Car clinic: Heavy-footed Laguna driver, windy 3-series sunroof, French road signs explained and checking vital fluids

The Car Clinic experts

TIM’LL FIX IT
Tim Shallcross used to train AA patrols to fix cars. Now he advises the Institute of Advanced Motorists.

INSPECTOR GADGET
Dave Pollard has written several Haynes manuals and has tested just about every car-related accessory.

LADY DRIVER
Emma Smith is a journalist specialising in consumer issues and is a regular Driving contributor

Q. My husband has the habit of depressing the accelerator pedal slightly on our Renault Laguna when stationary — at traffic lights and elsewhere. I think this is affecting the vehicle: I keep needing to replace the oxygen sensors, and the rev counter is erratic. Is my husband damaging the car?

MK, Leek, Staffordshire

A. Your husband’s habit burns extra fuel for no reason, but it is almost certainly not responsible for the problems you are experiencing. Oxygen sensors analyse the exhaust gas so that the engine management computer can adjust the amount of fuel being injected, to keep the air/fuel ratio exactly right.

Your Laguna has two sensors; properly fitted and with the correct specification, they should last for at least 70,000 miles, and most will be good for 100,000 miles or more.

Contamination is the main reason why sensors fail prematurely. If your engine is burning oil because, say, the pistons or valve seals are worn, the resulting ash will coat the sensors. The telltale sign of oil combustion is blue or grey smoke from the exhaust. Get your garage to check for this and make sure it fits good-quality sensors, as cheap ones are more likely to fail prematurely.

Expect to pay about £80 for the front sensor and £120 for the rear for a good-quality replacement part, plus labour (about one hour per sensor).

The problem with the rev counter is separate. The Laguna has a bit of a reputation for electrical faults, and erratic instruments are not uncommon. You may be able to live with this but if the problem spreads to the speedometer you will need to have the entire cluster repaired by a specialist. I recommend ecutesting.com or bba-reman.com/UK.

TS

Q. I’ve been enjoying the last of the sun through the sunroof in my BMW 3-series, but when it’s fully open the car vibrates and makes a lot of noise above 50mph. To reduce this I have to lower one of the rear windows. My dealer claims this is normal. Is he right?

RK, Sutton Coldfield, Birmingham

A. The phenomenon you refer to is known as “buffeting” or “wind throb”. It is caused by air being forced over the windscreen and then down through the open sunroof. This results in rapid pressurisation and depressurisation of the car’s cabin, creating a booming sound that can usually be felt as well as heard. Opening a window slightly prevents pressure from building up in the cabin and can eliminate the problem.

Your dealer is correct: this is a downside with most sunroofs, although the extent of the problem varies according to factors such as the angle of the windscreen and the location of the sunroof, as well as other aerodynamic features. Vehicle speed, and the wind direction and its velocity also have an effect on buffeting.

You could try fitting an external wind deflector to reduce buffeting. Climairuk.com has one for about £50.

DP

Q. I’m planning a driving holiday in France but know from past experience that it’s easy to be flummoxed by the country’s road-numbering system. Lots of routes seem to have two numbers and even a name, and these numbers seem to switch and change for no apparent reason. Can you shed any light?

RF, London

A. There are some basic rules you should be aware of that might make interpreting the signs easier. Any number prefixed by an A indicates an autoroute (motorway). Numbers prefixed by an N refer to routes nationales, part of the country’s strategic trunk road network (similar to A-roads here). Some of these roads — or parts of them — have returned to regional control and have been recategorised as routes départementales, or D-roads, which are maintained by the local département, or regional council. Smaller roads are designated as routes communales, or C-roads, and their numbers are often not displayed.

To add to the complexity autoroutes are sometimes given names as well as numbers, such as the Autoroute des Anglais (A26), which runs from Calais to Troyes, and the Autoroute du Soleil (A6 and A7), which goes from Paris to Marseilles.

Some roads also have an E (for European) number, which is written in white on a green background: the A6, for example, is also part of the E15. These routes are part of the international E-road network, a route numbering system devised back in the 1970s by the United Nations Economic Commission for Europe. The E15 runs from Inverness in the Highlands, through England and France to Algeciras in Spain. The UK chooses not to display E-roads on its signs, but France shows both national and European numbers alongside each other.

Finally, if you see a sign with the word Bis on it followed by a destination, this denotes an alternative or scenic route.

French motorists are not in a state of confusion because they tend to ignore road numbers and focus on destinations, or the next town on the route instead. I suggest you do the same.

ES

Q. According to my Vauxhall Vectra’s handbook, there is never any need to change the engine coolant, gearbox oil or power-steering fluid. It says they merely need to be checked at regular servicing intervals and topped up if necessary. Is this right, or should I ignore the handbook and get them changed?

DD, London

A. This may come as a surprise to amateur mechanics who have been used to working on their own cars, but your manual is correct. Many gearboxes, for example, are now in effect “sealed for life”, a change that started to come about more than a decade ago and is now the case on almost all new cars: providing no leaks or problems occur, the gearbox will never need to be touched.

Since 2005 there have been changes to the chemical structure of the coolant and power-steering fluids, which now no longer degrade in the way they used to. It means both should be capable of lasting the lifetime of the vehicle. It is, however, very important that top-ups of any fluids should be made with products approved by Vauxhall, as contamination with non-approved fluids could dilute their effectiveness. In addition, any topping up of the coolant should be carried out using a ready-mix coolant, not water.

With a used car it can be hard to know how closely previous owners have followed these guidelines, so it is worth asking your garage to check for contamination, particularly in the coolant.

DP

Got a car problem?

Email your question to carclinic@sunday-times.co.uk, or write to Car Clinic, Driving, The Sunday Times, 3 Thomas More Square, London E98 1ST, with a daytime phone number, your address and as much detail about your car as possible. We can’t reply personally, so please don’t send original documents or SAEs. Advice is offered without legal responsibility.