valve impact on piston? cause?
Discussion
Hi,
1st time poster on here
I have a problem for which i hope someone can help.
My mother is being pursued by a hire company regarding damage to a courtesy car given to her. The car wouldnt start one morning and they took it away. Months later they sent a bill of £4k!
The valves had impacted the pistons and the ECU codes apparently showed the car being driven over the limited speed. We find this very hard to believe, she is 50 years old and has been driving over 30 years.
My questions is, what other things could cause the valves to impact the pistons? Also, does i've been told by a family friend (not a mechanic though) that the ECU does not record when a car has been driven over a limited speed? Does anyone know how true this is?
The car was a vauxhall astra, over a year old with around 14000 on the clock when we had picked it up.
Thanks in advance and any help will be appreciated.
Simon
1st time poster on here
I have a problem for which i hope someone can help.
My mother is being pursued by a hire company regarding damage to a courtesy car given to her. The car wouldnt start one morning and they took it away. Months later they sent a bill of £4k!
The valves had impacted the pistons and the ECU codes apparently showed the car being driven over the limited speed. We find this very hard to believe, she is 50 years old and has been driving over 30 years.
My questions is, what other things could cause the valves to impact the pistons? Also, does i've been told by a family friend (not a mechanic though) that the ECU does not record when a car has been driven over a limited speed? Does anyone know how true this is?
The car was a vauxhall astra, over a year old with around 14000 on the clock when we had picked it up.
Thanks in advance and any help will be appreciated.
Simon
I don’t believe an ECU records speed but as has been said it may well record an over rev.
In order for a claim to progress you/yours should be asking for evidence to be presented demonstrating the damage and the data supporting their claim.
You/yours would then be asking where it is stated the max speed/revs that particular make/model should not exceed.
The first route is to deny the claim and ask for all the evidence.
Steve
In order for a claim to progress you/yours should be asking for evidence to be presented demonstrating the damage and the data supporting their claim.
You/yours would then be asking where it is stated the max speed/revs that particular make/model should not exceed.
The first route is to deny the claim and ask for all the evidence.
Steve
ks!Not sure whether this..
All said, I think that the OP has probably got it slightly wrong; the dealer wouldn't have complained about the vehicle speed but rather the engine speed, which can be recorded and can be a cause of valve impacting a piston.
Explanation for the benefit of "simonwalker":
Reason why an over-rev can cause damage is that the valves are pushed down using the cam shaft lobes, but pulled back up again using the springs. There's a limit to how fast the springs can pull the valves back up. If the engine revs too fast then the valve bounces off the cam lobe and stays down for longer than it should do, so it's still in the way when the piston comes back up. Have a look at this video for an illustration. Look carefully at the valve on the right side of the image, there's a point in the cycle where that valve only just closes before the piston comes up, so a bounce at that point would cause damage:
http://www.youtube.com/watch?v=MgklyL0wkVM
As for what would cause the over-rev in the first place, that's usually caused by shifting into too low a gear. Say you're doing 60mph in 4th gear and you're doing 3,000revs, then you need to slow down so you try to get 3rd gear but accidentally get 1st (as it's a hire car and you're not so familar with the gearbox layout). Well you lift the clutch and all of a sudden you're doing (about) 60mph in first gear which is LOTs of revs and will cause the over rev.
This is much more common than, say, 'ragging' a car and redlining each up change as you accelerate, as there's a rev-limiter which will cut the fuel (or spark) to prevent the engine accelerating to too high revs using the accelerator. But when you're coasting and lift the clutch in 1st gear, it's the wheels driving the engine round (not the sparks and fuel) so there's nothing the car can do to protect itself.
Unfortunately it is possible that damage was cause via a shift to a low gear at high speed which caused the mechanical over-rev, the springs can't then keep the valves under control and there's a valve/piston collision which results in expensive damage.
As others have said, make sure it's real, and that it will be fixed for a reasonable price, and that the damage was cause while you (whoever it was) was hiring the car, and not the week before. It's worth making sure for sure if it was you (or whoever) that was responsible. But just be aware that it IS possible to cause the damage described in this fashion, and it's not done by driving hard, it's just done by making an honest mistake when selecting gears.
Olivera said:
Many modern ECUs will record an over-rev along with a timestamp, so your friend isn't correct.
.. actually contradicts the OP which said..simonwalker said:
i've been told by a family friend (not a mechanic though) that the ECU does not record when a car has been driven over a limited speed? Does anyone know how true this is?
There's a difference between recording an over-rev and "a car being driver over a limited speed". The "limited speed" thing sounds like a 155mph speed limiter or something similar.All said, I think that the OP has probably got it slightly wrong; the dealer wouldn't have complained about the vehicle speed but rather the engine speed, which can be recorded and can be a cause of valve impacting a piston.
Explanation for the benefit of "simonwalker":
Reason why an over-rev can cause damage is that the valves are pushed down using the cam shaft lobes, but pulled back up again using the springs. There's a limit to how fast the springs can pull the valves back up. If the engine revs too fast then the valve bounces off the cam lobe and stays down for longer than it should do, so it's still in the way when the piston comes back up. Have a look at this video for an illustration. Look carefully at the valve on the right side of the image, there's a point in the cycle where that valve only just closes before the piston comes up, so a bounce at that point would cause damage:
http://www.youtube.com/watch?v=MgklyL0wkVM
As for what would cause the over-rev in the first place, that's usually caused by shifting into too low a gear. Say you're doing 60mph in 4th gear and you're doing 3,000revs, then you need to slow down so you try to get 3rd gear but accidentally get 1st (as it's a hire car and you're not so familar with the gearbox layout). Well you lift the clutch and all of a sudden you're doing (about) 60mph in first gear which is LOTs of revs and will cause the over rev.
This is much more common than, say, 'ragging' a car and redlining each up change as you accelerate, as there's a rev-limiter which will cut the fuel (or spark) to prevent the engine accelerating to too high revs using the accelerator. But when you're coasting and lift the clutch in 1st gear, it's the wheels driving the engine round (not the sparks and fuel) so there's nothing the car can do to protect itself.
Unfortunately it is possible that damage was cause via a shift to a low gear at high speed which caused the mechanical over-rev, the springs can't then keep the valves under control and there's a valve/piston collision which results in expensive damage.
As others have said, make sure it's real, and that it will be fixed for a reasonable price, and that the damage was cause while you (whoever it was) was hiring the car, and not the week before. It's worth making sure for sure if it was you (or whoever) that was responsible. But just be aware that it IS possible to cause the damage described in this fashion, and it's not done by driving hard, it's just done by making an honest mistake when selecting gears.
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