Timing belts are often considered the weak link in modern engines.
While timing chains give plenty of warning rattles before they break, belts may be quiet as a mouse before disaster strikes.
Most modern car manufacturers have forsaken overhead valve design and changed to overhead camshaft. This proved to be a sensible move in many ways, but there were snags.
With OHV there is a short distance between the crankshaft and camshaft, meaning that a short chain could be used between the two. With OHC, the distance between crank and camshaft is much longer and because chains tend to stretch with age, the longer the chain the more it stretches and rattles.
And so, while many of us were quite happy with the “fit and forget” chains, along came the belt and we have no more rattles, no need for lubrication and quieter operation.
JUST IN CASE
The downside, of course, is that belts do not last as long as chains and it is wise to renew them at or before the recommended intervals. In fact, I always reckon it worthwhile to fit a new belt when buying a used car with no service history - just in case.
This is even more important with diesels because the injector pump is belt-driven.
It’s worth noting that ignoring recommended belt change intervals is very risky and potentially expensive on diesels.
Ideally, the belt should be carefully checked at each service but, as many cars go for long periods without professional attention, it makes good sense to DIY check occasionally for cracks, oil contamination and signs of wear.
READ THE WORKSHOP MANUAL
On many cars a belt change is dead easy, while on others the engine has to come out!
Again, before buying a used car, enquire about the difficulty (and expense!) involved in carrying out normal service operations. Read the workshop manual and/or seek advice from technicians. While some belt changes may be done in about half an hour, others may involve several hours’ labour.
Timing is critical; with the belt off, the camshaft and crank must be locked so that they cannot move in relation to each other. Locking is normally done by inserting locking pins to hold them. The shop manual will tell you (as is often the case) if the tensioners and idler pulleys should be renewed along with the belt - as should the water pump if driven by the belt. If access is difficult and time-consuming, renew them anyway.
RENEWAL TIMES ARE MERELY A GUIDE
As for belt renewal times, bear in mind that they are merely a guide. A car that is used primarily for short journeys, such as taking the kids to school and shopping trips, will wear out the belt more quickly than a car that does more long-distance driving with fewer engine starts because it is during starting, especially cold starts, that the belt is under most strain.
High mileage is not the only reason for belt failure; some very low-mileage belts fail due to age alone. Granted, some belts last for five years or more and give no signs of wear, but even if the mileage recommended for renewal has not been reached, I would advise renewing anyway.
A belt that breaks at speed could mean a wrecked engine, and even a break while turning the engine via the starter could mean damage to valves and pistons. - Star Motoring