How to Know if a Timing Belt Is Bad

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A timing belt is a rubber belt that's usually mounted underneath and engine cover that's used to transfer power from the crank shaft to the cam shaft. Find out how to remove the engine cover to visually inspect a timing belt with help from an ASE-certified master automobile technician in this free video on timing belts.

Part of the Video Series: Auto Repair & Maintenance
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Video Transcript

So the question comes up, how do you know if your timing belt is bad? First off, what's a timing belt? A timing belt is a rubber belt typically mounted underneath an engine cover that is used to transfer power from the crank shaft to the cam shaft. And also maintain a relationship and keep the two things timed together. Where other vehicles might have a timing chain, vehicles that have a timing belt, it does the same job. The timing belt on this car is located on this end, and we've removed the cover, in order to make it easier to see it's operation and condition. And certainly that would be the easiest way to tell if indeed the timing belt was bad, is to remove the cover and get at it. But in many cases it may not be quite that convenient to actually do that because it could take hours worth of work. And you wouldn't even know if that's the reason why the vehicle won't start. So there are other symptoms that we can look at and determine whether it is likely that a timing belt is bad. First of, one thing that we can do is, we can actually take and hook up a vacuum gage to the intake manifold, and observe the vacuum as we crank the engine over. One of the first things is that you learn at trade school, is basically that you know, while an engine provides power, basically it's a pop. So the air comes in through the air cleaner, goes through the engine and basically leaves out the exhaust pipe. So we are looking for signs of suction at the intake manifold, as long as the timing belt is whole and working, we should have some vacuum. And if it's not, then we won't have any, we may actually have some pressure. So in this case, I've hooked up a remote starter switch and while it cranks over, we'll watch to see if there's some evidence of vacuum. There does seem to be, so this could be one easy way of telling if we potentially had a problem with the timing belt. There are other reasons why an engine could have low vacuum. But if the engine vacuum is there then we know that it is working as a pump and that part of it is good. The next thing we can do, is look for some evidence that the cam shaft turns. The cam shaft, directly or indirectly is responsible for opening and closing the valves in an engine, in this case we've actually removed the valve cover on this engine. And while we cannot see the cam shaft, the cam shaft actually is located underneath where these rocker arms are and the valves can be seen with their springs right across the front of this engine. And so one of the other signs that we would look for to find out if a timing belt was bad, is whether the valves operate or not. In many cases you could actually see these rocker arms through the fill hole for the oil. But one of the things we're looking for is, when the engine is cranked over to see if the rocker arm have to be removed, and they do. So that's a pretty good indication that the timing belt has not broken on this car. Customers will often ask, how do I know if my timing belt is going bad? Well, it's very difficult to tell whether it's going bad or not. Because say for instance, like with this timing belt, this is a typical one with the cogs on the inside. We can inspect this part of it and it looks pretty good, there's no cracks in it that are immediately obvious. But the truth of the matter is, if we look at this end of it here, the belt has actually broken in two. And this vehicle stopped running because the timing belt had actually broken, the one that this one came from. So basically one thing to take into consideration with the timing belt is, that they have a recommended service interval at which they should be changed. Depending on the car, that interval could be every sixty thousand miles up to a hundred and twenty thousand miles. So change it before you end up with this situation and the timing belt breaks. It's important because in some vehicles they have what is referred to as an interference engine. And if the timing belt breaks, you could end up with significant amounts of engine damage because the valves and pistons could collide. So change those timing belts when it's due.

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