Written by Robin Brown ▼

'Help, I´ve put petrol in a diesel engine' - Frequently Asked Questions

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As of 2012, figures from the Society of Motor Manufacturers and Traders showed that diesel models comprise more than half of all new-car sales.

Efficiency of the newest diesel engines are making them competitive with hybrid engines for fuel efficiency.

In light of fuel costs that are threatening to top £1.40 per litre nationwide, with that figure already passed in many rural areas, diesels are becoming more popular than ever.

New ´clean diesel´ engines such as those from Volkswagen´s BlueMotion range and diesels equipped with BMW´s EfficientDynamics programme hugely reduce fuel consumption, sooty particulate matter and carbon dioxide emissions.

The Polo BlueMotion returns around 70mpg and emits just over 100g/km of CO2 while many diesels now fall under 100g/km in the UK. Both models also feature Diesel Particulate Filters, making the new diesel a popular choice among those looking to purchase a ´green´ car.

But with the increasing number of diesel-engined cars on the road there´s a resulting rise in the number of incidents of misfuelling - filling diesel cars with unleaded petrol.

It´s thought to be happening more than 400 times a day with repair charges ranging from a few hundred to several thousand pounds.

So what do you do if you´ve made this mistake? Read on, and we´ll try to advise.

Misfuelling checklist

• Have you checked if the car you´re driving is a petrol or diesel model? This is particularly important if using a hire car or company car.

• Is everyone who drives your car aware of the fuel-type it uses?

• Have you labelled your fuel tank with a ´Diesel Only´ sticker?

• Do you double-check fuel pumps at petrol stations? Green may not mean unleaded, black may not mean diesel.

• Think you might have filled up with unleaded? Your receipt should tell you.

• Remember NOT to start your engine if you think you´ve filled up with unleaded. If possible do not unlock doors. If you have misfuelled the amount of your repair costs could depend on this.

• If you are attempting to dilute unleaded petrol with diesel ensure your car is not a modern common-rail diesel. If it is do not even consider diluting.

• It should only be possible to dilute 10% of unleaded or under. Make sure you know the capacity of your fuel tank and make a judgement about whether it´s worth the risk.

• You may be able to drain a fuel tank yourself but if in doubt seek expert help. There are specialist companies that will come to your house and drain your fuel tank.

How does the wrong fuel type affect engines?

Petrol in a diesel engine Breakdown organisations put the figure of petrol misfuelling at around 150,000 cars a year, with repair bills totalling hundreds of millions of pounds.

Petrol will strip your diesel engine of the lubricant used to keep it ticking over and cause metal components to grate against each other and disintegrate.

This runs the risk of damaging the fuel pump and fuel-injection systems, or wrecking an engine completely if used for a significant amount of time.

To make matters worse pre-ignition systems start fuel pumps in new diesel models when car doors are unlocked. This cuts out the time it takes for new diesel engines to warm up, but means that diesel engines can be ruined by petrol by simply unlocking doors.

The fuel pump and injection systems and filters will likely need to be replaced on new models, while an entire engine could be wrecked beyond repair if it has been run for long enough. Regardless of the extent of the damage, the repair bill could potentially run into thousands of pounds.

Avoiding turning the engine over or stopping as soon as possible is vital to prevent more damage to the engine in these circumstances. A fuel-tank drain could be sufficient in such cases to prevent damage.

How much will this cost?

If you´ve avoided turning on the engine or stopped quickly after turning on the engine a fuel-tank drain may be sufficient. This should cost between £100-£200.

If you´ve driven the car for several miles and your car has a common-rail injection system you may be looking at a bill that runs into quadruple figures.

Am I covered on my breakdown cover?

Misfuelling is not covered by your manufacturer, though they make tow your car to a dealer or garage.

Am I covered on my insurance?

Misfuelling is generally excluded from insurance policies, even those offering comprehensive protection.

Anyone with third-party insurance is looking at stumping up for repairs themselves.

What if I lease my car?

You´ll have to meet the cost of any repairs yourself if your car´s on PCP or contract hire - or any other form of leasing.

Will my warranty be invalidated?

That probably depends on the amount of damage. Your manufacturer may threaten to invalidate your warranty on your fuel system and related parts unless you let them do the repair work, but there may be room for negotiation.

Can I dilute the petrol with diesel?

Diluting contaminated fuel may be possible on older cars that lack the fine, high-pressure diesel injection systems of modern diesels. However, this is a high-risk move which could result in the destruction of your engine.

It may be possible to dilute a proportion of petrol to diesel under 10% in such cars, though there´s no guarantee this will work. Your car may run roughly for a while and you may risk long-term damage that is not immediately apparent.

We don´t recommend you try this but if you do risk it you need to be completely sure about your car´s fuel tank and engine system - try this on a common-rail diesel and you´ll simply wreck your engine.

How does this happen?

Modern diesel engines can be hard to differentiate for drivers with an untrained eye, or ear. Where traditionally diesel engines were clattery, dirty and slow to warm up, modern diesels drive and sound very similar to petrol engines. People simply forget they are driving a diesel as the associated audio and visual clues are simply not there.

In light of the dash for diesel manufacturers are turning their attention towards the problem of misfuelling.

Ford has been the first manufacturer to develop technology designed to prevent a misfuelling mishap, with its Easyfuel system, which prevents drivers filling up their diesel models with petrol.

An unleaded petrol pump nozzle cannot enter a diesel tank on Mondeo and Focus models equipped with the system. Typically diesel nozzles are too wide to fit into any petrol tank, so the problem rarely crops up.

To make matters more confusing the increasing uniformity of petrol pump colours does not provide motorists with a visual cue as to the fuel type.

Whereas diesel pumps were traditionally black, unleaded petrol green and four-star leaded petrol red, modern-day diesel pumps are often green to enhance fuel suppliers´ eco credentials, with BP being the most notable culprit.

With the massive rise in popularity of diesels misfuelling is an inevitable and growing problem which is only just starting to be addressed by manufacturers.

Diesel in a petrol engine

Theoretically it should be hard to fill a petrol tank with diesel, as diesel nozzles are too wide to fit into the tank. However using diesel in a petrol engine may destroy the engine's catalytic converter, necessitating a costly replacement.

How much biodiesel/ethanol can I safely put in my car?

If you´ve accidentally filled your tank with biodiesel or ethanol the likelihood is you´ll need to drain your tank, unless your car is correctly modified to receive these fuels.

What can I do to prevent this?

Aside from buying a Ford model with the Easyfuel system the best thing you can do is to clearly label your vehicle with a sticker above the petrol tank, as car-hire companies generally do. This is particularly important if your car is used by a number of drivers.

Until manufacturers or fuel suppliers address the issue it´s simply a case of having your wits about you and bearing those astronomical repair costs in mind.

Recent moves to introduce such a system by BMW across its model range suggest that manufacturers are intent on tackling the problem.