What Happens If You Put Gas in a Diesel Engine? Warning Signs & Fixes

Putting petrol in a diesel engine is a costly mistake. It happens more often than you might think, especially if you’re switching between vehicles.

But what happens if you put gas in a diesel engine? What actually goes wrong inside the engine – and what should you do if it happens?

If immediate action is not taken, there is a potential for further damage to the vehicle, which can lead to extensive harm to fuel system components and high repair costs.

At A1 Diesel, we specialise in diesel engine repairs. Here’s what you need to know if you (or someone else) accidentally fill up with the wrong fuel.

Why Gasoline and Diesel Don’t Mix

Diesel and petrol (gas) are two very different fuels. Diesel is heavier, oilier, and designed to ignite through compression, whereas petrol is lighter and ignites with a spark due to their different combustion properties.

That simple difference can lead to serious engine problems when the wrong fuel is used. Using the incorrect fuel type from diesel and gasoline pumps can result in significant engine damage, including misfiring and costly long-term repairs.

This is why knowing diesel engine basics and how to avoid fuelling mistakes is so important.

Here’s what happens:

• Diesel acts as a lubricant: It helps keep the fuel pump and injectors working smoothly. Petrol strips away that lubrication.
• Petrol burns differently: Diesel engines aren’t built to handle the way petrol combusts.
• Misfiring and knocking: The engine might run rough or not at all, leading to engine misfires.
• Incomplete combustion: Using gasoline in a diesel engine can result in incomplete combustion, causing performance issues and potential damage to engine components.
• Fuel system damage: The petrol can damage your fuel pump, lines, filters, and diesel injectors.
• Complete engine failure: If you drive it too far, you could end up needing a full engine replacement.

Signs you’ve put gas in a diesel engine:

There are several warning signs that may indicate you’ve accidentally filled your diesel engine with petrol. Trouble starting the engine is often the first red flag, especially if the fuel mixture is preventing proper combustion. You may also notice unusual knocking sounds, which can be a result of the engine misfiring due to the presence of gasoline.

Another common symptom is excess smoke coming from the exhaust. This is typically caused by incomplete combustion and may be accompanied by a strong, unfamiliar smell. Poor acceleration and reduced engine power are also typical signs, as gasoline does not provide the same lubrication or energy efficiency as diesel. In more severe cases, the engine may cut out entirely, especially if the petrol has reached the injectors and disrupted normal engine function.

Gasoline contamination in diesel engines can lead to all of these issues. It’s important to perform a thorough inspection and fuel test to detect any traces of gasoline and ensure the engine and fuel system components are functioning correctly.

What To Do If You Put Gas in a Diesel Engine

Mistakes happen – and misfueling is one of those errors that can go from bad to worse if not handled properly. Whether you realised the mistake before starting the engine or after driving a few miles, taking the right steps quickly can save you thousands in repairs. It is crucial to consult a qualified mechanic to handle the situation properly and avoid further damage.

If you haven’t started the engine:

You’re in luck. This is the best-case scenario. If you realize your mistake before turning the key, stop everything.

• Don’t turn it on: Starting the engine pulls the petrol into the fuel lines, injectors, and pump. At this point, your fuel system is still safe. Leave the ignition off to avoid spreading the petrol any further.
• Call for fuel recovery: Contact a professional fuel drain service or roadside assistance. Trying to syphon fuel yourself is risky and can damage components. A proper recovery team will have the tools to safely remove the petrol without harming the tank or surrounding parts. It is crucial to avoid contaminating the gas tank.
• Get the tank drained and cleaned professionally: Even if you caught it early, small traces of petrol can remain. We recommend a full tank flush followed by cleaning of the lines and filters. This gives your engine a clean start and avoids future issues. Make sure to drain the fuel tank completely to prevent any residual gasoline from causing damage.

Once the fuel system is clean, you’ll be good to fill up with diesel and drive off without worry.

If you’ve already driven off:

If you started the engine or drove for a while, the situation becomes more serious – but it’s still fixable. The damage depends on how much petrol you added and how long the engine ran.

• Stop the vehicle immediately: Pull over safely and shut the engine off as soon as you suspect the mistake. The longer the engine runs, the more the petrol spreads and the more parts get damaged – including the high-pressure pump, fuel injectors, and combustion chambers. Prolonged use of the wrong fuel can also damage the entire fuel system.
• Get it towed to a repair shop: Avoid trying to drive it any further. Towing your vehicle to a specialist repair centre is the safest option.
• Let the experts check the fuel system and engine parts: Petrol can damage seals, clog filters, and reduce lubrication in key engine components. Diesel technicians will inspect and test each part of the system – including the fuel rail, injectors, and pumps – to make sure everything is still in good working order. It is crucial to check these components to prevent long-term damage.

Depending on the level of contamination and damage, you may need parts cleaned, replaced, or recalibrated.

How Much Petrol Is Too Much?

Accidentally adding a small amount of petrol to a diesel tank doesn’t always mean instant disaster – but it depends on how much you’ve added, whether you’ve started the engine, and the type of diesel system your vehicle uses.

Even small quantities of gasoline can lead to substantial damage to diesel engines, primarily due to the differing properties of the two fuels and their intended engine types. If you’ve added a significant amount of petrol, it’s best not to start the engine and to seek professional help immediately.

However, even a few gallons mixed with diesel fuel can still lead to severe consequences such as engine sputtering, inconsistent ignition, and potential engine failure, emphasizing the need for caution to avoid catastrophic damage.

