What Happens If a Timing Belt Breaks? Breakage and Tooth-Skipping

In this article, we’ll talk about timing belt failures. How and why does a timing belt break? What happens if a timing belt breaks or slips a tooth, and how can we tell it’s broken? We’ll cover topics like these.

Timing Belt Failures

Timing belt failures show up either as what we call belt tooth-skipping or belt breakage. Tooth-skipping tends to cause problems like power loss in the engine, while a full break leads to much bigger problems.

The timing belt carries out an important, critical job in the vehicle’s engine — keeping certain engine components running in sync. That’s why it’s important to prevent timing belt failures before they happen.

To prevent timing belt failures, the vehicle’s periodic maintenance shouldn’t be neglected. The timing belt absolutely must be replaced at the intervals specified in the manufacturer’s manual. Although the exact figure varies by manufacturer, this typically falls somewhere between 64,000 and 96,000 km.

If the Timing Belt Breaks

An illustration showing what happens when the timing belt breaks. When the timing belt breaks, synchronization between the camshaft and crankshaft is lost. Even though the engine has stalled, the crankshaft — still trying to keep turning from its own remaining momentum (even half a turn is enough to cause damage) — causes the valves to strike the piston heads.

What Happens If the Timing Belt Breaks?

The situation illustrated in the previous image is shown as an animation in the video above. Watching this video should make it even clearer what happens when a timing belt breaks.

A broken timing belt is something you never want to happen — if you don’t have a spare, or don’t know how to remove and refit one, it’ll leave you stranded. If the timing belt breaks while the engine is running, the engine stalls suddenly, and at the same time the valve mechanism strikes the piston heads, damaging both the piston heads and the valves. This first strands you on the road, and later leads to a seriously expensive repair bill.

If the timing belt breaks, your vehicle’s engine can suffer damage expensive enough to burn a hole in your wallet (above).

In some timing belt failures, even a full engine replacement can end up being necessary. With that in mind, the smartest solution is to replace a timing belt that’s due for replacement without delay, rather than putting it off. The image above shows the cylinder bearings of an engine whose timing belt broke.

How Can We Tell the Timing Belt Has Broken?

It’s nearly impossible for a timing belt to break while the engine isn’t running. If the belt breaks while driving, you’ll most likely hear a sound similar to a rope snapping (though if your engine’s sound insulation is good and there’s a lot of outside noise, you might not hear it), and the engine will stall instantly. It won’t start again.

There’s another way to tell the timing belt has broken. If the engine suddenly stalled and you suspect a broken timing belt and want to confirm it, remove the distributor cap and try turning the crankshaft by hand. If it turns but the rotor inside the distributor doesn’t move, the belt has definitely broken.

Why Does the Timing Belt Break?

Whenever the front timing cover is opened, the belt should always be visually inspected. If anything looks off, the belt should be removed and checked. As long as the timing belt is in service, it wears down and loses elasticity. It hardens. And remember, a harder material is more prone to snapping.

I Just Had the Belt Replaced — It Shouldn’t Have Broken?

If your timing belt broke before its replacement mileage, check whether the belt was fitted correctly, and whether the timing belt cover gasket has degraded or was installed properly.

It’s more easily affected by foreign material in the environment. Debris that gets between the belt’s teeth and the pulley causes the belt — which is running at high speed — to wear down quickly, developing cracks and splits on its surface.

So how exactly does a timing belt break? After a while, these cracks cause the belt’s structure to deteriorate and weaken to the point where it can no longer withstand the tension on it, and that’s when what we call a “broken belt” actually happens.

Timing Belt Tooth-Skipping and Aging

What Happens If the Timing Belt Skips a Tooth?

If the tensioning mechanism we mentioned above doesn’t maintain the correct tension, what we call timing belt tooth-skipping can occur. As the belt reaches the end of its service life and hardens, one or more of its teeth can break off. A belt with broken teeth will then skip.

When the belt skips a tooth, motion isn’t transmitted with correct timing. Since the camshaft ends up moving with faulty synchronization, the engine experiences problems like power loss and poor traction.

Video: Timing Belt Tooth-Skipping

How Can You Tell the Timing Belt Is Wearing Out?

When a timing belt starts to wear, it can produce a ticking sound inside the engine. This warning sign can also be an indicator of low oil pressure or inadequate engine lubrication.

Sometimes the belt slips on the camshaft drive, causing a cylinder’s valve to open or close earlier than it should. This can cause the engine to misfire, and if the belt isn’t replaced soon, it can result in catastrophic engine damage.

If you hear squeaking sounds coming from around the timing belt cover, that’s a clear sign the belt is starting to feel the stress of long-term use and is aging. If you notice noise coming from the timing belt, you should definitely have it checked. You’ll probably need to replace it.

Oil leaking from the front of the engine can also be a sign the timing belt is wearing out. More precisely, you may have discovered a problem that’s causing the belt to wear. Oil leaking from the timing belt cover — usually caused by the gasket between the engine block and the timing cover wearing out, cracking, or being improperly installed — typically leads to the engine overheating, and to the timing belt aging prematurely.

Since the timing belt is so critical to your vehicle running at all, if you notice this warning sign, you should see a mechanic as soon as possible. If you ignore this warning or fail to notice it, know that you’ll be facing a much more serious repair bill once the belt actually breaks.

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