The timing: belt, chain and belt running in oil
It is the part that scares people most on a used car, and rightly so: if it snaps while driving, on most engines it is a big repair. Here is what it does, which types there are, when each one is replaced and how it warns you before it ruins your day.
What the timing does
At the bottom, the crankshaft moves the pistons; at the top, the camshafts open and close the valves. The timing is what links the two so they stay perfectly in step: each valve has to open exactly when its piston needs it, not a degree early or late. Illustration 1 shows it in magenta: the belt, its pulleys and the two camshafts.
If it snaps or jumps a tooth, the valves fall out of step with the pistons. On a great many engines, the so-called interference engines, a piston then hits an open valve: bent valves, sometimes a damaged piston and almost always a very serious bill. That is why you do not mess about with the timing.
Belt, chain or belt in oil
The classic rubber belt is quiet and cheap, but it is replaced by mileage or by age, whichever comes first: depending on the engine, between 60,000 and 240,000 kilometres (roughly 40,000 to 150,000 miles) and between four and ten years. Your car’s handbook is what counts. And it is the moment to change the tensioner, the idlers and, if the belt drives it, the water pump.
The metal chain is meant to last as long as the engine, but it does not always manage: over the years it stretches, especially if the oil was not changed on time or was not the right one, and it starts to make noise.
The belt running in oil sits inside the engine, lubricated, to be as quiet as a belt and last longer. Some modern three and four-cylinder engines use one. Its weak point is that with the wrong oil, or oil changed too late, the rubber degrades and sheds little pieces that can end up blocking the oil pump pickup. At that point it is no longer the belt: it is the lubrication of the whole engine.
How each one warns you
An old belt almost never warns you, which is why it is changed by the calendar. A stretched chain does: a metallic rattle for a few seconds on a cold start, which then goes away, is the classic warning, and often the engine control unit logs that the camshaft is not where it should be relative to the crankshaft. A belt in oil that is breaking up can end up triggering low oil pressure warnings.
The most common faults
- Stretched chain
Rattle on cold starts and, eventually, the engine control unit sees the camshaft out of phase with the crankshaft.
Typical codes: P0016, P0017, P0018, P0008 - Variable valve timing or its solenoid
The system that advances and retards the cams falls short or overshoots. Often it is the solenoid, or dirty or low oil.
Typical codes: P0010, P0011, P0012, P0014 - Crankshaft or camshaft sensor
Without its signal the engine starts badly, cuts out or will not start, even with perfect timing.
Typical codes: P0335, P0340, P0341 - Belt in oil breaking up
Bits of rubber in the oil blocking the pump pickup: low oil pressure warnings. Stop the engine and have it looked at now.
Typical codes: P0524, P0521 - Snapped belt
The engine stops dead and will not start again, or spins too freely when you turn the key. Do not try again: call recovery.
What you can see with Final OBD
Nobody can see a belt’s wear without stripping it down, and neither can Final OBD. What it does show you is how the engine warns: the crankshaft to camshaft correlation faults, which is how the engine control unit gives away a stretched chain, the variable timing ones and the oil pressure ones, all of them explained. And on engines that provide them, oil pressure and temperature live.
Open the bonnetthe in-depth explanation
Why they have to stay in step
A four-stroke engine needs two turns of the crankshaft for a full cycle in each cylinder (intake, compression, power and exhaust), and in those two turns each valve opens only once. That is why the camshaft turns at half the speed of the crankshaft, and its pulley has twice the teeth. The timing keeps that exact ratio, set at a reference point: cylinder 1 piston at the top and the cams in their place.
There is a moment in the cycle, at the end of exhaust and the start of intake, when both valves are slightly open at once with the piston almost at the very top. That is why, on an interference engine, if the timing jumps, the piston reaches the valves. Most diesels and a great many modern petrol engines are interference engines.
The belt: when and with what
Rubber ages even when it is not used: it cracks, hardens and loses teeth. That is why manufacturers give both a mileage and an age. If the car covers few miles, it is the years that will decide the change. It is done as a full kit: belt, tensioner and idlers, because a seized idler bearing breaks a new belt just as it would the old one. And if the water pump is driven by the belt, it is replaced too: if it starts leaking or its bearing goes a few months later, the whole lot has to come apart again.
When a mechanic inspects it, they look for cracks on the back, worn or missing teeth, a shiny glazed face, and oil or coolant stains, which eat it from the inside. An oil leak onto a dry belt is a death sentence for the belt.
The chain: why it stretches
A chain does not stretch because the metal gives, but because the pins and bushes in each link wear. A few tenths of a millimetre in each one, multiplied by more than a hundred links, make the chain several millimetres longer and leave the camshaft lagging behind the crankshaft.
It is tensioned by a tensioner that works on oil pressure and guided by plastic rails. On a cold start, until the pressure builds, a tired tensioner leaves the chain slack for a few seconds, hence the rattle. What wears it most is the oil: changing it late, using one the manufacturer does not specify, or letting it get diluted with fuel on short trips.
On engines with variable valve timing, the engine control unit learns how much offset there is between crankshaft and cams, and when it goes out of its margin it logs the correlation fault. That number, on engines that provide it, is the best way to follow the stretch without opening the engine. The fix is the full kit: chain, tensioner, guides and often the sprockets.
The belt running in oil
It was invented to get the best of both worlds: the quietness and low friction of a belt and a long life, because it is lubricated. Manufacturers give it long intervals, up to ten years or 240,000 kilometres. Examples are the 1.2 PureTech three-cylinder in Peugeot, Citroën and Opel or Vauxhall cars, Ford’s 1.0 EcoBoost and some EcoBlue diesels.
The problem is that this rubber is designed for one specific oil. With another oil, or with old oil full of fuel from lots of short trips, it swells and slowly breaks up. The bits it sheds travel round the sump and end up in the oil pump pickup, which gradually clogs until the pressure drops. On some engines that debris has even starved the brake vacuum pump of oil, which is why there were recall campaigns.
If you have an engine like this: exactly the oil grade the manufacturer specifies, oil changes earlier than the schedule if you do a lot of town driving, and have the belt measured at services, which is done without taking the engine apart. If it has already warned of low oil pressure, do not start it again.
A tip when buying a used car
Ask for the invoice for the last timing job. If it is a belt and there is no paperwork, budget to change it as soon as you buy: it is a known repair with a fixed price, far cheaper than an engine. If it is a chain, listen to it starting from cold, with the engine stood overnight. If it rattles, you know what you are dealing with.
All sections of the mechanics guide
See it in your own car
Almost everything in this guide can be seen with Final OBD and an adapter plugged in: faults explained, live data and the details of each system.
