What's the deal with timing belts?

Hey Zahntech --

My car has around 70,000 miles on it and I haven't replaced the timing belt that was due at 60k. Nothing appears to be going wrong as a result - do I really need to do this? It seems fairly expensive.

-- Late for a very (un)important date

Hey Late --

It's actually pretty important that you replace the timing belt at the suggested interval. Most vehicles have an interference motor, which means that the pistons will contact the valves if the timing belt breaks, causing severe engine damage. Such damage can cost you much, much more than just replacing the belt - but hey, perhaps it will be incentive to you to get a brand-new car! And if your vehicle doesn't have an interference engine, a belt breakage means you're stranded wherever it breaks, even in the middle of the 520 bridge between Seattle and Bellevue. Not good.

How likely is the belt to break? Well, it's a toothed belt made of rubber with fiberglass reinforcement. Like all belts, it stretches over time with use, and eventually could snap or strip the teeth. The replacement interval is somewhat arbitrary; however, few belts will break before this interval, and eventually all of them will need to be replaced. I believe that manufacturers have a greater interest in stretching out maintenance intervals because their vehicles will look cheaper to own (selling vehicles - and total cost of ownership as a component of that - is more important to them then getting your cash for oil changes and regular services). So that plus my experience tells me that the replacement interval suggested by your manufacturer is likely a good time to do the replacement.

Now, for the expense. It's not a cheap job - depending on the vehicle, it could go anywhere from $250 to upwards of $1,000. A low quote probably means that other parts such as tensioners, idler bearings, water pump, and cam and crank seals aren't included. These parts typically require replacement at the same time at the timing belt, especially in vehicles with long replacement intervals for the timing belt (90k+), because the odds of these parts lasting another 90k+ miles are fairly low. Failure of any one of these parts will guarantee a re-do of virtually the entire timing belt job, costing as much or more than the initial job. So when you're getting a quote (especially if you're comparing one service center to another), be sure to ask what it includes and why.