Put the wrong fuel in your car?
Here is exactly what to do right now.
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Why the Response Is Different Each Time
The correct action depends on two things: which fuel went into which engine, and whether the engine ran. Here is a plain-English breakdown.
Petrol in a Diesel Engine
Diesel engines rely on diesel fuel to lubricate the high-pressure fuel pump and injectors. Petrol strips that lubrication away. If the engine is not started, the wrong fuel sits in the tank and fuel lines but has not yet reached those expensive parts. A full tank drain usually solves the problem completely.
If the engine is started, the petrol circulates through the pump and injectors without lubrication. Even a few seconds of running can cause wear. The longer it runs, the worse the damage. Repair costs can range from a simple drain (best case) to a full pump and injector replacement (worst case).
Diesel in a Petrol Engine
Petrol engines ignite fuel with spark plugs. Diesel does not ignite the same way. When diesel enters a petrol engine, it can foul the spark plugs, clog the fuel injectors, and damage the catalytic converter. Symptoms appear quickly: rough running, black smoke, and stalling.
If the engine is not started, the situation is similar to petrol in diesel. A drain and flush usually fixes it. If the engine is started, the diesel may have reached the catalytic converter, which is one of the most expensive parts to replace. Speed matters here.
Typical Cost Ranges (for reference only)
| Scenario | If engine NOT started | If engine WAS started |
|---|---|---|
| Petrol in diesel | Drain and flush only. Usually the lowest cost option. | Possible pump, injector, or rail damage. Costs can rise quickly. |
| Diesel in petrol | Drain and flush. Spark plugs may need cleaning. | Possible catalytic converter damage on top of fuel system repairs. |
These are general estimates for common passenger cars. Actual costs depend on the vehicle, how much wrong fuel was added, and how far it traveled through the system.
Common Mistakes People Make
Starting the engine to "check if it runs"
This is the single most common and most damaging mistake. Even a brief start can push the wrong fuel through the system. If you have not started the car yet, do not. Leave it off.
Adding the correct fuel to "dilute" the mistake
Topping up with the right fuel does not fix the problem. It just means there is more liquid in the tank that still needs to be drained. Do not add anything. Call for help.
Driving the car to a garage
Driving circulates the wrong fuel further through the engine. If you need the car moved, push it or have it towed. Do not start it for any reason.
Waiting to see if symptoms appear
With petrol in diesel, damage can be invisible at first. The wrong fuel may sit quietly in the lines and then cause problems miles later. Act now, not later.
How to Avoid Misfuelling Next Time
Label your fuel cap
A small sticker that says "DIESEL ONLY" or "PETROL ONLY" on the inside of the fuel cap is one of the cheapest and most effective reminders. It is the last thing you see before opening the cap.
Use a fuel-cap adapter
Nozzle adapters are available that make it physically impossible to insert a petrol nozzle into a diesel filler neck (or vice versa). They cost very little and can save you hundreds.
Slow down at the pump
Most misfuelling happens when the driver is in a hurry. Take an extra five seconds to read the pump display and check the nozzle color before you start. Diesel nozzles are often green; petrol nozzles are often black. Colors vary by country, so do not rely on color alone.
Be extra careful with borrowed or rental cars
If you drive a petrol car every day and suddenly rent a diesel, your muscle memory works against you. Pause and think before you open the fuel cap. Check the rental agreement or the key ring, which often has the fuel type printed on it.
Helpful product:
Fuel Cap Reminder Sticker
A small adhesive label for your fuel cap that says the correct fuel type. Helps prevent misfuelling, especially when switching between vehicles.
Search on AmazonFrequently Asked Questions
Is petrol in diesel worse than diesel in petrol?
They are both serious, but in different ways. Petrol in a diesel engine strips away lubrication and can damage the high-pressure fuel pump and injectors. Diesel in a petrol engine can cause immediate misfiring and catalytic converter damage. Diesel in petrol tends to produce obvious symptoms right away. Petrol in diesel can be silent at first but cause costly damage later.
I already started the engine. Is my car ruined?
Not necessarily. If you turned it off within a few seconds, the wrong fuel may not have reached the sensitive components. Follow the engine-started branch of the checklist and call a drain service. Mechanics handle this regularly and can often prevent permanent damage if you act quickly.
Can I drain the fuel myself?
It is possible but not recommended. Modern fuel systems are pressurized and contain sensitive parts. A professional drain service has the right tools, knows how to avoid air locks, and can dispose of the fuel safely. The cost of a drain is almost always less than a repair.
What should I say when I call a drain service?
Be specific. For example: "I put about 15 litres of petrol into my diesel car. I did not start the engine. The car is parked at [location]." Clear information helps them bring the right equipment and give you a time estimate.
How can I avoid misfuelling in the future?
Put a fuel-type sticker on your cap, consider a nozzle adapter, slow down at the pump, and double-check before you start. Be especially careful with borrowed or rental cars that use a different fuel type than your own.