Snapped Cam Belt - instant engine death?

nottsphil

Many thanks for that - it's the first diagram of an ICE I've seen that actually shows what opens the valves. I always assumed the downward force of the piston caused a partial vacuum that pulled open a spring loaded flap on the tube from the carb, forcing in the mixture. But why would damage be caused if the top of the piston hits the valves? The diagram seems to indicate that the springs would just contract, allowing space for the valve stems to retract. 

Homer_home

nottsphil said:

Many thanks for that - it's the first diagram of an ICE I've seen that actually shows what opens the valves. I always assumed the downward force of the piston caused a partial vacuum that pulled open a spring loaded flap on the tube from the carb, forcing in the mixture. But why would damage be caused if the top of the piston hits the valves? The diagram seems to indicate that the springs would just contract, allowing space for the valves to retract. 

If the timing is correct yes however if the timing is off then the valve might open when the piston is on the upward stroke causing the piston to slam into it like a hammer and the valve gets bent and is buried into the head 

Homer_home

Carrot007 said:

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).

A 9 year old car is not old!!!

nottsphil

Oh yes, I can now see there wouldn't be room for contraction if the cam got stuck in its extended position.

Another thing I learned from the explanation under the original diagram is that glow plugs don't ignite the mixture in a diesel engine! 

AdrianC

nottsphil said:

Another thing I learned from the explanation under the original diagram is that glow plugs don't ignite the mixture in a diesel engine! 

They kinda do, at least to start with...
The original purpose of them was to warm the engine for a cold start, to allow that compression ignition to go ahead - hence the old rigmarole of turning the key, then waiting for the light to go out before trying to start.

In common-rail diesels (basically anything less than about 15yo now), that's only required at very low ambient temps, because of the pressure of the fuel at injection. They're mostly used these days for managing combustion temperatures for emissions reasons.

Homer_home said:

A 9 year old car is not old!!!

It's older than the average car in the UK (about 8.5yo), and about 2/3 of the way from manufacture to the average scrapping age (just under 14yo).

shinytop

It never ceases to amaze me how all these bits can spin so fast and for so long and not just explode or melt ...

AdrianC

shinytop said:

It never ceases to amaze me how all these bits can spin so fast and for so long and not just explode or melt ...

...and that's why proper servicing is important...