What Happens If You Put Diesel in a Petrol Car? (And What NSW Drivers Should Do)

It doesn’t happen often — but it does happen.

Putting diesel into a petrol car is less common than the other way around, mainly because diesel nozzles are usually larger. But after a long day, driving a borrowed car, or switching between vehicles, it’s surprisingly easy to reach for the wrong pump.

At Tynan Motors, we regularly assist drivers across the Sutherland Shire and Illawarra who’ve experienced misfuelling. The key is knowing what to do — and acting quickly.

First: Don’t Panic

If you realise you’ve put diesel into a petrol vehicle, the most important step is:

👉 Do not start the car.

Starting the engine can turn a relatively simple fix into a much more expensive repair.

Why Diesel and Petrol Don’t Mix

Petrol and diesel engines operate very differently.

• Petrol engines rely on spark plugs to ignite a petrol-air mixture.
• Diesel engines rely on compression — no spark plug required.

Diesel fuel has:

• A higher flash point
• A thicker, oilier consistency
• Different combustion properties

When diesel enters a petrol system, it doesn’t ignite properly.

What Happens If You Start a Petrol Car with Diesel in the Tank?

If the vehicle is started, you may notice:

• Rough running
• Misfiring
• Heavy smoke from the exhaust
• Difficulty starting
• Engine warning lights

Here’s what’s happening inside the engine:

1. Fouled Spark Plugs

Diesel can coat the spark plugs, preventing proper ignition.

2. Blocked Fuel Injectors

Diesel’s thicker consistency can clog petrol injectors.

3. Fuel Pump Issues

The petrol fuel pump isn’t designed for diesel’s viscosity, which can restrict flow.

4. Running Lean

If fuel delivery is compromised, the engine may run “lean” — meaning not enough fuel enters the combustion chamber. This can cause overheating and serious internal damage.

5. Risk of Hydro-Lock

Repeatedly trying to start the car can flood cylinders with diesel, leading to hydro-lock — a condition that can bend connecting rods or damage the crankshaft.

That’s when a simple mistake can become a major engine repair.

What Should You Do Immediately?

If you realise before starting the car:

✔ Do not turn the ignition on ✔ Do not try to “dilute” the diesel with petrol ✔ Call roadside assistance ✔ Arrange towing to a qualified service centre

If the engine has already been started:

✔ Pull over safely ✔ Turn the engine off immediately ✔ Do not attempt to restart

The sooner the vehicle is switched off, the less potential damage.

How Is It Fixed?

If caught early, the repair is usually straightforward:

• Drain and dispose of contaminated fuel
• Flush the system
• Replace the fuel filter
• Inspect spark plugs and injectors

In many cases, if the engine hasn’t run for long, the damage can be minimal.

However, if the vehicle has been driven extensively, repairs may involve:

• Injector replacement
• Fuel pump repairs
• Spark plug replacement
• In worst cases, engine work

How Much Does It Cost?

If the vehicle hasn’t been driven:

• Fuel drain services typically range from $200–$500

If damage has occurred, costs can increase significantly depending on the components affected.

It’s also important to note:

❌ Most Australian insurance policies do not cover misfuelling ❌ Incorrect fuel use can void manufacturer warranty

That’s why acting quickly matters.

Prevention Tips for NSW Drivers

✔ Double-check the pump label before filling ✔ Know your vehicle (especially if switching between diesel and petrol cars) ✔ Don’t rely on handle colour alone ✔ Avoid fuelling when distracted or in a rush

Taking a few extra seconds at the pump can save thousands in repairs.

Maintenance Is Key

Regular servicing ensures:

• Fuel systems are clean
• Filters are in good condition
• Injectors are operating correctly
• Engine diagnostics are up to date

At Tynan Motors Service, our factory-trained technicians can inspect your vehicle if you suspect misfuelling or contamination.

Early diagnosis can prevent costly long-term damage.

Think You’ve Put the Wrong Fuel In?

Contact your roadside assistance provider or call a tow truck and speak to our Tynan Motors Kirrawee, Miranda or Wollongong service teams for a booking.

Mistakes happen. Acting fast makes all the difference. 🚗