Timing is everything – advice that is very pertinent for an engine, the internal parts of which are kept millimetres apart, often while rotating at high speed, which demands a precise degree of synchronisation.
Many years ago, chains were employed to transmit motion between the engine crank and camshafts but these tended to be located within the engine, because a supply of oil was essential to reduce wear. From the 1970s, many car manufacturers replaced their chains gradually. Toothed belts, called either camshaft or timing belts, are a popular option these days because they are lighter and quieter in operation. Not only do they require routine replacement, however, they can also fail suddenly and without warning. The old chain types would become noisy as they stretched and wore out. Some manufacturers have continued using timing chains that are intended to last the life of the engine, even though some types do not.
Typical causes of timing belt failure vary from old age to contamination by an adjacent oil or water leak. Bad fitting, such as either under- or over-tightening a new belt, can cause it to fail prematurely, as can failure of neighbouring components.
The timing belt tends to operate other components, the condition of which affects the belt’s life. Excessive slack may be compensated for by one or two tensioners and idlers. Over time, their bearings may dry out, causing a squeak that should be heeded. In some cases, idlers and tensioners can seize, throwing the timing belt from its pulleys and causing extensive damage to the engine, with bent valves and even holed pistons resulting.
Many engines also utilise the timing belt to drive the water pump, which can also seize with the same consequences as failed tensioners, so you should not think that your garage is profiteering by suggesting that the water pump needs to be replaced as well. If you suspect leakage, which can be evident by a pool of engine coolant emanating from within the timing belt cover, budget for imminent replacement of both the pump and the timing belt.
Most car manufacturers specify time and mileage intervals but these stipulations are for whatever arrives first.
Worn timing belts tend to fail at sudden engine speed changes, such as moving off, changing gear, or revving the engine in neutral. As diesel cars have their emissions tested during an MoT Test, by the engine being revved to its maximum speed, an examiner may enquire if the timing belt has been changed recently. If the belt breaks during the test, causing serious engine damage, the garage will not be held liable.
In an interference-type engine, timing belt failure usually causes the pistons to collide with the inlet and exhaust valves. At the very least, the valves will be bent and a technician will need to remove the engine’s cylinder head to replace them.
In some cases, pistons can be holed, cylinder heads can be damaged and camshafts can be broken. To repair a severely damaged modern engine, the cost will be in excess of several thousands of pounds and a full replacement unit may prove to be the most economical solution.
In many cases, a driver will not notice any difference in the car’s performance after a timing belt change but, fortunately, most people realise that the extra peace of mind is well worth the additional periodic outlay.