Many people have questions about timing belts. Not every engine has one, but those that do need to have them replaced at a specific mileage or time interval. Timing belts offer several advantages over timing chains, such as light weight and precise timing over their lives. The timing belt is also far less affected by lubrication problems than a timing chain or gear drive. Like any piece of rubber, a timing belt will deteriorate over time. To know if your engine has a timing belt, it is best to refer to the owner’s manual or ask to a trusted professional.
How Timing Belts Work
They design a timing belt with teeth that mesh with grooves in the crankshaft and camshaft sprockets. This arrangement allows the timing belt to keep the camshaft and crankshaft in perfect time. Engineers normally provide a tensioner which keeps the timing belt tight.
Timing belts are made of a very durable rubber, with layers of cord to prevent stretching. This arrangement provides years of trouble free service and precise valve timing. Eventually the rubber and cords begin to deteriorate. The recommended replacement interval for a timing belt varies, depending on several factors. The design of the engine and the age of the belt determines when it should be replaced, and vehicle manufacturers provide guidelines which must be followed.
Interference and Non-Interference Engines
We often hear engines referred to as interference and non-interference types. An interference engine is one where the engine valves, and pistons occupy the same place, just not at the same time. On an interference engine, the valves will hit the pistons, if the timing belt breaks or slips. The result of this misalignment will be engine damage.
They use interference designs because they allow more horsepower. The drawback is an elevated risk of the belt breaking or slipping. Pushing the timing belt in these engines beyond the recommended time or age interval is very unwise.
When they build an engine for lower performance, the valves do not extend as far into the cylinders. They call these engines non-interference type. This is a bit misleading as under the wrong conditions, any engine may still suffer damage if the valve timing is off. The name, non-interference sometimes lulls people into a false sense of security.
When a timing belt slips or breaks, at the very least the engine will stop running and other damage may occur. An engine listed as non-interference does NOT mean it is safe to neglect timing belt replacement. Timing belts are normally recommended between 60,000 and 105,000 miles. The interference type engines are closer to the lower end and the non-interference type closer to the higher end.
The Age Factor
Time is perhaps more important than miles, in the life of a timing belt. Seven years is the maximum life of the timing belt, recommended by most manufacturers.
For instance, even if the engine has only 30,000 miles, but is seven years of age, the belt should be replaced. Because timing belts deteriorate with age, time is even more critical than miles.
More timing belts break due to age than mileage.
Parts that may be replaced with a timing belt.
Replacing a timing belt normally involves a good deal of effort. Because of this, there are several other components in near proximity, that might also be replaced. For instance, the water pump, if under the timing cover, can be replaced at a greatly reduced price. Because water pumps wear out, many people see the wisdom of replacing it when the belt is replaced. Not replacing these components often results in an expensive re-do of the timing belt job. Doing a complete job is far less expensive and less dangerous than having to do the job over.
Other components often recommended include:
Timing belt tensioner(s)
Cam shaft seal(s)
Front crankshaft seal
Oil pump seal (if present)
Balance shaft belt (if present)
Balance shaft seal (if present)
Some engines may have other components and wear items. Spark plugs, PCV valves, air and fuel filters may also be replaced if needed.
Beware the timing belt quick-change
When pricing a timing belt replacement, you must know what you are buying. Some auto shops quote a price for replacing the timing belt alone. Quality auto repair shops refer to this as a belt-slap. This may seem like a savings, until they call back with the real cost.
Perhaps worse is replacing the belt, only to have another component fail. For instance, if the water pump fails ten-thousand miles after the timing belt is replaced. Repair means another full disassembly and the water pump can actually damage the replaced timing belt or cause it to break.
A seized idler pulley bearing will cause the belt to break and may even melt the nylon pulley.
Another problem is a seal that starts to leak after timing belt replacement. The seal could have been replaced for a minimum cost while the timing belt was off. Now many of the same components must again be removed. Worse, the oil from the leaking seal can ruin a new timing belt.
A timing belt is not the same as a serpentine belt.
Vehicles are also equipped with exterior belts and serpentine belts. Folks sometimes confuse these and think replacing the outside belts is the same as the timing belt. The timing belt is inside of the engine. If your vehicle is seven years old or older, check to see if it is equipped with a timing belt. If so, be sure the timing belt has been replaced. In almost all cases replacing the timing belt before it breaks will save lots of money. If the timing belt breaks extensive repairs to the engine or even replacement the engine may be required.