Misfuelling—putting diesel in a petrol car—happens more often than you might think. It’s an easy mistake, especially if you’re distracted or driving an unfamiliar vehicle. But what happens when you do it? Can it ruin your car? How do you fix it? This guide answers those questions with clear, actionable advice. We’ll break down the effects, signs, and steps to take, plus tips to avoid this costly error. Let’s dive in.
Why Diesel and Petrol Don’t Mix?
Petrol and diesel engines work differently. Petrol engines use spark plugs to ignite a thin, highly flammable fuel. Diesel engines rely on compression to ignite a thicker, oilier fuel. When you put diesel in a petrol car, the mismatch causes problems. Diesel won’t burn properly in a petrol engine. It’s too thick and not flammable enough for spark plugs to ignite. This leads to clogs, misfires, and potential damage.
Key Differences Between Fuels
- Petrol: Thin, volatile, ignites with a spark.
- Diesel: Thick, oily, ignites under compression.
- Result: Diesel in a petrol car disrupts the fuel system.
What Happens If You Put Diesel in a Petrol Car?
The effects depend on how much diesel you add and whether you start the engine. Here’s what happens step-by-step.
If You Don’t Start the Engine
If you catch the mistake before turning the key, damage is minimal. Diesel sits in the tank and doesn’t reach the engine. You’ll need to drain the tank, but the car should be fine after that.
- Signs: No immediate symptoms since the engine hasn’t run.
- Damage: None, as long as you act quickly.
If You Start the Engine
Starting the engine pumps diesel into the fuel system. This is where trouble begins. Diesel clogs the fuel lines, filter, and spark plugs. The engine may misfire, stall, or produce smoke. You might hear odd noises or feel a loss of power.
- Signs:
- Engine misfires or sputters.
- White or black smoke from the exhaust.
- Car stalls or won’t restart.
- Damage: Clogged fuel system, fouled spark plugs, possible harm to the catalytic converter.
If You Drive for a While
Driving with diesel in the tank worsens the damage. The thick fuel coats engine parts, causing excess heat and pressure. This can harm the fuel pump, injectors, and catalytic converter. In severe cases, you’ll need major repairs.
- Signs:
- Reduced acceleration.
- Loud knocking sounds.
- Engine shuts off completely.
- Damage: Potentially thousands of dollars in repairs.
How Much Diesel Is Too Much?
A small amount—less than 5% of the tank—might not cause noticeable harm. If you fill the rest with petrol, the car may still run. But anything more than 5% risks serious issues. The exact threshold depends on your car’s make and model, so it’s best not to chance it.
- Safe Limit: Up to 5% diesel diluted with petrol.
- Risk Zone: Over 5% requires draining.
Signs You’ve Put Diesel in a Petrol Car
How do you know if you’ve misfuelled? Watch for these clues:
- At the Pump: Diesel nozzles are larger and often green, unlike petrol’s smaller, black nozzles.
- After Starting: Engine struggles, smokes, or stops.
- While Driving: Power drops, noises increase, or the car stalls.
If you spot these, act fast to limit damage.
What to Do If You Put Diesel in a Petrol Car
Caught the mistake? Here’s how to handle it based on when you notice.
At the Fuel Station
- Don’t Start the Engine: Keep the diesel in the tank.
- Move the Car: Put it in neutral and push it to a safe spot (ask for help if needed).
- Call for Help: Contact roadside assistance or a mechanic to drain the tank.
- Drain the Fuel: A pro will remove the diesel and flush the tank.
After Starting the Engine
- Turn Off the Engine: Stop as soon as you realize the error.
- Don’t Restart: Avoid pumping more diesel through the system.
- Get a Tow: Have the car taken to a garage.
- Full Flush: The mechanic will clean the tank, fuel lines, and replace the filter if needed.
After Driving
- Stop Safely: Pull over and shut off the engine.
- Arrange a Tow: Don’t drive further.
- Professional Repair: Expect a deeper inspection—fuel system and engine parts may need fixing.
How Much Does It Cost to Fix?
Costs vary by situation:
- Tank Drain Only: $100–$300 if you don’t start the engine.
- Fuel System Flush: $300–$800 if diesel reaches the lines.
- Major Repairs: $1,000–$3,000+ if the engine or catalytic converter is damaged.
Can You Prevent Misfuelling?
Yes! Try these tips to avoid putting diesel in your petrol car:
- Check the Pump: Diesel nozzles are larger and often green; petrol is usually black.
- Read Labels: Look at your fuel cap—it says “Petrol” or “Unleaded.”
- Stay Focused: Avoid distractions like phones while filling up.
- Use a Reminder: Stick a “Petrol Only” note near the fuel cap.
- Know Your Car: If it’s new or borrowed, double-check the fuel type.
Some cars have misfuel protection—like Ford’s filler neck design—that stops diesel nozzles from fitting. Check your manual to see if yours does.
Common Questions
.Can a Petrol Car Run on Diesel?
No. Diesel won’t ignite properly in a petrol engine. The car may start briefly but will soon stall or misfire.
Is It Worse to Put Petrol in a Diesel Car?
Yes. Petrol in a diesel engine causes more damage because petrol lacks diesel’s lubricating properties. It wears out pumps and injectors faster.
How Long Can You Drive With Diesel in a Petrol Car?
Not long—minutes at most. The engine will sputter and stop as diesel clogs the system. Stop immediately to avoid worse damage.
Will Insurance Cover Misfuelling?
Maybe. Some policies cover accidental misfuelling, but not all. Check with your provider. Repairs from driving with the wrong fuel are rarely covered.
Final Thoughts
Putting diesel in a petrol car isn’t the end of the world, but it can be a pricey headache. If you catch it early, you’re looking at a simple fix. Drive with it, and you risk serious damage. Stay alert at the pump, know your car’s fuel type, and act quickly if you mess up. Your engine—and wallet—will thank you.