Timing belts have a limited service life. That's why belts should always be replaced at the vehicle manufacturer's recommended mileage interval. But a belt change alone may not be enough because the majority of belt failures are actually caused by faulty belt tensioners and pulleys. Consequently, the new timing belt you've just installed may fail after a few thousand miles if the tensioner components are worn or loose.

The eccentric idlers and/or pulleys that are used on many OHC engines to maintain belt tension also have a limited service life -- which in some cases may be less than that of the belt! Their lifespan depends on their location, operating temperature, load, engine rpm and other environmental factors. Most tensioner components have a design life of around 2,000 hours, which may be the equivalent of 40,000 to 60,000 or more miles of service depending on how the vehicle is driven. Unless these components are also replaced when the belt is changed, it could lead to premature failure of the new belt and a costly comeback for you.

Tensioner bearings are "sealed for life" and are not serviceable. So there's no way to clean, inspect or relubricate the bearings when a timing belt is changed. Over time, the grease inside breaks down and oxidizes, accelerating bearing wear and increasing the risk of a bearing failure or seizure. And the hotter the operating environment, the shorter the lifespan of the grease and the bearings. The location of the tensioner components under the timing belt cover prevents them from receiving much cooling, so they tend to run quite hot.

If a tensioner bearing seal fails, dirt and humidity can enter the bearing. At the same time, a leaky seal will allow the grease inside to escape, dooming the bearing to rapid failure.

On many newer engines, spring-loaded automatic tensioners are used to keep the timing belt tight. A slide plate allows the tensioner roller to move against the belt. As the miles accumulate, though, wear can cause the plate to stick against the engine block preventing the tensioner from maintaining proper pressure. High underhood temperatures can also cause a loss of spring tension over time, too.

If the belt loses tension, it may jump time or slip. And if an idler pulley or tension seizes, it may cause the belt to suddenly snap.    Unfortunately, you can't always determine the true condition of the tensioner components by a simple visual inspection. Any pulley bearing that has visible looseness, grease streaks radiating outward from a damaged seal, makes noise or does not turn freely obviously needs to be replaced. But many tensioner pulleys that appear to be okay may be at or near the end of the useful service life. Reusing them is a risky proposition. That's why the tensioner components should always be replaced when installing a new belt.

Timing component kits are now available from NAPA Belts/Hose that make it much easier to replace the tensioner idlers and pulleys along with the belt. The kits include a new timing belt, idler and/or tensioner assembly along with detailed installation instructions.