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What do fuel additives do? And are they any use?

The market for fuel additives is small, but if the additive makers are to be believed, they could transform your wheezy old engine into a peak performer, just by pouring the right additive in your fuel tank. These additives are designed to unclog blocked injectors, improve ignition efficiency, and even reduce or eliminate knocking (the burning of fuel in the cylinder after combustion), and generally help your engine run more smoothly.

Fuel supplements are available for both petrol and diesel engines (there are specific additives for each fuel type), and they’re poured directly into the fuel tank when it’s full, so the solution is diluted with your fuel. This mixture then flows along the fuel lines, fuel pump, into the injectors or carburettor and into the combustion chamber, treating them all along the way. In reality, these additives tend to work best with older cars that have been around for 10 years or more, or vehicles that have been neglected and missed out on regular maintenance.

Additive types

There are a few different kinds of fuel additives on offer, and they do different things. You’ll find different additives for petrol and diesel engines, and there are additives designed for older cars that need leaded fuel. As unleaded has been the only petrol available on forecourts since 2000, these lead supplements are a lifeline for owners of older cars, which can suffer excessive valve wear and poor power delivery if they run on unleaded fuel.

Coldest roads Kolyma Highway refuel

If you live in a cold climate and run a diesel car, you can get additives that stop the diesel fuel in your tank from congealing, which can cause blockages in the fuel system. And if you’re putting a car into storage, another fuel additive can help preserve the fuel so that the car is easily started when you come back to it.

An engine flush could also prove useful if you’re trying to get a car that hasn’t run in a long time back on the road, as it should loosen off any residues and crust that has built up in the fuel system over time.


While these additives do help in certain situations and with different engines, the additives you should steer clear of are those that claim to improve fuel economy. Even with the best will in the world, the reality is that any savings you make in economy will be more than cancelled out by the price of the fuel additive in the first place.

What’s more, independent tests by consumer group Which? have revealed that these supplements don’t tend to deliver on their promises of adding oomph to your engine – some can even increase the cost of a petrol refill by 50 per cent! Most of these products claim to increase a fuel’s octane rating (a higher number can make the engine run smoother), but none managed even to equal the rating of premium-grade petrol.


The reality is that if you own a car that’s less than 10 years old that has been well maintained, there really isn’t any need to use any of these kind of additives, as the car’s electronics and fuel system will be optimised to deliver the best mix of performance and efficiency in the first place. And if that’s the case, it begs the question – if fuel additives are so good, why don’t car makers recommend them? And why are they not commonly found at petrol stations across the country?

Fuel station

In reality, they sort of are. If you feel like your car could do with a bit of a boost, then a better value alternative than an additive could be to use a higher octane forecourt fuel than regular 95 octane. Even supermarket chains offer 99 octane fuel these days for a few pence more than regular petrol, and leading fuel retailers make grand claims about the effective ‘cleaning’ that high-octane fuels can achieve.

These fuels are a necessity for highly tuned sports cars, but some firms even offer higher octane diesel fuel, and running a tankful through a regular car won’t do any damage, and may even give your car the boost it needs.