RACQ Motoring


Filling a vehicle’s fuel tank with the wrong fuel appears to be an increasingly common mistake. Here is some information about the consequences of this mistake and what needs to be done to correct it.

Petrol in diesel vehicles

Filling a vehicle's fuel tank with the wrong fuel is an increasingly common mistake.

This is a very common situation and depending on the vehicle, potentially the most costly to rectify.

Older style diesel engines may tolerate a small amount of petrol mixed with diesel without too much damage, although this is definitely not recommended. However newer Common Rail diesel fuel systems are far less tolerant of the wrong fuel and any level of petrol risks damage, as the high-pressure pumping elements rely on the lubrication properties of the diesel fuel. Petrol is a solvent which strips away the diesel fuel from the highly loaded pumping elements causing catastrophic damage.

If you’ve put petrol in a vehicle fitted with a Common Rail diesel injection system, don’t even turn the key, as the electric fuel pump will circulate the petrol through the whole system, necessitating a major repair.

The extent of the work required to rectify petrol in a diesel vehicle will depend on the type of vehicle and how long it has operated on the wrong fuel.

In the worst case, expect a bill for several thousand dollars (repair costs of in excess of $10,000 are common), as rectification could require repair or replacement of much of the fuel system, as well as repairs to the engine itself if it has suffered damage. The best case, where the fuel has been put in the tank but the engine wasn’t started, will involve draining the system and adding the correct fuel.

The RACQ Online Shop sells a device that helps prevent accidental mis-fuelling of diesel vehicles. Click here for more information: Diesel Mis-fuelling Device

Diesel in petrol vehicles

While this is less common, it does happen. Diesel fuel in petrol vehicles generally doesn’t cause too many problems, as often the engine simply won’t run. If it will run, typical symptoms that should not be ignored are pinging, exhaust smoke, loss of power and oil contamination due to incomplete combustion.

If the engine has operated on a diesel / petrol mixture for some time it is possible to damage pistons, along with the crankshaft and bearings. In most cases however, rectification will only involve draining the fuel, changing the fuel filter, flushing the lines to remove the diesel and refilling with petrol.

Incorrect petrol in petrol vehicles

Australia’s current standard fuel is 91 octane unleaded, with most cars sold locally since 1986 designed to accept it, however, an increasing number of vehicles require 95 octane or 98 octane fuel.

Using a high-octane fuel in a vehicle that only requires normal ULP will at worst hurt your pocket, but using 91 octane ULP in a vehicle specified for a high-octane fuel may damage the engine.

In the event of a mis-fuel with a lower octane fuel, it will be necessary to check the fuel specification in the owner’s handbook. Some vehicles specified for high-octane fuels will operate satisfactorily on ULP, albeit with reduced power and economy. However, others risk extensive engine damage so it’s important to read the recommendations carefully.

Providing the vehicle hasn’t been driven too far, it’s likely that the only action needed will be to drain the tank and add the correct fuel.

If you found this fact sheet helpful we have over 100 other Motoring fact sheets to assist you.

Was this information helpful?

Help us make this fact sheet more useful - please let us know what you think.
Don’t forget to leave your email address if you’d like a reply.

Remaining characters:
The information provided in this fact sheet is intended as a general guide only. You should carry out further research and investigations into the topics outlined above prior to implementing any actions or recommendations