Less Than 5% Petrol in the Tank

If you’ve added just a splash – say, 1 to 2 litres of petrol into a near-full diesel tank – there’s a chance you might get away with it. Understanding fuel usage is crucial here, as even small amounts of the wrong fuel can impact vehicle performance and efficiency.

Some older diesel engines can tolerate this small amount without immediate damage. However, newer diesel engines (especially common rail systems) are far more sensitive and precise. Even low levels of petrol can cause poor lubrication and increase wear. It is essential to follow the manufacturer’s fuel requirements to avoid misfueling and ensure the engine operates optimally.

Our advice: Don’t take chances. Even if you’ve added less than 5%, get the tank checked and flushed professionally – especially if you’ve already turned the engine on.

Between 5% and 10%

Now things are riskier. At this level, the petrol starts to thin out the diesel too much. It reduces the fuel’s lubricating quality, which can lead to:

• Excess friction in the fuel pump
• Injector damage
• Poor combustion
• Engine knocking or misfiring

Additionally, using gasoline in a diesel engine can cause inefficient combustion due to the differing combustion properties of the fuels.

Even if the vehicle still runs, you’re likely shortening the lifespan of key components. Continuing to drive can turn a manageable issue into a full engine repair job, potentially resulting in increased wear on essential engine components.

More Than 10% Petrol in the Tank

This is serious. If more than 10% of the fuel mix is petrol, you’re looking at potential long-term damage – especially if the engine has already run. Reduced performance is a common issue, with symptoms like misfires and poor acceleration becoming evident.

• Lubrication is almost gone
• Pump and injector seals may wear or crack
• Combustion becomes erratic and harsh
• The risk of complete fuel system failure is high

Engine performance can significantly decline due to gasoline contamination, leading to reduced efficiency and potentially costly repairs.

Important: If the petrol-to-diesel mix is this high, do not start the engine. Tow the vehicle immediately to a diesel repair centre like A1 Diesel.

Full tank of petrol in a diesel engine

Filling up a diesel tank entirely with petrol and driving off is one of the worst scenarios. This can lead to:

• Total breakdown within minutes
• Severe damage to the fuel pump and injectors
• Metal-on-metal wear inside the engine
• Possible engine seizure

Prolonged use of the wrong fuel can inflict irreversible damage on vital parts, including the entire fuel system.

In most cases, this level of contamination calls for major repairs – and in some cases, a full engine rebuild. It is important to check for damage to crucial components to avoid further complications.

Rule of Thumb

• Under 5% petrol and engine off? Get it drained, and you might avoid damage. Using the correct fuel is crucial to prevent any potential issues.
• More than 5% or engine started? Stop immediately and call a professional. Consulting a professional mechanic is essential to assess the damage and determine the necessary repairs.
• Driven far on the mix? Expect repairs to fuel system parts at a minimum.

Can You Fix It Yourself?

It might be tempting to sort it out on your own, especially if the tank isn’t full or you haven’t started the engine yet. But before you grab a hose or start watching DIY fuel-drain videos, here’s what you should know:

Draining a Diesel Tank Isn’t As Easy As It Sounds

Modern diesel engines aren’t like the old-school setups. Most use high-pressure fuel systems with advanced technology that’s sensitive to contamination, making the management of the fuel tank crucial. It’s not just about removing the petrol – it’s about making sure every part of the fuel system is clean and undamaged.

Here’s what you’re dealing with:

• High-pressure fuel pumps that can cost thousands if damaged
• Delicate injectors that can clog or corrode with even small petrol traces
• Fuel filters and return lines that need proper flushing
• Sealed tanks that are hard to access without specialist equipment

One wrong move – like turning on the ignition or using the wrong tool – can send petrol deeper into the system and multiply the cost of repairs.

Common DIY Mistakes:

• Using makeshift siphon tubes that don’t reach all the way
• Spilling fuel and damaging wiring or sensors
• Contaminating new diesel with leftover petrol
• Starting the engine thinking “a little bit won’t hurt” – it does
• Not properly using the drain plug to remove contaminated fuel, which can prevent thorough cleaning and cause damage to the vehicle

Final Thoughts

Putting petrol in a diesel engine can be a big headache, but it’s not the end of the road. The key is acting fast to ensure optimal performance. If you think you’ve made the mistake, stop the car and call the professionals.

At A1 Diesel, we’re ready to help with expert repairs and fast service. Whether it’s a minor flush or a full fuel system rebuild, we’ve got the tools and know-how to get your diesel engine running right again.

Need help now? Contact A1 Diesel today.We’re the diesel repair experts you can trust.

Frequently Asked Questions

1. Why are diesel engines more sensitive to misfueling than gasoline engines?

Diesel engines rely heavily on fuel for both lubrication and combustion. Petrol lacks the necessary lubrication, leading to rapid wear in high-pressure diesel components and damage to crucial components such as injectors and fuel pumps. Petrol engines, on the other hand, don’t depend on fuel for lubrication in the same way, making them slightly more forgiving in reverse scenarios.

2. What’s the difference between misfueling and contaminated diesel?

Misfueling means using the wrong fuel type entirely, like putting petrol in a diesel car. Contaminated diesel refers to diesel that’s been compromised – often with water, dirt, or small amounts of other substances – and can still damage the engine, though usually less severely than full misfueling. Gasoline contamination in diesel engines can also be a serious issue, manifesting through symptoms such as unusual engine noises, lighter exhaust smoke, and reduced performance.

3. Can a diesel engine run on petrol temporarily in an emergency?

No – even short-term use can cause lasting damage. Petrol doesn’t provide the lubrication diesel parts need, and the combustion process is entirely different. Using petrol even once can damage the pump, injectors, and engine internals, leading to increased wear on essential components.