Snapped Cam Belt - instant engine death?

TadleyBaggie

My daughter's Fiesta (11 reg, 1.4 diesel, 135K miles) broke down last. night. She pulled away from a junction and the car hesitated and then went into limp mode. She drove for around a mile or so at 40 and then the engine cut out completely and wouldn't restart. RAC attended and reading engine codes showed crankshaft and camshaft sensor errors. RAC guy said probably snapped cam belt. 

I have a lingering doubt as I thought a snapped cam belt would kill the engine immediately, not go into limp mode for a short while. Thoughts?

chrisw

The belt snapping would stop the engine instantly, however, sometimes they start to shred and jump teeth before finally snapping.

You can often tell when it's snapped as the starter makes a different sound, more like a motor whirring. Otherwise you'll need to try and have a look in the timing belt covers.

Ebe_Scrooge

You're right that a snapped cambelt would mean the engine wouldn't run at all.  I guess it's possible that the initial fault was something else, then the cambelt subsequently went.  Your best bet is to get a mechanic to take a look - it won't be difficult, once the car is in a workshop, for them to tell whether it's just a knackered sensor or whether the cambelt has in fact gone.  A proper diagnosis shouldn't cost much, you can then decide whether it's worth repairing.

A snapped cambelt (if that's what it turns out to be) doesn't necessarily mean the end of the world - it just depends on how much damage has been done.  It can mean a huge bill, or it can mean just a few hundred.  Years ago I had one go, and was lucky - got it repaired for I think it was about £400, there was far less damage than there could have been.  I was lucky enough to have a very decent local independent mechanic who I always used, he was world-famous locally :-)

Were_Doomed

As this is a diesel (which probably injects the air/fuel mix into the cylinders and compression does the rest) then there may be less chance of piston stroke coming into contact with other parts at the top (or bottom) of the cylinder. (In a petrol engine, for example, the lower/closed position of an inlet valve may be lower than the piston top position - in such a case a cam belt failure may lead to the piston crashing against the valves = expensive repair).

lordmountararat

Some cars (Mazda MX5 for example) have what are called 'non-interference' engines so that cataclysmic damage does not occur when the cam belt snaps. Perhaps your daughter's  Fiesta has a a non-interference engine.

AdrianC

It's very unlikely any diesel will be non-interference, simply because of the much higher compression ratios compared to petrols.

My money's on the warning being when it jumped a tooth. At that stage, simply replacing the belt would have resolved the problem. Now, it's probably uneconomical.

nottsphil

AdrianC said:

My money's on the warning being when it jumped a tooth. At that stage, simply replacing the belt would have resolved the problem. Now, it's probably uneconomical.

Does that mean that if any car I drive goes into limp mode, then I mustn't limp home in it just in case? I've really no idea what a cambelt is or does, but from what I've read on the thread, the only way I might be able to tell is by the sound of the starter motor. 

macman

The cambelt, via the camshaft, synchronises the opening of the valves with the movement of the pistons. Get that timing wrong, and the resulting engine damage can be terminal.

chrisw

The trouble is, a number of problems can throw the car into limp mode, some minor, some more serious. Most of the time you won't know whats causing it until you have the diagnostic codes read or you have a failure.

All you can really do is ensure the cambelt is changed at or before the manufacturer's specified age and mileage limit. It's usually around 10 years or 100k miles so if your daughter's has never been changed its probably been running on borrowed time.

Carrot007

chrisw said:

All you can really do is ensure the cambelt is changed at or before the manufacturer's specified age and mileage limit. It's usually around 10 years or 100k miles so if your daughter's has never been changed its probably been running on borrowed time.

Do also mention that a lot are 4 or 5 years or 70K.

However with a car so old the garage taken to for mot may not mention is and they may think it's being run into the ground (were serviceing also being done I would hope they would mention it).

AdrianC

nottsphil said:

AdrianC said:

My money's on the warning being when it jumped a tooth. At that stage, simply replacing the belt would have resolved the problem. Now, it's probably uneconomical.

Does that mean that if any car I drive goes into limp mode, then I mustn't limp home in it just in case?

If my car goes into limp mode, I'll be limping to the first safe place to stop and call recovery - not to attempt to drive home.

I've really no idea what a cambelt is or does

Basic four-stroke combustion cycle...
Suck, squeeze, bang, blow.
Intake, compression, combustion, exhaust.
Suck the air in, compress it with fuel, ignite it and it pushes the piston down to produce power, get rid of the burnt gases. Start again.

That's one of four pistons in your engine.
The rotating shaft at the bottom is the crankshaft. Your rev counter tells you how many times that's going round per minute - roughly 900-6,000 times per minute, 15-100 times per second.
The rotating shafts at the top are camshafts - there may be one doing both inlet and exhaust valves, or two. They go round at half the speed of the crank.
So that two-crank-revolution, four-stroke cycle is happening roughly 30-200x per second inside your engine.

The cambelt is a rubber belt which ties the cam(s) and crank together. If the piston meets the valves, the valves get bent or other internal bits get damaged.

Some engines use a chain instead. That isn't foolproof, either...

The combustion chamber size is compressed to about 1/8th of its volume in a petrol, and about 1/20th in a diesel. The main difference between the two is that there's no spark plug in a diesel to start the ignition - that increased compression alone is enough to ignite the fuel/air mixture.