Broken crank

George Hogg

George Hogg

A2OC Donor
A friend has a bluemotion polo and the crank has broken at 152k miles. He habitually drives in the highest possible gear at 1200-1400rpm in 4th gear. Claims it’s best for mileage, but that’s not the case in my experience though.
It always felt like the engine was really unhappy and rough.
Another friend, a diesel specialist told him he was courting trouble.
Any thoughts/experience of low revs under load causing crank problems?

George
 
I thought that was always considered an absolute no-no - the ignition impulse of a tdi is such that getting below "happy" revs is presumably putting violent torque impulses from one cylinder into the crank at exactly the time that another cylinder is on the compression cycle with maximum resistance. Isn't this exacerbated by the fact that the three cylinder needs the balancer shaft to try to even some of those impulses out?
 
Minimum I find in 4th with the jdd in mine is 1800 rpm having changed from 3rd gear no less than 45 mph unless your then going slightly down hill 40 mph gear change will work okish

Correction
45 mph is around 1600 rpm not 1800 sorry
I love the JDD but it's clear to see it has to be used with caution! Can't wait to get the MZN in Mr Black to compare them (not sure when that will be thou lol)
 
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George Hogg said:
A friend has a bluemotion polo and the crank has broken at 152k miles. He habitually drives in the highest possible gear at 1200-1400rpm in 4th gear. Claims it’s best for mileage, but that’s not the case in my experience though.
It always felt like the engine was really unhappy and rough.
Another friend, a diesel specialist told him he was courting trouble.
Any thoughts/experience of low revs under load causing crank problems?

George
Click to expand...
A diesel is a diesel 😳
Having spent 3k on emissions for one van before finally getting it right and a new turbo in the other van…. to which that van now has emission problems… think dpf etc.
Wife is now banned from using top gear in any of them…. and only third gear around town - why? She has been driving the vans just like your friend. Have instructed her to drive like a F1 driver everywhere to stop the darned dpf’s from going ballistic 😂
Forgot to add, my wife is main driver of the vans😹
Of course, she has her own A2 as well😍
 
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Robin_Cox said:
I thought that was always considered an absolute no-no - the ignition impulse of a tdi is such that getting below "happy" revs is presumably putting violent torque impulses from one cylinder into the crank at exactly the time that another cylinder is on the compression cycle with maximum resistance. Isn't this exacerbated by the fact that the three cylinder needs the balancer shaft to try to even some of those impulses out?
Click to expand...
Worded better than I would Robin but that’s exactly it.

G
 
I don’t have the science to support this but I had this discussion with my son preparing for the L test, low revs = low fuel consumption?
Not at all I explained, an engine has a sweet spot, it is around peak torque with peak fuel efficiency just before peak torque. So for a diesel (AMF) peak efficiency is around 2000 rpm with peak torque around 2200 rpm. By comparison a VW petrol (PB) produces peak torque at 4000 rpm so peak fuel efficiency is over 3000 rpm.
What influences fuel efficiency is how you get there. Start at low revs is probably best for efficiency but a petrol will labour and a diesel feel like it is being hit with a hammer. Not good for the engine. Best for the engine, but not fuel efficiency is to spin the engine above peak torque and change up to achieve efficient cruising speeds.
So back to A2, AMF as I say no science to support this:
Sweet spot 2000 to 2200rpm
Spins happily to around 3200rpm
Feels like a bag of spanners less than 1700rpm
Aim for
30mph in 3rd
40mph in 4th
50mph and above 5th
80mph + try not to go there
Finally treat it gently until it is warmed up.
All of my engine have gone to mega high milages over the years. The exception will be my AMF but that is because I was supplied 6v glow plugs. Despite extensive combustion chamber damage it still runs efficiently and cleanly.
 
philward said:
I don’t have the science to support this but I had this discussion with my son preparing for the L test, low revs = low fuel consumption?
Not at all I explained, an engine has a sweet spot, it is around peak torque with peak fuel efficiency just before peak torque. So for a diesel (AMF) peak efficiency is around 2000 rpm with peak torque around 2200 rpm. By comparison a VW petrol (PB) produces peak torque at 4000 rpm so peak fuel efficiency is over 3000 rpm.
What influences fuel efficiency is how you get there. Start at low revs is probably best for efficiency but a petrol will labour and a diesel feel like it is being hit with a hammer. Not good for the engine. Best for the engine, but not fuel efficiency is to spin the engine above peak torque and change up to achieve efficient cruising speeds.
So back to A2, AMF as I say no science to support this:
Sweet spot 2000 to 2200rpm
Spins happily to around 3200rpm
Feels like a bag of spanners less than 1700rpm
Aim for
30mph in 3rd
40mph in 4th
50mph and above 5th
80mph + try not to go there
Finally treat it gently until it is warmed up.
All of my engine have gone to mega high milages over the years. The exception will be my AMF but that is because I was supplied 6v glow plugs. Despite extensive combustion chamber damage it still runs efficiently and cleanly.
Click to expand...

I totally agree, they needed to be treated with care to last
 
JavaGreen said:
A diesel is a diesel 😳
Having spent 3k on emissions for one van before finally getting it right and a new turbo in the other van…. to which that van now has emission problems… think dpf etc.
Wife is now banned from using top gear in any of them…. and only third gear around town - why? She has been driving the vans just like your friend. Have instructed her to drive like a F1 driver everywhere to stop the darned dpf’s from going ballistic 😂
Forgot to add, my wife is main driver of the vans😹
Of course, she has her own A2 as well😍
Click to expand...
Good luck with that, I’d be hiding keys…lol
 
George Hogg said:
I totally agree, they needed to be treated with care to last
Click to expand...
I would not say. There are engines that need to be treated with care, e.g. VW R5 TDI PD or 2.5 TD from VM motori. These engines (cast PDs, like AMF) need to be treated with respect same as almost every other engine. If you are deaf and run it in stupidly low RPMs with load, then you get what you ask for...
 
BigBang said:
I would not say. There are engines that need to be treated with care, e.g. VW R5 TDI PD or 2.5 TD from VM motori. These engines (cast PDs, like AMF) need to be treated with respect same as almost every other engine. If you are deaf and run it in stupidly low RPMs with load, then you get what you ask for...
Click to expand...
I had a R5 td until recently what a piece of work that is! Silky smooth ( ihad a good loved engine!) sold it at 170K and was still like new. A true marvel of engineering. THats had a high rev sweet spot, felt unatural to the ear but natural to the foot.
 
BigBang said:
I would not say. There are engines that need to be treated with care, e.g. VW R5 TDI PD or 2.5 TD from VM motori. These engines (cast PDs, like AMF) need to be treated with respect same as almost every other engine. If you are deaf and run it in stupidly low RPMs with load, then you get what you ask for...
Click to expand...

We totally agree, I mean servicing and sympathetic driving. Not kid gloves

George
 
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