Your car's engine is a mass of fast-moving parts. It works by sucking in air and fuel, burning it, then chucking out the exhaust gases, and it's the valves which control when all this happens. At the heart of this valve system (known as the valve train) is the cam shaft, which spins round as the engine turns.
In the old days, the cam shaft was driven by a chain, but in the 1970s the timing belt started to become popular, as they're significantly quieter. The problem is, while timing chains would last pretty much the life of the engine, a timing belt (otherwise known as a cam belt) doesn’t. They have to be replaced, and if they're not they'll fail, usually destroying the engine in the process.
If the belt goes, your car could be fit for scrap only
Not all modern cars have a timing belt (some still have a chain), but if one is fitted it'll need to be replaced every five or six years or every 60,000-80,000 miles; they wear with use as well as age. Some car makers suggest replacement after a much longer distance or time, but in some cases, belts have snapped before they’re scheduled to be renewed, lunching the engine in the process. The problem is that a garage will often charge £400-600 to replace a timing belt because of the time it takes, but the parts cost is often only £25 or so.
The replacement cost is often pushed up by the fact that the timing belt drives (and is driven by) a series of pulleys, one of which will probably be the water pump. In these days of throwaway parts, when a timing belt is replaced the water pump should also be renewed – because if the latter wears out and fails, that too can lead to the timing belt being thrown off, wrecking the engine in the process.
So when you’re quoted several hundred pounds to replace a timing belt, you’re not necessarily being ripped off – although you must shop around to make sure you’re getting a good deal. It’s also worth noting that some engines are designed in such a way that if the timing belt snaps it doesn’t wreck the engine – but most engines aren’t designed this way. Sadly…
So when you buy that second-hand car, ask when the timing belt was last replaced and look in the service records for evidence that the work has been done. You can’t really check visually that the work has been done (although a mechanic could probably do so), which is why if you’re unsure as to whether or not the belt has been replaced in the last five or six years, it can be worth replacing it anyway. Yes it’s a lot of money to lay out, but if the belt goes, your car could be fit for scrap only…