Around 150,000 drivers misfuel every year in the UK. This equates to someone putting the wrong type of fuel in their car every 3 minutes!
It happens to experienced drivers as well as new ones and more often than you might expect at unfamiliar pumps or when driving a hire car. If it has just happened to you, the most important thing is to stay calm and not start the engine.
This guide walks you through exactly what to do if you put petrol in a diesel car and vice versa.
Plus, we’ll explain something most guides miss: what misfuelling does to your fuel filter and why that matters for the long term health of your vehicle.
Petrol In A Diesel Car Vs Diesel In A Petrol Car: Which is worse?
The two scenarios are not equally serious, so it is worth knowing which situation you are in.
Petrol In A Diesel Car
This is the more common and more damaging mistake. Diesel acts as a lubricant for the fuel pump and injectors. Petrol removes that lubrication and if the engine is run, causes metal-on-metal wear in components designed to operate on the oily, viscous properties of diesel. Even a relatively small quantity of petrol (as little as 5% contamination) can be enough to cause damage if the car is driven. The fuel filter will trap some of the contaminated fuel, but it is not designed to cope with the wrong fuel type and will need to be replaced as part of the drain and flush process.
Diesel In A Petrol Car
Less common and generally less damaging because petrol engines have larger fuel injector nozzles and the diesel nozzle is usually too large to fit a petrol filler neck in the first place. If it does happen, the diesel will foul the spark plugs and cause the engine to run poorly or stall. As with petrol in a diesel, the fuel filter will be contaminated and should be inspected after the fuel system is flushed.
What To Do Immediately
1. Do not start the engine. This is the single most important step. Starting the engine causes the wrong fuel to circulate through the fuel pump, fuel lines, injectors and filter. The damage caused by running the engine even briefly can turn a recoverable situation into a much more expensive one.
2. Do not put the key in the ignition. On modern vehicles, inserting the key or pressing the start button can activate the fuel pump even without starting the engine, beginning the circulation process. Keep the key out.
3. Put the car in neutral and push it away from the pump if possible. If you are blocking the forecourt, alert the petrol station staff. Many forecourts have dealt with this before and can help you move the vehicle safely without starting the engine.
4. Call roadside assistance or a fuel drain specialist. The RAC, AA and dedicated fuel drain companies can usually attend within the hour. A mobile fuel drain service will drain and flush the tank at the roadside, which is far cheaper and quicker than a garage recovery in most cases. Costs typically range from £100 to £200 depending on tank size and contamination level.
5. Inform the fuel drain technician how much wrong fuel was added. Even if you only put a small amount in before realising, tell them. They need to know the approximate ratio of contamination to assess what the fuel system requires.
What Happens During A Fuel Drain
A fuel drain service involves removing all fuel from the tank via a pump, flushing the tank to clear residue, and refilling with the correct fuel. On a diesel vehicle where petrol has been added, the technician will also typically flush the fuel lines and, depending on how far the contaminated fuel has travelled, may inspect or replace the fuel filter.
If the engine was started before the mistake was noticed, a more thorough inspection at a garage is advisable. Components, including the high pressure fuel pump and injectors, should be checked for wear.
Why The Fuel Filter Matters After Misfuelling
This is the part most guides do not cover, and it is important.
Your fuel filter’s job is to trap contaminants before they reach the injectors and engine.
During a misfuelling incident, the filter is one of the first components the contaminated fuel passes through. On diesel vehicles in particular, the fuel filter will often absorb some of the petrol before the tank is drained.
Even after a successful drain and flush, a filter that has been exposed to the wrong fuel type should be replaced. Here is why:
- Petrol degrades the filter media. The different chemical composition of petrol can break down the filtration material in a diesel fuel filter, reducing its effectiveness and potentially releasing particulate matter into the fuel system.
- Residual contamination. A used filter will retain traces of the wrong fuel. When new diesel flows through a compromised filter, those traces can still reach the injectors.
- Reduced flow rate. A filter that has been stressed by contamination may become partially blocked, starving the high-pressure pump of fuel and causing performance issues that are not immediately obvious.
If your vehicle has gone through a misfuelling incident, replacing the fuel filter as part of the recovery process is straightforward and inexpensive relative to the cost of injector or pump damage down the line.
At Filter Services, we stock fuel filters for a wide range of vehicles, including cars, vans, HGVs, agricultural machinery and more. We stock all of the top brands, including Fleetguard, Donaldson, Mann, Baldwin and Hifi.
If you need a replacement fuel filter following a misfuelling incident, our team at Chesterfield, Leeds or Leicester can help you find the right part.
What To Check Before Driving Away
Once the fuel has been drained and the system flushed, do not simply assume the vehicle is ready to go. Before setting off, confirm the following with the technician or your mechanic:
- The fuel tank has been fully drained and flushed, not just diluted with the correct fuel
- The fuel filter has been inspected and replaced if necessary
- The fuel lines and injectors have been checked for residual contamination
- On diesel vehicles, the high-pressure fuel pump has been assessed to determine whether the engine was run on the wrong fuel
If the engine was started on contaminated fuel for more than a few seconds, request a more thorough inspection at a garage rather than relying solely on the roadside drain.
Will Insurance Cover Misfuelling?
Standard car insurance policies do not typically cover misfuelling, as it is considered driver error. Some comprehensive policies include accidental damage cover that may extend to misfuelling costs. Check your policy wording or call your insurer before you assume you are covered or not covered. Some breakdown cover add-ons include misfuelling assistance, and a small number of insurers offer it as a standalone add-on.
Need A Replacement Fuel Filter?
If your vehicle has been through a misfuelling incident, a new fuel filter is one of the most cost-effective steps you can take to protect the rest of the fuel system.
Filter Services supplies fuel filters for cars, vans, HGVs, agricultural vehicles and plant equipment from leading brands including Fleetguard, Donaldson, Mann, Baldwin and Hifi.
With branches in Chesterfield, Leeds and Leicester, we can usually get the right filter to you quickly.
Call us on 01246 802 822 or browse our fuel filter range online.