Motorists in Northern Ireland are running the gauntlet of contaminated fuel when they buy petrol or diesel from some filling stations.
Criminals had been removing a marker dye from red diesel, which attracts a low duty payment, and selling it at a much higher price at the roadside.
It is known as laundered diesel and can cause damage to the more sensitive parts of diesel engines.
But BBC Northern Ireland's Newsline programme has discovered that several substances are also being added to adulterate both diesel and petrol.
The programme was able to purchase so-called bio diesel from a road front garage that had been raided a few weeks earlier for selling illegal fuel.
But the bio diesel turned out to be little more than ordinary or illegal laundered diesel with a substantial amount of kerosene or home heating oil added.
'Stretching'
It is also known that some filling stations are "stretching" their diesel by secretly adding heating oil or kerosene to the underground tanks.
They do this after taking a delivery of conventional diesel which helps mask the special tell-tale dyes that are in the heating oil. It is hard to spot in a casual check.
The fuel that does the most harm is that contaminated with paraffin, hydraulic oil or even gear oil. Adding paraffin to diesel can cause a diesel engine to breakdown.
So the people who make it add the oils as a lubricant. The oils are chosen because they do not produce much smoke when burned and are less likely to give the game away.
Some people add substances like surplus white spirits or paint thinners.
It is all to do with bulking out the diesel. The crooks can make it for a few pence a litre and sell it at many times that.
It is often sold below the price of supermarket diesel. Often the garages who buy the fuel may be unaware that it is illegal.
Its thought that one filling station in three sells contaminated fuels. While the bulk of it is contaminated or laundered diesel, the criminals have now turned their attention to petrol.
They hope to increase their profits by adding a small amount of paraffin to every litre of petrol.
However, increasingly, the more sophisticated petrol engine is rejecting the fuel and it is this that is starting to tip off the authorities.
Security measures
The problem is seen as a threat by legitimate fuel suppliers and one local fuel supplier, Maxol, has set up special security with their retailers to prevent the fraud.
"We, as a company, lock all the tanks. We hold the keys and it is only our personnel who can make deliveries. We carry out sporadic and ad hoc checks" says Brian Donaldson of Maxol.
It is a difficult problem to solve. Some of the unbranded diesel originates across the border in the Republic of Ireland.
It is almost impossible for independent retailers to test the fuel beforehand.
Motorists can only reduce the chance of buying this fuel by avoiding it if that they think is too cheap to be legal.
They can also ask if VAT receipts are available and if credit cards are accepted. Even then there is no guarantee.
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