Petrol acts as a contaminant in a diesel fuel system and can cause severe damage.

If you have accidentally put petrol in your diesel vehicle, do not start the engine.

Starting the engine allows petrol to circulate through the high-pressure fuel system, increasing the risk of costly damage.

Today, it is even more common, thanks in part to Australia’s love for diesels. Petrol is a contaminant in a diesel fuel system, and internal damage cannot be accurately assessed without proper inspection.

The result, however, can be catastrophic. It can lead to the destruction of the fuel system or the engine. In severe cases, engine failure at speed can create serious safety risks. In my 30-year career, I’ve witnessed failures that have resulted in court cases 12 months after the incident initially occurred.

How Does The Damage Occur?

The fuel injection system in a diesel engine works on hydraulic principles. The fuel must have the right consistency to ensure optimal operation, lubrication, and temperature control. This will provide the correct fuelling for the engine.

Petrol is far less lubricating and does not have the hydraulic capabilities to operate the diesel injection. Diesel fuel systems operate with extremely fine tolerances — often less than a human hair in thickness — and rely on diesel’s lubricating properties. Petrol does not provide this lubrication.

If You Have Not Started the Engine

If the engine has not been started, damage may be avoided.

• Do not turn the ignition on.

• Do not unlock and relock repeatedly (some vehicles prime the fuel pump automatically).

• Arrange for the vehicle to be towed to a workshop.

In many cases, the tank can be drained, the system flushed and the fuel filter replaced before damage occurs.

If the Engine Has Been Started

If the engine has been started or driven, petrol may have circulated through the high-pressure pump and injectors.

Damage can occur quickly due to insufficient lubrication and increased friction. In some cases, complete fuel system replacement may be required.

In some cases, the engine may continue to run briefly. As the petrol concentration increases, fuel pressure drops and the engine may shut down. By this time, the petrol has entered the engine, which will contribute to actual engine failure.

What Damage Can I Expect?

Damage can occur to the high-pressure pump and injectors when the engine has been idling or driven only a couple of kilometers. When the engine increases rpm, an increase in temperature follows and, soon, hot spots appear on the internal metal components. The chance of failure is greater than ever. In some situations, the vehicle will either shut down or, worse, rev out of control. This will cause catastrophic damage to the fuel system and engine components.

The valve, image right, is from a Toyota V8 Land Cruiser diesel. After the owner filled the diesel with petrol on several occasions, due to the high octane petrol combined with a much higher combustion temperature engine, the failure occurred.

Can the System Be Cleaned?

If the engine has not been started and petrol has not circulated beyond the fuel filter, the system may be drained and cleaned successfully.

However, once petrol has passed through the high-pressure pump and injectors, the risk of internal damage increases significantly.

Before attempting cleaning, a workshop must confirm:

• The engine has not been started

• The in-tank pump has not circulated fuel

• Petrol has not returned through the high-pressure system

Only then can the tank and fuel lines be drained, the fuel filter replaced, and the vehicle safely refilled with diesel.

Repairs can exceed $15,000 depending on the extent of damage. In severe cases, engine failure at speed can create serious safety risks.

If you have put petrol into your diesel vehicle, do not start it. Arrange for it to be towed to a workshop.

Prevention is always less costly than repair.

We have created a technical bulletin to support the automotive industry and diesel owners for this specific issue. Feel free to download and share Wrong Fuel: Why This Can Destroy Diesel Fuel Systems and Engines.

For information on maintaining diesel fuel systems, see our guide on Common Diesel Problems Explained.