A timing belt is a critical component in an engine. It has a limited life span, is intended to be serviced with regularity, and can have severe consequences if it fails.
Not all engines have timing belts. A timing belt is a rubber cog-toothed belt that transfers rotational motion from the crankshaft to, at minimum, a camshaft. Some vehicles have multiple camshafts, and some vehicles also have other components that are driven by the timing belt. Most commonly that would be a water pump.
In addition to transferring rotational motion, because it is a cog-toothed belt, it is designed to keep rotating engine components in a specific position in relation to each other. That is why it is called a "Timing Belt". It keeps the engine in time, mechanically. Without the belt in place, or in the correct position, the engine will run poorly, or not run at all. And it could cause severe engine damage.
There are two major designs for engines in regards to how the valve train is operated. One design is known as "overhead valve". This generally means the camshaft is in the engine block and operates the valves by way of push rods, rockers, and so forth. These engines have a timing chain system. The other major design is "overhead valve". In these designs the camshaft (or camshafts) are in the cylinder head. These designs can have a timing chain, a timing belt, or a combination of both -- although that is rare.
Engines have another major division worth noting. Engines can be "non-interference" or "interference". These may also be known as "freewheeling" or "non-freewheeling". If an engine is non-freewheeling or interference, if the engine components become out of time then engine damage is very likely to occur. In the other case, non-interference or freewheeling, that is generally not the case.
A timing belt moving positions is also known as a "jumped" timing belt. At minimum a jumped timing belt will cause a noticeable running problem and/or a Check Engine Light. In many cases a jumped timing belt will cause some engine damage. A timing belt can also strip teeth off, shred, or break entirely. On an interference engine this will cause engine components such as valves and pistons to collide which is every bit as bad as it sounds. Generally the cost of these kinds of repairs starts in the thousands. In the worst case scenarios the damage to the engine is too great to be repaired in a cost-effective manner.
Below are some pictures from one such case. This vehicle was a VW Passat with a 1.8L engine. The customer had gone in excess of the recommended mileage for the timing belt. The customer said the vehicle suddenly shut off on the freeway and would not restart. It was towed in where the broken timing belt was discovered. A complete timing belt job, using factory components and including the water pump, on this vehicle would run around $1,100. The cost to repair this vehicle after the timing belt failed was in excess of $3,800. The vehicle ended up in a scrap yard shortly afterwards, unfortunately.
All of the intake valves are bent and unable to close. You can see in the second picture that they are so bent over that they have contacted the head itself. You can also see in the third picture that the valves were bent over because they came in contact with not only the piston but the cylinder wall, as well. Because the valves were unable to close properly, no compression could be created, and so the engine could not run. Because this engine has an interference design, the valves and the piston occupy the same space but at different times -- unless there is a problem with the timing belt. Damage such as is shown in the pictures is not uncommon for interference engines that experience timing belt issues. In some cases the valves actually break off and embed themselves into the cylinder walls and/or pistons.
An example of an entirely catastrophic failure was a Kia that suffered a timing belt breakage. Very similar story as the Passat: Customer was beyond their maintenance interval for the timing belt, was driving when the engine suddenly stopped and would not restart, and had it towed in. In this case we used a Video Borescope to look inside the cylinders and the damage was very obvious.
The picture on the left is looking straight down onto the piston. It should be smooth like the picture of the VW piston. Notice it looks like the surface of the moon. You can see an almost circular shape, which is the head of a valve embedded in the piston. You can see another triangular shape, which is a piece of another valve. They bounced around inside the cylinder causing extreme damage before they were finally embedded. The picture on the right is of the same area but taken from the side with a 90 degree mirror. You can see the same triangular piece. A timing belt service on this vehicle runs about $750. The cost to repair this damage required engine replacement with a minimum price of $4,500. This vehicle also ended up in the scrap yard.
Sometimes failures that can not be prevented or predicted occur. In both of those cases the vehicle owner did not service the vehicle correctly. Sometimes customers are not aware of this necessary service, and other times they can not afford it. We at John's Bascom Automotive encourage our customers to be educated about their vehicle. We also work to educate our customers whenever possible. If you are unsure if your vehicle has a timing belt, or are unclear as to when it needs to be serviced you can feel free to call us and ask. We would be happy to advise you as best we can.