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Timing is Everything

What you need to know about your timing belt

Real world experience: A mini-truck was cruising a rural road. The engine died mid-shift. One minute it was purring right along, the next minute the only sound was the wind whistling in the open window. Turn on the ignition and nothing, absolutely nothing.

Time's Up

After the truck was dragged to a mechanic, the diagnosis: The timing belt broke; the truck was lucky it occurred with the tranny disengaged or the engine may have received terminal damage. So, what's all the fuss about the timing belt? How could its demise wreak such havoc on the engine and what can be done to avoid this unfortunate automotive surprise?

For those who skipped auto shop class, the timing belt is a nylon-reinforced rubber serpentine belt with square teeth on the inside surface. Its function is to transfer the rotation of the engine crankshaft to the camshaft-the component that moves the piston valves up and down.

Even the non-mechanically minded know the valves admit air and fuel and expel exhaust gases; without functioning valves, your engine is dead in the water. The timing belt, true to its name, governs the precise timing of the opening and closing of the cylinder's valves. By controlling the ignition of the fuel/air mixture, it secondarily controls the motion of the pistons. Unfortunately in the case of a broken belt, the pistons will complete their cycle even with inert valves.

Damage Control

To understand the degree of damage, let's take a quick refresher course in Auto 101 (or at least the class you skipped). When the piston is at the bottom of its travel, the valves are open; as the piston hits its high point, the valves are closed.

Some engines do not have any extra clearance between the piston at its high point and the closed valves. So, if the belt that regulates the valve motion breaks, the piston will continue to travel but the valves do not. The piston can collide with the valve mechanism causing damage to any number of costly components: valves, valve stems, pistons, cylinder head, cylinder walls.

Belt or Chain

Before you go into serious timing belt angst, check your owner's manual to determine if your vehicle has a timing belt or timing chain (the latter is much more durable). Most vehicles, both domestic and imports, have belts, but the rate of failure is greatest on vehicles built before the mid-1990s.

A timing belt is not to be confused with fan belts, positioned around the engine fan and alternator, which are easily inspected for wear and tear. They also give you advance warning: the alternator/generator gauge goes flat and the engine starts to heat up-before serious damage occurs.

Not so with the timing belt. In most engine configurations, this belt is protected with a cover so visual inspection is impossible. Out of sight, the timing belt is aging. Heat and friction turn the supple rubber hard and brittle. The reinforcing cords fatigue and weaken. Even if you're ambitious enough to remove the cover, the belt could look like it has plenty of life left in it from the outside, but those nylon or fiber reinforcing cords could be shot without any obvious indication on the belt surface.

Get a Clue

One indication of a bad belt is a glazed or glossy appearance. This is an indication that the rubber is starting down the hard and brittle path. The other test is to push your fingernail into the rubber. If the rubber is supple enough to indent, you're okay. If there's no fingernail impression in the rubber, replace it.

If removing the cover is too daunting, the safe thing to do is stick to the manufacturer's recommendations for replacement. This is where that pesky owner's manual comes in handy. The manufacturer will include mileage limitations on the equipment. The mileage indication—usually 60,000 miles but often as high as 90,000 in later models—is a conservative estimate, but well worth heeding.

Also, some engine conditions, such as overheating, a damaged camshaft or crankshaft oil seal, a jammed tensioner etc., can cause premature timing belt damage. If your vehicle has been diagnosed with problems in the systems impacted by the belt, have your mechanic check on the belt's health. A good mechanic will do that automatically, but one who is juggling too many cars at once may need reminding.

HSN

Starting in the mid 1990s, some manufacturers started making a better timing belt out of tougher materials, a high temperature grade of neoprene called highly saturated nitrile (HSN). The result is a belt with up to a 50 percent increase in replacement intervals.