Timing is everything for an internal combustion engine to provide the most power for the least amount of fuel. The efficiency of the combustion process relies on either a timing belt or a timing chain to keep things on track.
In order to power the engine, each cylinder relies on a camshaft to open and close the valves, which take in fuel and lets out spent gases. The valve timing, or camshaft timing, is crucial to the engine running smoothly, and if the timing is thrown off, a valve can open at the wrong time and cause a number of problems.
The camshaft is controlled by the crankshaft with a timing chain. In some vehicles, the chain is attached to the camshaft with a plastic sprocket. If this piece melts, the timing chain can be thrown off and the car will not run. The chain can also wear down over time and become loose, or teeth can come loose from the chain. Missing teeth will cause the valves to malfunction and the engine will likely run roughly.
When the chain is so worn that it doesn't sit properly on the gears of the cam shaft and crankshaft, the engine will not start. It is important that drivers do not attempt to start their vehicles until the issues have been addressed, or they could risk causing excessive damage to the engine. Drivers may notice a rattling sound coming from near the engine when the chain needs to be replaced.
Not all vehicles are equipped with timing chains, however. Roughly half of the vehicles on the road today rely on timing belts. This method of connecting the camshaft and the crankshaft is ideal for vehicles where the two components are not near one another, such as with an overhead cam engine.
It is important to replace the timing belt or chain at regular intervals. The length of time a vehicle can go before this part needs to be replaced can vary from model to model, and the owner's manual will provide the necessary information for a particular vehicle.