Car Clinic: Pressure washing

I’ve recently read several articles that suggest conserving fuel by driving at low speeds in high gears, keeping the engine speed/revs low. My 2.5-litre V6 petrol Ford Mondeo appears to be fine driving at 30mph in sixth gear, but is it? Could I damage the engine?
MR, Reading

Modern engines produce more torque (pulling power) at lower revs, so cars are now more “flexible” to drive than they used to be. Consequently, most of today’s cars are comfortable being driven at low road speeds in high gears, where engine speed is necessarily also low.

Diesels are particularly good for this as they develop their torque at lower engine speeds than do petrol engines. Also, whether diesel or petrol, the more cylinders there are, the greater the low-speed torque, and therefore the driving flexibility. This is also improved by having a larger engine capacity, and as your Mondeo’s engine is not only a larger-than-average 2.5 litres, but also has six cylinders rather than the usual four, you’re off to a good start.

Whereas older engines need at least 1500rpm to keep the car motoring comfortably, most modern power units are happy to drive the car along a flat road at 1000rpm. Maintaining 30mph in sixth gear is therefore perfectly acceptable — at least until you reach a gradient or need to accelerate. Then you’ll need a lower gear to increase the engine speed, and so the torque available at the wheels. On a flat, clear road, running the engine at minimum speed is an effective way of saving fuel, and it won’t cause any damage.

One word of caution, though, to drivers of cars equipped with a diesel particulate filter (DPF): if the filter “regeneration” light comes on, you will need to maintain a high engine speed (2500-3000rpm) for half an hour or so to clear the filter of soot, which will almost certainly mean using a lower gear — even at 70mph.

This will use some extra fuel, but that’s preferable to the alternative of having to buy a new DPF that could cost as much as £1,000. TS

My parents’ 12-year-old 4-litre Range Rover SE has become a money pit, recently needing £7,000 worth of repairs to the suspension and radiator systems. What would be a good replacement? My father no longer drives and they do no more than 100 miles a month, but they’d like to retain the higher driving position you get with a 4x4. They need an automatic and nothing too complicated. Their budget is around £20,000.
KC, Cobham, Kent

If your parents would like to stay with a 4x4, we would recommend the Honda CR-V diesel, which is available with an automatic gearbox, has a great reputation for reliability and should prove very economical in comparison with the Range Rover. A 2009 59-registered SE auto should be just on budget. A slightly newer alternative would be the Hyundai Santa Fe. A 2.2 CRDi Premium automatic 59 plate should be available at a price they can afford. Also, take a look at the Nissan Qashqai. The 4x4 versions feel very much like regular hatchbacks to drive. Available with automatic transmission, a 2010 59-registered Tekna model is also just on budget.

All these cars will still be covered by the manufacturer’s original warranty, which should assure your parents that they won’t get unexpected bills. In the case of the Hyundai, the warranty runs for five years from the date of first registration; for the Honda and Nissan it is three years. JD

There is a recurring fault with the antilock braking system (ABS) on my BMW 523i SE. When driving on ice or snow, the ABS light comes on, indicating that the ABS is inoperative, frustratingly, just when it is needed. It sometimes stays on overnight before righting itself. Any ideas?
BJ, Buckie, Moray

If the light stays on with the ignition off, it suggests that a wiring fault is supplying power to the ABS’s electronic control unit (ECU) constantly, even though everything is apparently turned off. This could be a complicated and tricky-to-find loom fault, which will need careful investigation by an expert — a dealer or an electrical specialist.

If it is going off overnight but comes back on again when you turn on the ignition in the morning, it could be a more conventional fault, albeit intermittent and therefore trickier to pin down. Next time it happens, take the car straight to your dealer. The dealer can plug a diagnostic computer into the car and read its list of fault codes, which should reveal what’s happening — or not happening. The most common ABS problem is that a sensor at the wheel is faulty. This will show as a fault code.

When the warning lamp is on, you will need to take extra care to avoid harsh braking. If you do need to brake very hard, “cadence” braking (applying and releasing the brakes rhythmically to get a compromise between steering and braking performance) is your best bet to avoid locking up the wheels and thus retain some steering ability. DP

The Car Clinic panel

Jason Dawe is our used-car expert and has appeared on Top Gear, Watchdog and The Morning Show.
Dave Pollard has written several Haynes manuals and has tested just about every car-related accessory.
James Mills is a motoring magazine editor with 18 years’ experience of evaluating new cars.

Got a problem? Email your question to carclinic@sunday-times.co.uk, or write to Car Clinic, InGear, The Sunday Times, 3 Thomas More Square, London E98 1ST. Please supply a daytime phone number, your town or city and as much detail about your car as you can. We cannot send personal replies, so please do not send original documents. Advice is offered without legal responsibility.