Written by Simon McBride ▼

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

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Have you just put the wrong fuel in your car? It doesn't matter if you have put Petrol in a Diesel, or Diesel in a Petrol... this guide can help

Have you noticed your error at the Petrol Station?

Don't start the engine, do not put your keys in the ignition, and don't unlock the car! Turning the key can prime the fuel pump and contaminate engine components, even if you don't turn all the way. Some modern cars also prime the fuel pump when you unlock the car doors. Call a breakdown assistance service, grab a coffee, and wait for them to come and pump all of the fuel out of your tank. You can then fill up properly and be on your way.

Have you noticed your error after you've left the Forecourt?

We'll assume your reading this having noticed your mistake and pulled over. Ensure you have left your vehicle in a safe location, as it's probably better to wreck your engine than it is to put your life, or the lives or others at risk. Remember that if you are on a motorway, ideally you should park next to one of the emergency phones which are located at 1 mile intervals. You should put your hazard lights on, and if it's dark leave your sidelights on too. It's advisable to stand on the verge so that you are well out of the way of fast moving traffic. From here you can call a breakdown service, but remember to always face the traffic when you're using the phone. If your car is under warranty, you could invalidate the warranty unless you let the manufacturer fix the problem. Bear this in mind before calling in a local breakdown company and letting them do any work on the car, it could cost you more in the long run.

Have you called the breakdown company and gasped at the price of rescue?

Probably best you pay them and avoid wrecking your engine. However, theoretically if you were near empty and you only squirted in a small amount of wrong fuel, you could then completely the fill the tank with the correct fuel and be on your way. Some engines can handle up to 10% contamination, so if you only added 1 litre of the wrong fuel and still have space for 50 litres of the correct fuel you could risk it. However, even a small amount of contamination will wreck modern common-rail diesel (CRD) engines for example. Check your vehicle handbook if you're unsure, or call the manufacturer and get their advice.

Disclaimer: We take no responsibility for your actions. We are not trained mechanics. These tips are provided as a guide, bringing together advice from around internet and various forums. You should always seek the advice of a trained vehicle mechanic or breakdown service when dealing with car maintenance issues such as this.

Are you unsure if you have misfuelled?

Aside from the obvious, i.e. checking your receipt, typical symptoms include:

  1. The vehicle is difficult to start, especially when the engine is warm
  2. The engine gets very noisy under load
  3. Your exhaust is smoky when driving
  4. The engine won't stop once started
  5. Sudden loss of power
  6. Complete loss of power

Do you need cheering up?

If it makes you feel any better, 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. This trend has continued to the point that as of 2014, diesels now account for 34.5% of all vehicles on Britain's roads.

Whilst running diesel cars can be good for peoples wallets, with the increasing number of diesel-engined cars on the road comes the increased number of cases of misfuelling - filling diesel cars with unleaded petrol. It is thought to be happening as many as 400 times a day, with repair charges ranging from a few hundred to several thousand pounds.

A report in 2014 showed that the british police force spend around £300,000 a year as a result of officers not paying attention at the pumps, which is an average of 4 police cars damaged per day! Unbelievable.

Misfuelling Checklist - Preventing Misfuelling

  1. 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.
  2. Is everyone who drives your car aware of the fuel-type it uses?
  3. Have you labelled your fuel tank with a 'Diesel Only' sticker?
  4. Do you double-check fuel pumps at petrol stations? Green may not mean unleaded, black may not mean diesel, especially when driving abroad! Unbelievably, they're the opposite way around in some countries. So much for standards.
  5. Think you might have filled up with unleaded? Your receipt should tell you.
  6. 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.
  7. 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.
  8. 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 strips a diesel engine of the lubricant it needs to keep it ticking over, as well as damaging seals. It will almost always lead to the engine seizing, by which point the whole engine block and many vital components will likely be destroyed beyond repair.

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 simply by unlocking the doors. Modern technology doesn't always help when it comes to human error.

How much will this mistake cost?

Brace yourself.

If you've avoided turning on the engine or stopped quickly after turning on the engine, a fuel-tank drain may be sufficient. This can cost anywhere from £130+. It really depends on the area you've misfuelled in, and therefore which breakdown companies are within range. Don't be surprised if you're charged more if you're somewhere remote, as you're paying for their hourly rate inclusive of the time taken to reach you, as well as for safe disposal of your tank full of contaminated fuel afterwards.

If you've driven the car after misfuelling, it is going to be costly. Vital components will be damaged, and unless you're driving a go-kart the bill for repair will easily stretch well into the thousands.

Am I covered on my breakdown cover?

Misfuelling will not be covered by your manufacturer, though they may help tow your car to a dealership for you.

Am I covered on my insurance?

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

What if I lease my car?

We are sorry to say that 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 contract. Misfuelling and then firing up your engine really is about the worst thing you can do to your car, expect bills of at least several thousand pounds for a replacement engine and fuel system. You will be crying yourself to sleep for weeks, and you'll never hear the end of it from your mates. Chances are you'll never let it happen again though.

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 older 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. You can also buy little gadgets which you stick inside your fuel flap that are light activated, and shout out "Diesel, Diesel...." when you open the fuel cap. These really exist, but to be honest we've never seen anyone using them.

Until manufacturers work with fuel suppliers to address the issue, it's simply a case of having your wits about you and bearing the 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. Otherwise, you could always just buy an electric car and never have to worry about this problem again.