Unfortunately, the timing belt in my car has failed. I was driving quite fast in the outside lane of a three lane motorway (the M50) when it happened. The engine simply stopped and the car immediately slowed down to a halt, but luckily I had enough time to indicate, change lanes and move on to the hard shoulder. There was no bang or loud noise when it happened, the engine just stopped working. The computer display in the car simply stated ‘check fuel injectors’, so I wasn’t sure it was the timing belt at first. But I suspected it as the belt was due to be changed. I had the car booked in to have it changed the following week. The tow truck took my car to the nearest garage where they confirmed that the timing belt failed; this surprised me a little as I thought that when a timing belt broke you would hear a loud bang, but not always as it turns out.
The garage said it would be very costly to fix, depending on how much damage had been caused to the engine, but they couldn’t be sure of the final cost until the engine was opened up and inspected. This particular garage wasn’t close to where I lived, so I asked a local mechanic, who my friend knew and vouched for to look at it. He opened the engine up and found that while some damage had been caused, it luckily wasn’t as bad as expected. Four valves had been damaged. He is fixing it at the moment, but as I have learned a lot about timing belts in the last week, I thought I would share it with you all.
What does a timing belt do?
A timing belt connects the camshaft to the crankshaft and keeps the engine valves opening and closing in time with the movement of the pistons. The belt itself is black toothed rubber belt which because of wear needs to be replaced at regular intervals.
Why is it so catastrophic for the engine if it fails?
Sometimes the engine can be so badly damaged by a timing belt failing that it is cheaper to replace the engine. A belt failure damages the engine because the belt controls the timing of the opening and closing of the cylinder valves. Once the belt fails, the valves will remain in the position they were in at the instant the belt fails; but the pistons will continue to move, because of their momentum, and hit and damage any open valves. Sometimes, the pistons will hit the valves so hard that the camshaft will be damaged as well as the valves. This is when the mechanic starts talking about replacing your engine with a second hand one. Modern cars have 16 valves, four per cylinder. Two are to let fuel into the cylinder and two are to remove exhaust gases.
Why are timing belts not designed to last as long as the car?
Timing belts are made from rubber because it is lighter and quieter than any other alternative like a chain. A rubber timing belt doesn’t rust or require oil, whereas a chain would, but it does need to be changed at regular intervals. The timing belt in my car also powers the water pump, and is usually changed at the same time as the belt.
Does it actually snap when it fails?
When the belt fails it is unlikely to actually snap, instead it is more likely that the teeth of the belt will break off. The following is a good animation from YouTube of an engine which shows how the timing belt operates and is connected to the valves by the camshafts.
Below are pictures of the inside of the engine of my own car, with the relevant parts. My advice, for what its worth, is to have your timing belt checked and replaced when your mechanic/garage says to and not when it suits you. Also if buying a second hand car you should make sure you receive written proof when the timing belt was last changed. The final cost to fix my timing belt was €750, but the dealer who I didn’t use quoted me between €1,500 and €2,000.