What the timing system does
Inside an engine, two sets of moving parts must stay in step. The crankshaft turns as the pistons move up and down. The camshaft (or camshafts) opens and closes the intake and exhaust valves. The valves must open and close at precisely the right moments relative to piston position, and the timing belt or timing chain is what links the crankshaft to the camshaft and keeps them synchronized. In most engines the camshaft turns once for every two turns of the crankshaft, and the timing drive is built to maintain that ratio exactly.
Without correct timing, the engine runs badly or not at all. If the drive fails completely while the engine is running, the camshaft stops while the crankshaft may still turn for a moment, and the valves and pistons can collide.
Belt or chain
Timing belt. A toothed rubber belt reinforced with internal cords, often running in a covered area at the front or side of the engine. It is quiet, light and inexpensive to make. Rubber ages, though, so manufacturers give belts a replacement interval based on distance, time, or both.
Timing chain. A metal chain, similar in principle to a bicycle chain, usually running inside the engine and lubricated by engine oil. Chains are often intended to last much longer, sometimes the life of the engine, but that is not guaranteed. They can stretch, and the guides and tensioners that keep them tight can wear or fail, especially if oil changes have been neglected.
Some engines use a belt, others a chain, and a few use both in different places. The owner’s manual or a mechanic can tell you which your car has.
Why scheduled replacement matters
A belt that has reached the end of its life often gives little or no warning before it breaks. The fact that a car seems fine does not mean the belt is fine. Age matters as well as mileage, because rubber hardens and cracks with time and heat even in a car that is rarely driven. This is why many manuals state an interval in years as well as distance.
The consequences of a failure depend on the engine design:
- Interference engines. The valves and pistons occupy some of the same space at different times. If timing is lost, they can collide, bending valves and damaging pistons or the cylinder head. Repairs can be very expensive, sometimes more than an older car is worth.
- Non-interference engines. The parts have enough clearance that a broken belt simply stops the engine, usually without internal damage, though you would still need towing and a new belt.
Whether your engine is interference or not is something a mechanic or the manufacturer’s information can tell you. Because many engines are interference designs, treating the interval seriously is the safest course. When people discuss costly repairs, a neglected timing belt is a common example.
What gets replaced with it
Replacing a belt involves removing much of the front of the engine, so the labor is a large part of the cost. It therefore makes sense to replace related parts at the same time, which typically include the tensioner, idler pulleys and, if the belt drives it, the water pump. A shop quote often bundles these into a “timing belt kit”. Exact contents vary by engine, so ask what is included and why.
Some engines also drive other items off the same belt, such as an oil pump. If you are building a maintenance budget, add this item well in advance, because it is a larger single expense than most routine services.
Finding the interval
- Check the owner’s manual and the service schedule, which often lists the timing belt with a mileage and a time limit.
- If the manual is missing, a dealer or reputable mechanic can look it up for your engine.
- When buying a used car, ask for evidence of timing belt replacement. If there is none, and the car is near or past the interval, assume it needs doing and factor that into the price. Checking history during a used car inspection is a smart habit.
Be cautious about cars with unknown history. The manufacturer’s interval is a maximum, so some mechanics may suggest earlier replacement in harsh conditions.
Signs of chain or belt trouble
- A rattling sound from the front of the engine, especially at startup, can suggest a stretched chain or a worn tensioner. See the notes on car noises for how to describe sounds to a mechanic.
- A check engine light with codes relating to camshaft or crankshaft correlation.
- Rough running, misfires or poor performance.
- Oil debris, or visible fraying and cracks on a belt when a mechanic inspects it.
A belt may show no signs at all before it fails. Do not wait for symptoms.
If it breaks while driving
If the engine suddenly cuts out or you hear a loud mechanical noise, pull over safely and do not try to restart it. Restarting a damaged engine can make things worse. Arrange a tow and have a qualified mechanic assess it.
The bottom line
A timing belt or chain quietly does one of the most important jobs in the engine. Follow the schedule, replace related parts together, and treat strange noises from the front of the engine seriously. Doing so is far less expensive than repairing the damage after a failure.