Timing belts have been getting better, and any new OE-quality belt on a late-model engine can be expected to last at least 60,000 miles. But long gone is the free-wheeling engine, in which a snapped belt would simply stop the engine. Tightened emissions controls have forced a reduction in the clearances between pistons and combustion chambers, so a piston rising to top dead center is sure to crash into open valves. The best-case scenario is bent valves, where it's possible for a machine shop to recondition the head...still not a quick, cheap fix. If the vehicle was leased, the original driver had a warranty and no concerns about this. But the off-lease buyer is looking at a vehicle nearly off warranty. If a belt snaps not far down the road, he or she would have to foot the bill.

That's why it makes good sense to include a timing belt inspection with any other underhood service on an engine with 30,000 miles or more. Access to the belt depends on the engine and the layout of the engine bay, of course. However, in most cases the timing belt cover-at least the top half-can be removed or at least pushed away so you can inspect the belt.

With rare exceptions, what you want to find is a properly tensioned belt in good condition. Anything else calls for belt replacement.

The first thing to check for is belt looseness. Most engines today have an automatic tensioner for the belt; many have a manual preset, with the adjuster itself being automatic. You should be able to determine looseness by feel, although there are available some special tension gauges for precise measurements. But even if you have one, you'd have to remove the complete belt housing to position the gauge midway between pulleys. That's impractical for a basic inspection in most powertrain compartments.

If a belt is loose, either it has stretched excessively (somewhat less likely with today's belt designs) or the automatic tensioner is loose or seized, or has a broken spring. A simple repositioning of the tensioner and tightening a loose retainer bolt is possible if the problem were caught early-that is, if the belt teeth hadn't been damaged by the loose running. However, odds are the belt took a beating and should be replaced.

What if a belt seems adequately tight? Put a chalk mark or piece of masking tape on the back of the belt as a reference, then turn the engine by hand and inspect the complete belt, looking for worn or sheared teeth, sidewall cracks (particularly at the tooth edges), cracks on the back of the belt and any signs of oil/grease or coolant soaking.

Any of these problems require belt replacement, but you also have to trace and correct the root cause. Yes, it could be that someone spilled oil or coolant on the belt, rather than contamination caused by a leak. But remember, belt housings have gasket seals, and they could be deteriorated or the housing covers may be warped.

Damage to belt teeth typically occurs when belt tension is low (even if the belt isn't obviously loose) and there's dirt in the sprocket grooves. Here, again, check the gasket seals.

Belt cracking most often is caused by a lot of underhood heat or severe cold. As a case in point, some manufacturer's belt replacement recommendation on its latest models is 105,000 miles, but goes down about 40% (to 60,000 miles) with operation in hot or cold climates, and, in any case, not more than six years. Observe appropriately shorter replacement intervals on older vehicles, which may carry "normal" timing belt life of only 60,000 miles/4 years or 90,000 miles/5 years.

It's rare, but teeth and the belt back also can be cracked if the tensioner applies too much tension, or if the tensioner is misaligned even while still being able to apply adequate tension. A misaligned tensioner produces uneven tooth wear and a lot of noise, which is uncharacteristic for a timing belt and a good tipoff to misalignment problems.