Most Frequently Asked Question

Is it worse to put petrol in a diesel car, or diesel in a petrol car?

Unfortunately a lot of the information provided on this question is based on opinion or theory - rather than actual mechanical experience. I would like to set the record straight once and for all.

Summary: The TL:DR answer for this question is that there is no answer: it totally depends on the ratio of contamination, age and condition of the affected fuel system, how long the engine is run on the contamination, etc. There is no “one answer fits all”.

So, let me explain in detail:

Liken a vehicle engine to a human body. Putting the wrong fuel in the vehicle is like putting a poison into the body. Every person will have a slightly different reaction to the poison: some will be greatly affected immediately, others will have less of a reaction, and some might never even notice it at all.

To really understand the problem of mixed fuel it is important to know the differences between petrol and diesel.

Petrol (definition): a highly volatile, flammable solvent made from a mixture of refined petroleum and natural gas by-products (such a naphthenes, pentanes, octane, isobutene, etc). It is manufactured specifically for use in internal combustion engines. Its basic general chemical composition is C9H2O.

Diesel (definition): Referring specifically to the petroleum derived distillate of petroleum (aka Petrodiesel): a heavy fuel oil made by mixing heavy petroleum distillate with various other chemicals to obtain a petroleum oil that combusts under compression (as opposed to petrol/gasoline which ignites with a spark). Its basic general chemical composition is C14H3O.

A Petrol/Diesel Mixture (aka Contaminated or Mixed Fuel)

As mentioned above, petrol is a solvent; diesel is an oil. From this we can see that in a mixture of petrol and diesel, the petrol will dissolve the diesel and will therefore reduce its lubricity and change its chemical composition. Additionally, the octane rating and cetane number of the fuel will be changed. This will greatly affect its ability to function correctly in a diesel or petrol engine.

People who tell you that the “petrol is lighter” and that “you can separate the fuels by letting one fall to the bottom” unfortunately are not familiar with the properties of the two fuels. The ONLY way to separate a petrol/diesel mixture is fractional distillation – exactly the same way they were originally separated in the refinery.

Putting Petrol in a Diesel Engine:

A diesel engine relies on diesel’s ability to ignite when compressed to a certain pressure (cetane number). Mixed fuel will have a different cetane number and therefore will not ignite correctly. Fortunately adding petrol tends to lower the cetane number. If it went the other way you would get premature ignition which could potentially damage the entire drive-shaft.

However, the major problem with Mixed Fuel in a diesel engine is the fact that Diesel Oil, in addition to being the fuel in a diesel engine, is also the lubricant for the system. Petrol removes this lubrication and causes the delicate lift pump and high-pressure pump to wear. It also tends to “dry out” the injectors which can cause them to fail.

Worst case scenario problems commonly associated with using Mixed Fuel in a diesel engine include:

• Corrosion of rubber and plastic fuel system components (pipes, seals, etc).
• Wearing out of lift and high-pressure pumps.
• Injectors ceasing due to “drying out”.

Fortunately you would really only expect to see these problems on a vehicle run for a very long time on Mixed Fuel.

The probability of this “worst case scenario” happening is, according to our experience, is about 1:500 (0.20% of known cases of misfuelling).

Most people notice the following symptoms and get the vehicle fixed before there is any chance of doing any real damage:

• Vehicle not starting or constantly stalling
• Loss of engine power
• Engine coughing or shuddering
• Excessive smoke

Putting Diesel in a Petrol Engine:

The most common fault encountered when using Mixed Fuel in a petrol engine is that the sparkplugs become coated in fuel oil and will no longer spark correctly. This, as well as an incorrect octane rating, can cause the fuel to not burn, or to burn incorrectly. The excess smoke caused by this can damage the catalytic converter and ECU sensors. Also, diesel oil can enter the oil sump and thin out the vehicle’s oil.

The combination of thinner oil and incorrect ignition timing (often resulting in engine knocking) can, in the “worst case scenario” result in complete engine failure/seizure, bent piston-rods and cam-shaft, burnt-out cat and sensors.

The probability of this “worst case scenario” happening is, according to the same experience, is even lower at about 1:750 (0.13% of known cases of misfuelling).

Again, the most commonly noticed symptoms which let the driver know something needs to be fixed are:

• Vehicle not starting at all
• Loss of engine power
• Engine coughing or spluttering
• Engine knocking
• Excessive smoke

Conclusion:

If caught early and dealt with quickly there are NO major long-term problems associated with using Mixed Fuel in either a petrol or a diesel engine. Additionally the cost of getting a fuel drain is about the same whether you have petrol in a diesel car, or diesel in a petrol car.

In the “worst case scenario” a Petrol in Diesel fix will involve replacing the entire fuel system at a cost of anywhere between £1,000 and £9,000 depending on the vehicle.

The same “worst case scenario” for a Diesel in Petrol fix will involve replacing the entire engine (not the fuel system). This can incur the same or even higher costs, and will often result in the vehicle being written off.

Wrong Fuel SOS

Wrong Fuel Advice

• Petrol in a Diesel Car? Diesel in a Petrol Car? Wrong Fuel in Car?
• Don't start the engine (not even to move the car off the pump). Keeping the contaminated fuel in the tank and away from sensitive fuel system components will reduce the risk of damage to your vehicle.
• Call Wrong Fuel SOS now on
  0800 711 7172.
• Get professional advice on what to do next. Our call operators are on hand 24/7 to help get you going again.
• If you have started the engine or driven the car, don't panic. Almost 99% of vehicles run on contaminated fuel can be fully cleaned out without any lasting effects.

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