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At Auto Express in Sacramento, CA, we occasionally run across customers who are a little confused about timing belts and what they actually do. We’d like to take a little time to clear that up…

Q: Is the timing belt the same as the serpentine belt?

A: In a word, no. Older vehicles had multiple belts to drive the water pump, power steering pump, air conditioning compressor and other accessories. In the 90s, the serpentine belt was introduced; the serpentine belt is a much longer belt that loops around to drive all the accessories at once. This saves parasitic drag on the engine and saves space under the hood, since the accessories and multiple belts don’t have to all be offset from each other. The timing belt, on the other hand, is concealed under the engine’s front cover and isn’t visible.

Q: What does the timing belt do?

A:The internal combustion engine’s bottom end consists of pistons, connecting rods and a crankshaft. The pistons are driven down when the spark plugs detonate explosions of fuel/air inside the cylinders and generate power. The upper end consists of valves that supply fuel/air and then allow the spent exhaust gases to be expelled. The valves are opened by the lobes of a camshaft and closed again by their own springs. The timing belt, then, is what connects the upper end and lower end of the engine (or, the camshaft and crank) and allows the valves to work in sync with the pistons for the four strokes of the combustion process. Older engine designs used a bicycle-style chain rather than a belt, and many still do; timing chains generally last the entire life cycle of the vehicle..

Q: Why is it so important to change the belt at the manufacturer’s specified interval?

A: The timing belt, like any other elastic belt, will stretch and eventually fail over time. If the pistons and valves suddenly go out of sync with each other, that means that the pistons are traveling upward in the cylinder at a moment when exhaust or intake valves are open. The pistons’ crowns will  crash into the opened valves, bending them, punching holes in the pistons, bending the pistons’ connecting rods and generally wrecking the engine’s internals. Some engines are “non-interference” engines, specifically designed to prevent this from happening, but many aren’t. That’s why if the manufacturer says to change the engine at 60,000 miles, every mile past that is borrowed time.

Q: How involved is it to change the timing belt?

A: It’s a fairly big job. It requires removing accessories, often removing the radiator, cooling fan and hoses, and the engine’s front timing cover. On about 60 percent of engines, the water pump is also driven by the timing belt. Many technicians will advise replacing the water pump as well at that time, since everything’s already disassembled. It’s also advisable to replace the belt’s pulleys and tensioner at the same time; the tensioner, in particular, can wear out and seize, causing the timing belt to jump from its pulleys and causing major engine damage.

Q: Is there a way to check for wear on the timing belt?

A: Not really. “Inspecting” the belt will usually mean disassembly, at which point you may as well just go ahead and replace it. Sometimes you might hear a rattle or squeak from the front end of the engine, but more often than not the belt will just fail without warning. A car that has a broken timing belt will just make a steady “whirrrrrr” when it cranks, rather than the usual rhythm of an engine that’s trying to start.

We hope this clears up any questions you might have about timing belts and their purpose. Check your owner’s manual for manufacturer’s recommendations on timing belt service intervals, then check your odometer. If you’re coming up on the recommended mileage for a timing belt change, don’t put it off this important auto maintenance detail. Give us a call at Auto Express Sacramento and make an appointment!