Drivers who mis-fuel their diesel cars with petrol can expect a costly bill to repair the damage, according to UK company Lex Vehicle Leasing.

The firm's maintenance department, which spends around £50 million a year on the upkeep of its 123,000 vehicle fleet, is seeing repair costs range from £300-£3,000 depending on the type of diesel engine fitted to the car.

Most at risk are the modern common rail diesels which are built to more exacting tolerances than standard diesel engines. If petrol fuel gets into the common rail diesel system it can mean replacing both low and high pressure fuel pumps, injectors, rail, line filters and tanks, which can easily cost a few thousand pounds to fix.

"The cost of the repair depends on whether the driver has simply started the engine or they have driven it for a few miles with the wrong fuel in the system. The longer petrol has been in the system, the more money it will cost to repair," said a spokesman.

"Manufacturers are now giving their dealers detailed advice as to which stage the mis-fuel has reached and the correcting action they must take. We had two Mercedes common rail diesels to repair recently which cost £3,000 per engine," he added.

All costs to repair mis-fuels have to be met by the consumer as manufacturer breakdown cover does not cover mis-fuels, although they are likely to come out to the car and take it to the nearest dealership. If a car is on a PCP or contract hire agreement it will still be up to the driver to pay for the damage, rather than the finance company.

Lex advises lease drivers to copy rental companies and label the car's fuel access flap with the correct fuel type, or place a discreet label inside the car.

"When repairs cost up to £3,000 it's worth drivers taking some precautions to prevent them from mis-fuelling. Rental companies have seen mis-fuels come down significantly by taking these precautions so it's worth the investment in clear labelling for the driver," said the spokesman.

Among the reasons why repairs following a mis-fuel cost so much is the fact that diesel oil is a lubricant, while petrol is a solvent. Diesel fuel injection equipment needs to be lubricated by the diesel oil when the engine is running. Not all the fuel delivered to the fuel injection system is used; around 70% of fuel is returned to the fuel tank. As petrol is a solvent it strips the lubricant off the engine's working parts so causing metal to come into contact with other metal parts which can generate debris inside the engine.

Any debris from the injection system may be returned to the tank and then drawn back up and passed through the system again.

After filling a diesel engine vehicle with petrol a common misconception is that just turning the ignition on will not be a problem and that no damage will occur. Most systems now use a low pressure electric pump fitted in the tank or sender unit, contaminated fuel is immediately circulated through the pump and fuel rail, when the ignition is turned on. Where the vehicle has a mechanical gear driven pump the same level of contamination will occur if the engine is run.

Repirs require the fuel tank to be drained completely, preferably off the vehicle, without using the electric pump, as it is lubricated by diesel. Running with petrol causes the internal bushes to fail which transfers metal swarf to the rest of the fuel system. As most tanks have baffles in them, the tank must be partially filled with clean fuel and swilled repeatedly until there is no further evidence of debris in the expelled solution.

The fuel lines (feed and return) must be blown through to evacuate any residual fuel. Fresh fuel should be purged through the lines with the injectors disconnected.

Common rail diesel vehicles will not tolerate any petrol in their fuel systems, in fact the manufacturers of the fuel injection system state that any petrol contamination should necessitate replacing the low pressure pump, high pressure pump, injectors, rail, lines filters and tanks. This is not really practical and usually costs more than a fully dressed engine.