Snap, said my car engines timing belt

By The Helpful Engineer / On / In Technical & Discussion

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.

Top of my engine - the camshafts
The cylinder valves - reverse side up
My car engines cylinders

14 thoughts on “Snap, said my car engines timing belt

  1. My timing belt is due must book it in. Just one thing if a mechanic offers to inspect the timing belt it is pretty meaningless as the belt need to be just replaced at the intervals specified by the car manufacturer. Also never accept just a belt replacement the water pump and belt tensioner should also be replaced.

    1. Many manufacturers (and shops who benefit tremendously) seem to tell people to change timing belts regularly. While in the past the rubbers dried out and cracked for newer materieal belts, this is unnecessary. For example original vw pages say the belt should be checked and replaced IF NECESSARY and lists what should be checked if your belt should be replaced or not. See translation:
      https://translate.google.fi/translate?hl=en&sl=de&u=http://www.volkswagen.de/de/servicezubehoer/VolkswagenOriginalTeile/original_teile/zahnriemen/wann_ist_der_richtige_zeitpunkt_fuer_den_zahnriemenwechsel.html&prev=search

      1. My timing belt just went on a Ford Galaxy; I didn’t realise that the Galaxy has two timing belts, the main one and one that sits in a full contained unit that is full of oil.
        I had the main timing belt replaced to avoid this issue and for some reason the mechanic didn’t replace this belt that is in oil (perhaps he didn’t know it existed, and I wasn’t experienced enough to know this as well). Anyway, the damage was immense; and I had to get a new engine as the valve heads and piston chamber was damaged beyond repair. Total cost of €2200 and this was mates rates; to put in a second hand engine plus new timing belt kit.

        So the best advise I can give:
        – change it at less than recommended mileage (mine went at 100k, recommended was 120k on Galaxy).
        – ask the mechanic to change all the components, so all timing belts (if >1 exists) plus the water pump. The timing belt in the oil is now a chain; so these last longer apparently.

        I hope my misery can save someone elses pain ;(

    1. Thanks but I took the facebook like button off the site. It was slowing down the website a little. You can use the facebook share button instead if you like.

  2. Ahh the Beauty of good engineering. Many toyota’s (and other models) have what is called a “non – interference” engine, where the valves and pistons cannot contact even in the event of a timing belt snap. Why more manufacturers don’t use this approach is beyond me.

      1. Because it causes poor fuel economy. You can have higher compression ratios in interference designs. Therefore it is better to buy a car with an interference design engine. Considering many places advice you to change belt 100k KMs, if you save little fuel per km, you would get the belt change for free 😉

  3. Great forum.
    I just after replacing 8 valves in my 2011 saab 95. I bought it 7 months ago and was told belt was done . Yep problem here was 2 yrs ago when car had 135,000 km done . Garage never changed it 100k later . It’s cost me know apprx €1400 now . As I was mislead at sale the garage never changed the belt but told me apprx €1600 Euro of service work done . I do think I still have a case to follow the Garage who sold me this car that cost apprx €14k with a false service record.

  4. Hi
    I have 2013 Peugoet 208 bought it last June from a car sales company through finance had 6 months warranty but a month out of warranty now and the timing belt has gone at 47,000 miles it is stuck In a local garage and they said they can’t repair it as they don’t know how to as they cannot find any data on it as it is a fairly newest engine and there will be damage to the engine
    I really don’t know what to do could you please advise thanks janer

  5. I was driving my Renault scenic 2007 when it suddenly slowed down and stopped will not turn back on at all got it towed to the garage they have told me it is the timing belt but have no idea if there is any other damage to the car is there any way I could find this out?

    1. Unfortunately, the only way to definitively determine damage to the engine is to open up the engine case and inspect the interior for damage. Damage can range anywhere from minimal marring on valves all the way to cracked engine blocks.
      Visualize that, while the piston heads and connecting rods in modern engines are made to be lightweight, the momentum mentioned in the article takes place at thousands of revolutions every minute. It’s not the weight of the piston head striking the valves that causes the damage, it’s the speed.
      An alternative to this is aligning the cam shafts, replacing the timing belt, and turning the engine over to see what happens, but this may cause additional damage. For this reason, it is unlikely to find a mechanic who would ever recommend this method.

      1. thanks for replying I decided just to scrap it was a lot easier than trying to get it fixed thank you again for your reply

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