What is a timing chain? It is a metal chain inside the engine that links the crankshaft to the camshafts, so the valves open and close at the right moment as the pistons move. A timing belt does the same job with a toothed rubber belt, and the main difference for an owner is that a belt is replaced on a schedule while a chain is replaced only when testing shows it is worn.
What a timing chain does
In a four-stroke engine the crankshaft turns twice for every one turn of the camshafts. The timing chain keeps that two-to-one relationship exact. It runs from a sprocket on the crankshaft to sprockets on the camshafts, and on some engines it also drives the oil pump, a balance shaft or a high-pressure fuel pump.
A timing chain looks like a narrow, precise version of a bicycle chain. It lives inside the engine behind a sealed cover and is lubricated by engine oil. Three other parts work with it:
- The tensioner. A spring and oil-pressure plunger that pushes on the chain to take up slack.
- The guides. Rails faced with hard plastic that the chain slides along. One of them usually pivots, and the tensioner pushes on it.
- The sprockets. Toothed wheels on the crankshaft and camshafts. On most modern engines the camshaft sprockets are part of the variable valve timing units.
Timing chain vs timing belt
| Timing chain | Timing belt | |
|---|---|---|
| Material | Steel links | Rubber reinforced with fibre cords, with moulded teeth |
| Where it runs | Inside the engine, in engine oil | Usually outside the oil, behind a plastic cover. A few newer engines run a belt in oil |
| Replacement | No scheduled interval on most engines. Replaced when testing shows wear | Scheduled item, by mileage and by age, as set by the carmaker |
| How it wears | Slowly. Links wear, guides and tensioner wear | Rubber ages, teeth crack or shear, cords weaken |
| Warning before failure | Often a startup rattle or a check engine light | Often none |
| What it depends on | Clean oil at the correct level | Age, heat, and the condition of the tensioner, idlers and water pump |
| Noise | Slightly noisier | Quieter |
| Replacement job | Usually larger, since the chain is inside the engine | Usually simpler, since the belt is behind a cover |
Neither design is the right answer for every engine. Carmakers have moved back and forth between the two, and some models were sold with a belt on one engine and a chain on another in the same year. The maintenance schedule for your exact engine is the reliable source.
How a timing chain wears
A chain rarely fails without something else wearing first. Four things work together.
Chain stretch. The steel does not really stretch. Each link pivots on a pin, and every pin and its bore wear a tiny amount. Added up over all the links, the chain gets longer. A longer chain sits differently on the sprockets, so the camshafts run slightly late compared with the crankshaft.
The tensioner. It takes up slack as the chain lengthens, but only as far as its travel allows. Most tensioners rely on oil pressure, which drops away when the engine is off. A worn tensioner, or one that does not hold its position, leaves the chain loose for the first seconds after a cold start.
The guides. The plastic faces wear into grooves and become brittle after years of heat. A loose chain slaps against them and can crack them. Broken pieces fall into the oil pan, where they can restrict the oil pickup.
Oil condition. This is the part an owner controls. Old oil carries soot and fine metal that act as an abrasive on the chain pins. Low oil level and thin, fuel-diluted oil reduce the pressure the tensioner needs. Engines that see long oil change intervals and many short trips show chain wear sooner. South Florida traffic and heat make this worse, which is why we advise changing oil on time with oil that meets the carmaker’s specification. See our European oil change service.
Some engines are known for early chain or tensioner wear, including certain BMW four-cylinder engines and early Audi and Volkswagen 2.0T engines. Our Audi timing chain repair page covers the Audi cases, and our BMW repair page covers the BMW ones.
Symptoms of a worn timing chain
- A rattle for a few seconds at cold start, from the front or rear of the engine depending on where the chain sits. This is the most common early sign.
- A check engine light with crankshaft and camshaft correlation codes. P0016, P0017, P0018 and P0019 belong to this family.
- Rough idle, hesitation or reduced power, because the valves are opening late.
- Slow or difficult starting.
- A steady rattle or whine that rises with engine speed, on more advanced wear.
- Plastic or metal fragments in the oil at an oil change.
None of these proves the chain is worn. A failing variable valve timing solenoid, a cam adjuster, a sensor fault or low oil can cause the same codes and some of the same noises. Our guide to BMW VANOS solenoid faults shows how close the symptoms can be.
What happens if a timing chain fails
It depends on the engine design.
In an interference engine, the valves and the pistons use the same space in the cylinder at different moments. The timing drive is the only thing keeping them apart. If the chain jumps several teeth or breaks, open valves are left in the path of the pistons. The usual result is bent valves, and sometimes damaged pistons, valve guides or the cylinder head. The repair then goes well beyond a chain.
In a non-interference engine, there is clearance even when timing is lost. The engine stops and will not restart, but the valves and pistons do not touch.
Most modern engines, and nearly all modern European engines, are interference designs. A chain that jumps a single tooth may leave the engine running badly with a warning light on. That is the last practical warning, and the car should not be driven further.
How a shop checks a timing chain
A chain is inside the engine, so most of the diagnosis is done before anything is taken apart.
- Listen to a true cold start, after the car has sat for several hours. A warm engine can hide the noise.
- Read the fault codes and freeze-frame data from the engine computer.
- Read cam and crank timing values with a scan tool. Many engines report how far each camshaft is from its expected position, and a chain that has lengthened shows as a steady offset.
- Check the oil level, condition and service history, and look at the old oil filter or drained oil for plastic or metal.
- Inspect the tensioner where the engine allows it. Some engines have an inspection opening that shows how far the tensioner has extended.
- Rule out the look-alikes: timing solenoids, cam adjusters, sensors and oil pressure.
- Open the engine further only if the data supports it, for example by removing a valve cover to look at the chain and guides directly.
At Prestige Auto Repairs & Tires, the findings go to you as photos with a written estimate before any repair. When a chain does need replacing, the tensioner, guides and any worn sprockets are replaced with it, because they wear together. Our timing chain and timing belt replacement page explains the repair itself.
If you think your chain is worn
Check the oil level first and top it up if it is low. Note when the noise happens, how long it lasts and whether a warning light is on. Then book a check engine light diagnosis or request an estimate with the details and any codes you have.
Prestige Auto Repairs & Tires is at 250 NE 44th St, Oakland Park, open every day 8 AM to 7 PM, (954) 303-0758, and at 930 NE 4th Ave, Fort Lauderdale, open Monday to Saturday 8 AM to 7 PM, (954) 562-8045.
