(Updated on February 22, 2022)
Timing is the process of synchronizing the camshafts with the crankshaft. This allows the air to enter and exit the engine at a precise time. This also tells the ECU when it should fire each spark plug for optimal combustion.
Modern vehicles use timing chains instead of timing belts. While timing belts need to be replaced every 60-80,000 miles, timing chains are typically supposed to last the life of the vehicle. Timing chains will only be serviced if there is a problem, or if they are removed for other engine work.
How a Timing Chain Works
Timing chains work a bit like bicycle chains. If you’ve ever ridden a bicycle, you know that the chain connects the pedals to the rear wheel. As you push, the torque from the pedals is transferred to the rear wheel using the chain.
On a four stroke engine with a timing chain, the torque of the crankshaft is transferred through the timing chain to the camshafts. The timing chain also has the responsibility of keeping the crankshaft and camshafts perfectly synchronized with each other.
This allows the valves to open and close at precisely the right time for each piston as the pistons move up and down.
Bad Timing Chain Symptoms
1) Chain Noise on Startup
When you go to start your car, you typically shouldn’t hear the timing chain. If you hear an unusual rattling or grinding noise from the engine on startup that goes away after a moment, you may have a failing timing chain component.
Note that some vehicles have more engine noise than others, so you’ll want to listen for any new sounds that come up. If you’ve had the car for years and it’s always made a slight timing chain rattle, it might be fine. It never hurts to get the vehicle checked out if you aren’t sure, though.
2) Misfires or Rough Idle
Timing chains optimize the combustion process by controlling the ingress and egress of air in the combustion chamber. If you have a timing chain that jumped a tooth, valves may be open or closed when they shouldn’t be. This causes a rough running engine, particularly at idle.
3) Can’t Start Engine
If the timing chain has broken, you won’t be able to start your engine. There will be no means to lift the intake valves to bring fresh air into the engine, and no way to evacuate exhaust gases without the timing chain.
What Causes Timing Chain Noise?
Timing chain noise is often caused by a loose timing chain tensioner. The timing chain tensioner is responsible for taking up slack on one side of the timing chain.
Many timing chain tensioners are hydraulically actuated. On these engines, there is an oil passage that runs through the front of the engine into a little piston. When the oil is pressurized, this piston puts pressure on the timing chain tensioner and takes the slack out of the timing chain.
If oil change intervals are neglected, the oil passages that lead to this little piston can become clogged, preventing the piston from taking the slack out of the timing chain. This increases rattling noise from the chain.
There is another type of timing chain tensioner that is mechanical instead of hydraulic. Mechanical tensioners use a spring and a pawl instead of hydraulic pressure.
The spring puts pressure on the tensioner’s piston, and this piston pushes against the chain guide. The pawl then locks the tensioner in place. This holds the chain guide in place with something a little more permanent and reliable than the spring pressure on its own.
Since there is a small distance between pawl teeth, the tensioner piston could stop in a number of slightly different positions when you turn off the engine. The engine may stop with the timing chain resting right up against the pawl, or it may stop with a little slack in the tensioner.
If there is a little slack in the tensioner, this could lead to a small amount of timing chain noise on first startup for a fraction of a second.
Related: 6 Causes of Timing Chain Noise
What Causes a Timing Chain to Break?
Bad Timing Chain Tensioner
A bad timing chain tensioner will lead to a loose timing chain. This puts a dynamic load on the chain as you start and stop the engine, increasing wear if it exceeds the tolerances the chain was designed for. Over time, this could cause the timing chain to snap from the extra stress.
Broken Timing Chain Guide
A broken timing chain guide may allow the timing chain to move out of position. This would either snap the timing chain if it contacts another component or disconnect it from the timing gear if it moves too far out of position.
What Happens If the Timing Chain Breaks While Driving?
If your timing chain breaks while driving, your engine will die and your car will come to a stop. You will not be able to drive the vehicle again until the timing chain is replaced.
It’s important to note that certain vehicles may suffer severe engine damage if the timing chain breaks. This depends on whether or not the engine is an interference or non interference type.
An interference engine is an engine where the pistons and valves could possibly make contact if the vehicle is not timed correctly. Valve-to-valve or piston-to-valve contact would likely ruin the engine.
When Should a Timing Chain Be Replaced?
While a timing chain won’t typically become stretched or worn over time like a timing belt might, timing chains may require replacement under certain circumstances.
If a component in the timing chain assembly has failed such as a chain guide or tensioner, it is often wise to replace the whole timing chain assembly. This includes the timing chain itself.
Some manufacturers may recommend replacing the timing chain if it needs to be removed for other work, such as a valve job, front main seal replacement, or head gasket replacement. If you are performing the work yourself, check the factory service manual to see if replacing the timing chain is recommended.
If you are taking the vehicle to a shop, your mechanic should know to replace the timing chain if it is necessary and give you an estimate for the extra labor.
Timing Chain Replacement Cost
Timing chain replacement cost will vary depending on the year, make, and model of the vehicle. Expect to pay about $800 to $1500 for this maintenance at an independent shop.
Should You Replace a Timing Chain Yourself?
Since timing chains aren’t meant to be serviced regularly, they are often more difficult to replace than a traditional timing belt. You should not attempt to replace the timing chain yourself unless you are qualified to do so.
It’s best to leave this job to a professional with some experience. This is particularly important if your vehicle has an interference engine.