Diesel in a Petrol Car – What to Do
Putting diesel in a petrol car is a common mistake — especially when switching vehicles or driving a hire car. The good news is that, in many cases, serious damage can be avoided, particularly if the engine hasn’t been started.
This guide explains exactly what to do, what can happen, and when to get professional help.
First things first – Do NOT start the car
What to do immediately if you put diesel in a petrol car
If you realise diesel has been added to a petrol vehicle:
Do not start the engine
Do not turn the ignition on
Do not attempt to drive
Get professional advice as soon as possible
If the engine has not been started, the situation is often much easier to resolve.
Why diesel causes problems in petrol engines
Diesel fuel is thicker and does not ignite the same way petrol does. When diesel enters a petrol fuel system, it can:
- Prevent the engine from starting
- Cause misfiring or rough running
- Block fuel filters
- Affect injectors and spark plugs
Compared to petrol in a diesel car, this situation is usually less damaging, but it still needs to be handled correctly.
Started the engine with diesel in a petrol car?
If the engine was started, or the car was driven:
- Stop the engine as soon as it’s safe
- Do not continue driving
- Get advice immediately
Driving with diesel in a petrol car can spread contamination further through the fuel system, increasing repair costs.
Does a small amount of diesel matter?
Even a small amount of diesel can cause issues, depending on:
- How much diesel was added
- Whether the engine was started
- How long the vehicle ran
Because petrol systems are designed for lighter fuel, it’s safest not to assume dilution will fix the issue.
Is diesel contamination repairable?
Yes — in many cases.
A professional solution may involve:
- Draining the fuel tank
- Flushing the fuel system if required
- Refilling with the correct petrol
- Checking the vehicle before restart
When dealt with early, long-term damage is often avoided.
Common mistakes to avoid
Do NOT:
- Try to “burn off” the diesel by driving
- Dilute it by adding more petrol
- Start the engine “just to test”
- Attempt DIY fixes without proper equipment
- Avoid the 6 Wrong Fuel Myths
These actions often make the situation worse.
When to call a wrong fuel specialist?
You should seek professional help if:
- Diesel has been added to a petrol car
- You’re unsure if the engine was started
- The vehicle won’t start
- Warning lights appear
A specialist can advise you immediately and attend your location if required.
Put the wrong fuel in your car? Don’t start it.
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