Car Doctor: Can fuel clean an engine?

Q. My N registration Nissan Micra has a Delta car alarm system, model 120A supplied by Winn, in West Hampstead.

Q. My N registration Nissan Micra has a Delta car alarm system, model 120A supplied by Winn, in West Hampstead. But one of the two key system fobs is faulty, and I cannot contact the dealer because they seem to have left their premises. Who can supply a replacement key?
Mr C Burgess, Hampshire

A. Nissan say any of their dealers can supply one. Your closest dealer is Hilsea Nissan, 023 9266 4000. You can also check out their website at www.hilseanissan.co.uk

Q. My car is an old Peugeot 306 XL which has just passed its MOT. A friend suggested using BP Ultimate fuel. He said it cleans and maintains the engine, reducing the cost of the next MOT. It seems quite strange that a fuel can clean, is this true?
Elizabeth Newsome, London

A. The Car Doctor still needs a lot of convincing that they make a difference. Big oil companies spend a lot of money promoting them, in particular BP Ultimate and Shell's Optimax. That is not surprising; it can cost up to 20p more a litre than unleaded petrol.

These fuels may boost performance, because normal unleaded is rated at 95RON and Optimax is 98RON and Ultimate, 97RON. Whether they boost performance is questionable and Shell got into trouble with the Advertising Standards Authority for claiming Optimax gave "an extra burst of power when you need it". Higher RON-rated fuels are recommended by manufacturers who sell high-performance cars.

The more expensive fuels have more detergents, which help reduce emissions and engine deposits. The oil companies say they work, certainly long-term although recent independent assessments suggest they make little difference.

Modern engine management systems automatically adjust the ignition timing to combust the fuel at just the right moment, so why pay 20p more for the privilege? A Peugeot 306 XL will not run any better, and only proper servicing and maintenance will help it through the next MOT.

The Car Doctor runs cars on the cheapest fuel available and to significant six-figure mileages without ignition or fuelling problems. But then if you want to use the same sort of fuel as Mikey Schumacher, go ahead.