This is a familiar subject, but one worthy of a fresh review. A key part of any car engine is the drive mechanism that operates the camshaft, which in turn operates the inlet and exhaust valves. With nearly all diesel engines, the high compression ratio, and therefore the relatively small combustion space, means that if the timing goes wrong, for some reason, the piston can hit any of the valves and cause very expensive engine damage, if a valve is open when the piston is at the top of its stroke.

So the systems that operate the valve gear must be prevented from any such occurrences, which means that the drive system can never allow this situation. But if a toothed drive belt or a chain should break, then such a disaster can, and does quite frequently happen, although cam belts have become more robust in recent years. So correct maintenance of cam belts (or timing belts) and timing chains is essential. In the case of cam belts, this involves replacement of the belt at specific intervals notified by manufacturers, and we should note that they are car model and engine specific rather than engine specific, and a 1.6-litre HDi or 2.0-litre TDI engine may have different replacement intervals in different cars. The frequency of replacement is specified in mileage, or time intervals, and a low mileage car may need changing at five years, even though it has not reached its specified mileage of, say, 80,000 miles. Many belt failures are not down to actual belt breakage, but to possibly belt deterioration, maybe due to oil contamination, or more likely failure of the equipment that maintains belt tension, usually a system with tensioning rollers that take up any stretch that may develop in the belt’s lifetime. Quite often the bearings in the rollers of the belt tensioning system may fail, which is why replacement of the tensioning unit is essential when having the belt replaced. It’s also advised to change the water pump at the same time, another item that has a similar sort of life to a cam belt, and which can cause costly engine damage if it fails. It’s something that is quite costly to replace on its own, so doing the job whilst the belt and tensioners are being replaced simply involves the cost of the parts and only a small element of labour costs, and is a good insurance policy.

It’s worth shopping around when it comes to a cam belt, tensioner, and water pump replacement job, and of course you really should make yourself aware what the cam belt replacement mileage or time is for your car. Sometimes it may be a good move, particularly if you’re thinking of changing your car anyway, to move the car on a few thousand miles before it is due, since the cost is typically likely to be somewhere between £400 to £700, including labour. If you’re happy to take your car to an independent garage, specialist nationwide group Mr Clutch are experienced operators and quote good competitive prices for belt replacement. Cam belt replacement is not a job that we would advise doing yourself, unless you are a really experienced mechanic, and the downsides of getting it wrong (like the belt fitted a tooth or two off correct timing) are really huge.

A final word on timing chains. Space does not allow us to mention all the diesel engines that have timing chains rather than belts, (in brief, all BMW, Mercedes-Benz, Honda, Kia, Hyundai, Subaru, Fiat/Vauxhall 1.3, plus more) but maintaining good chain life is mostly related to regular oil services. Neglect of regular oil and filter changes can cause premature chain wear, and the potential damage resulting from failure and significant costs of replacement are certainly not to be ignored. If you do make a habit of lifting your bonnet occasionally (as you should!) and listening to your engine ticking over, a rough idle and a regular clicking or rattling noise might just warn you of timing chain problems, but you’ll get no such helpful warning from a cam belt that’s on its last legs.