to choose the best time to sell, and make informed choices about investments in your car’s maintenance.

    Need to sell your car?

    Want to learn more about owning, maintaining, and selling your car? Check out more of our guides here, covering everything from Clean Air Zones to car tax, and plate changes to part exchange. 

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