Timing belts and timing chains

Belt or chain?

Every engine has one or the other. A timing belt is reinforced rubber and has a replacement interval in the maintenance schedule. A timing chain is steel and is meant to last, but its tensioners and plastic guides wear and the chain itself stretches. Give us your VIN and we will tell you which your engine has and what its schedule says.

Timing belts

• When. By the maker's schedule — usually somewhere between 70,000 and 120,000 miles — or by age, because rubber hardens whether the car is driven or not. A ten-year-old belt on a low-mileage car is overdue.
• Why it cannot wait. On most engines a broken belt lets the valves meet the pistons. The engine stops at once, and the repair is a cylinder head or an engine.
• What we replace with it. The tensioner and the idler rollers — a seized roller destroys a new belt — and the water pump when the belt drives it, with fresh coolant. It is the same labor, done once.

Many Volkswagen and Volvo engines have one, as do older Audis and plenty of domestic and Asian engines.

Timing chains

A chain gives warning, if you know the signs:

• A rattle for a second or two on a cold start that goes away as oil pressure builds.
• A check-engine light with a code saying the camshaft and crankshaft are out of step.
• Rough running or a no-start once the chain has jumped a tooth.

We measure before we open an engine: the stored timing values read with a scan tool, and the tensioner's travel where the engine lets us see it. When the chain is stretched we replace the chain, the guides and the tensioners together, and the sprockets and camshaft adjusters where they are worn, and time the engine with its own locking tools.

Timing work we have done here includes timing chains on Land Rover and Jaguar engines, chain and sprocket repairs on MINI, camshaft adjusters on Mercedes-Benz and Volkswagen, and variable valve timing (VANOS) parts on BMW. The seals around the timing gear are a repair of their own: the upper timing cover on Audi, Volkswagen and Mercedes-Benz engines is a common oil leak.

The other belt: the drive belt

The drive belt — the serpentine belt — is not the timing belt. It runs on the outside of the engine and turns the alternator, the A/C compressor and often the water pump. When it breaks you get a battery light, heavy steering on some cars and, where it drives the pump, an engine that overheats within minutes.

We inspect it at every visit and replace it at 80,000 miles, sooner if it is cracked or noisy. A chirp or a squeal on a cold morning is usually the belt, its tensioner or an idler pulley; they are replaced together when they are worn.

Timing questions

How do I know if my car has a timing belt or a chain?

Tell us the VIN or the plate and we will look it up. As a rough guide, most BMW, MINI, Mercedes-Benz, Jaguar and Land Rover engines use chains; many Volkswagen and Volvo engines, and older Audi engines, use belts.

My timing belt is due. Can I put it off?

You can, the way you can put off a smoke detector battery. Belts rarely give warning, and the cost of one breaking is many times the cost of replacing it.

Is a rattle on start-up always the timing chain?

No. A camshaft adjuster, a loose heat shield or a tensioner slow to fill can all rattle. That is why we read the engine's timing values before quoting a chain.

Why replace the water pump with the timing belt?

On many engines the belt drives the pump, and the pump sits behind the same covers. A pump that fails later means paying the timing belt labor a second time — and a leaking pump ruins the new belt.