Wrong Fuel & Misfuelling Diesel in Petrol

What happens if you put diesel in a petrol car?

Q: What happens if you put diesel in a petrol car?

Diesel in a petrol car prevents the engine from firing correctly and can foul spark plugs and injectors. Do not start the engine — if you catch it at the pump, call a fuel drain service immediately. If the engine has not been started, a roadside drain is usually sufficient. Starting the engine and driving can cause more serious damage requiring professional repair.

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Putting diesel in a petrol car is less common than the reverse, but it can still cause serious damage if you start the engine. The key rule is: do not turn the ignition on — stop at the pump, keep the engine off, and call for a fuel drain immediately.

Why Diesel in a Petrol Engine Is a Problem

Diesel won't ignite properly: Petrol engines use a spark to ignite fuel, but diesel requires compression to combust. Diesel in a petrol engine won't fire correctly, causing misfires, stalling, or the engine refusing to start. 3

Diesel coats the spark plugs: The oily nature of diesel can foul the spark plugs and fuel injectors, reducing their effectiveness and potentially requiring replacement if the engine is run. 2

Less catastrophic than petrol in diesel: Unlike petrol in a diesel car — where the lack of lubrication can destroy the high-pressure fuel pump — diesel in a petrol car is generally less destructive, provided the engine is not started. 4

What To Do Immediately

Do not start the engine: This is the most critical step. If the engine has not been started, the diesel is still contained in the fuel tank and has not circulated through the fuel system. 3

Push the car to safety if needed: If you realise your mistake at the pump, put the car in neutral and push it clear of the forecourt rather than driving it even a short distance. 2

Call a fuel drain service: Contact a breakdown provider or roadside assistance. They can drain and flush the tank at the roadside, usually within an hour, without needing to tow the vehicle. 4

What Happens If You Started the Engine

Symptoms to watch for: If diesel has circulated, you may notice excessive smoke from the exhaust, rough running, heavy misfiring, or the engine cutting out shortly after starting. 3

Repair costs can be significant: Running the engine with a diesel-contaminated fuel system may require professional cleaning of injectors and spark plugs, and in worse cases, a full fuel system flush — costs can run into several hundred pounds. 2

Stop driving immediately: If you have already driven the car, stop as soon as it is safe to do so and call for recovery. The longer you run the engine, the more contamination spreads through the system. 4

Avoiding Misfuelling in Future

Diesel nozzles are larger: Diesel pump nozzles are physically wider than petrol nozzles, which actually makes it harder to accidentally put diesel into a petrol filler neck — but it can still happen with older or modified nozzles. 2

Stick a reminder label on your filler cap: Misfuelling is most likely to happen when you are distracted or driving an unfamiliar vehicle. A simple fuel-type sticker on the filler cap or dashboard is a cheap preventative measure.

Check fuel prices before you go: Knowing which station you are heading to before you leave — and what fuel type you need — keeps you focused at the pump. CheckFuelPrices shows live prices so you can plan your fill-up in advance. 1