Timing chains are an essential component to engine timing. The first “stroke” in a typical four-stroke engine is the opening of the intake valve. The piston pushes downwards and the intake valve opens, letting air and fuel into the cylinder. The piston pushes back upwards, compressing the air and fuel and the intake valve closes. Next, the spark plugs fire creating combustion, sending the piston back downwards where the exhaust valve opens letting the air out of the cylinder.
For engines to function properly, the timing of the intake valve opening/closing, the combustion taking place, and the exhaust valve opening/closing must be precise. The timing chain is responsible for ensuring proper timing of the valves opening and closing.
Timing chains are a maintenance item. Over time they wear down and stretch which can lead to various performance issues and even cause catastrophic engine damage. This article will go into detail on timing chain function and how to tell when your timing chain starts to fail.
What is a timing chain?
A timing chain is responsible for opening and closing the intake and exhaust valves in an engine. The timing chain is connected to the camshaft and crankshaft by a set of gears, usually located on the front of the engine block.
The camshaft turns once for every two turns of the crankshaft. As the camshaft turns, small lobes also called cams push up against the valves to open them.
Here is an example of a timing chain system with the chain, guides, tensioners, sprockets, etc.
Additional timing chain components
The timing chain is made of metal and turns on metal gears or sprockets, so it needs to be lubricated with oil. Therefore, the timing chain sits within a timing chain cover, which bolts up to the block and protects the timing chain from road dirt and debris while also lubricating it with engine oil.
Additionally, the timing chain also has timing chain guides, which help keep it on track and prevent it from slipping off of the sprockets.
The last important piece to timing chains is a tensioner. The timing chain tensioner makes sure there is no slack in the chain. Most tensioners are spring-based and hydraulically actuated, meaning they use oil pressure to keep the chain tight.
Why do timing chains fail?
The primary cause of timing chain failure is timing chain stretch. Because the timing chain sits right near the block it is subject to lots of heat. Over time the heat causes the metal chain to slowly begin to stretch. At the chain stretches, it becomes loose on the gears and can then “jump teeth” or even completely fall off of the sprockets.
Additionally, timing chain stretch can be caused by low oil levels. Since the metal chain runs on metal gears, it creates heat and friction when not properly lubricated. When oil levels get too low the chain can be starved of lubrication which also causes it to stretch.
Timing chain tensioner failure
Hydraulically actuated tensioners are known to fail due to low oil levels or oil pressure leaks. The tensioners use oil pressure to keep tension on the chain. When oil levels get low the oil pressure drops and therefore the tensioner doesn’t have the pressure it needs to keep the slack out of the line.
Timing chain guide failure
Timing chain guides also commonly fail. The guides are usually manufactured of plastic which means they easily break over time from heat, engine vibration, etc.
When the guides fail, the chain gets off track and can jump teeth or completely fall off.
Timing chains vs. timing belts
At the simplest level, timing chains are made of metal and turn on gears or sprockets. Timing belts are made of rubber and do not run against sprockets. Additionally, chains sit within a timing chain cover whereas the belts are usually open in the engine bay.
Timing chains have a longer life span than belts, however, they also have more failure points. Timing chain covers can leak, the tensioners and guides can break, etc. However, timing belts also stretch over time and tend to have a shorter life span, albeit they have less supporting parts that can fail.
Failing Timing Chain Symptoms
- Cylinder misfires
- Metal shavings in engine oil
- Knocking or rattling noises from engine
- Rough idling
- Engine no start
When the timing chain stretches, jumps teeth, or completely fails, it causes the valves to open and close at the wrong times. If the chain just jumps a little then you will likely just notice rough idling, poor performance, misfires, etc.
However, if the chain jumps a lot or completely falls off the valves can go crashing into the pistons as there is nothing controlling the opening and closing of the valves. When this happens it can cause severe internal engine damage.
How long do timing chains last?
Most timing chains nowadays are manufactured to last the life of the engine, although, it is common for timing chains to go bad around the 150,000 mile mark.
Timing chains tend to show some symptoms before they completely fail. Misfires and poor idling are common symptoms that let you know your timing chain is on the way out but hasn’t completely failed yet.
Low oil levels and engine overheating are two most common things that can cause the timing chain to fail prematurely. Both of these situations will put extra heat and stress on the timing chain which can cause it to stretch.
Preventing timing chain failure
- Inspect your timing chain cover frequently for leaks
- Make sure engine oil levels don’t get low
- Don’t let the engine overheat; if it does, do not drive it
Can I drive on a bad timing chain?
As mentioned, the timing chain stretches over time. You can drive on an old timing chain but we recommend replacing the chain immediately once you have noticeable stretch and symptoms of failure.
If you drive for too long on a bad chain and it continues to stretch then it can jump a lot of teeth or completely fail and lead to catastrophic internal engine damage.
Timing chain replacement costs
Most timing chain parts will only cost $100-$200. The most variable piece is labor. Labor typically ranges from $250-$1000, but it can be significantly more in some cases.
For example, the timing chain on Audi’s 4.2 V8 is located at the back of the engine requiring the whole engine to be pulled to be replaced. This leads to this being a $5k+ replacement job.
While that is an extreme example, the point is the cost to replace is predominantly driven by the difficulty of replacing it and not the cost of the parts themselves.