Water Pump

[ub7rm]

I believe the failure mode is that the impellers shear off the spindle, and there is a theory going about that the stress of detensioning and retensioning with a new belt is actually the prime culprit in causing this.  Thats why you should get a new pump with a new belt.

....That's what I was independently told and later confirmed by my friendly VW dealer.

Not sure about that.

I think the prime culprit is the hot and cold incessant cycle the plastic has to endure, which makes the plastic age harden and  brittle.  The Bearings would take the brunt of the De tensioning/Tensioning.

The impeller is on the shaft and unless the shaft has the capability to force the impeller to be stressed against the orifice it sits in, the the only forces the impeller is subject to is the rotational shear force the coolant can induce, and the afore mentioned H/C cycling.

Regarding the H/C cycle; there is some truth in what you say there, it contributes to the failure but is greatly accelerated by the new belt.  This isn't unique to VAG, its a common problem with most water pumps / engines.  The most common time for pump failure is shortly after the belt is changed. 

To be pedantic its the impeller that induces a force on the fluid, the fluid merely provides resistance.   

I suspect that the tensioing / retensioning causes the shaft and impeller to move slightly off centre which adversely changes the flow around the impeller, concentrating stresses where they are not designed to be and ultimately resulting in failure. 

So the impeller is rotating as you as fit the belt?  It doesn't matter where the impeller sits within the void within the engine block, unless it touches the block, nothing will affect it.  Unless the seal is gapped, then you will get a leak, the pump will still be rotating.  The impeller will still be doing its job.

I have changed 2 VW water pumps recently, and both broke up and crumbled in my hands, the plastic has become very brittle due to H/C cycles, nothing to do with differing flows around it.

you are right, the fluid will induce a resistance to rotation, this is a shear force trying to twist the impeller off,  but I think we are talking about the same thing.  Tomatoes and red things!

in the pic below you can see the cracks and how aged, brittle and crumbly the plastic has become.

I'm not saying the pump is spinning when the belt is changed.  And its clear from that picture that H/C cycling is the cause!  But I believe the problem usually manifests itself after a belt change so the tensioing / detensioning has some effect - maybe the movement of the shaft is the final nail in the coffin.

I agree that the impeller will continue to operate if its off its axis slightly ain the way I'm suggesting, but this will definately change the stress distribution in the part and change the turbulence around the part.  I'm talking about the kind of change you would see if you did a CFD analysis - not something that will be immediately obvious...

Is that a VW part in the picture?

[TagnuT]

I believe the failure mode is that the impellers shear off the spindle, and there is a theory going about that the stress of detensioning and retensioning with a new belt is actually the prime culprit in causing this.  Thats why you should get a new pump with a new belt.

....That's what I was independently told and later confirmed by my friendly VW dealer.

Not sure about that.

I think the prime culprit is the hot and cold incessant cycle the plastic has to endure, which makes the plastic age harden and  brittle.  The Bearings would take the brunt of the De tensioning/Tensioning.

The impeller is on the shaft and unless the shaft has the capability to force the impeller to be stressed against the orifice it sits in, the the only forces the impeller is subject to is the rotational shear force the coolant can induce, and the afore mentioned H/C cycling.

Regarding the H/C cycle; there is some truth in what you say there, it contributes to the failure but is greatly accelerated by the new belt.  This isn't unique to VAG, its a common problem with most water pumps / engines.  The most common time for pump failure is shortly after the belt is changed. 

To be pedantic its the impeller that induces a force on the fluid, the fluid merely provides resistance.   

I suspect that the tensioing / retensioning causes the shaft and impeller to move slightly off centre which adversely changes the flow around the impeller, concentrating stresses where they are not designed to be and ultimately resulting in failure. 

So the impeller is rotating as you as fit the belt?  It doesn't matter where the impeller sits within the void within the engine block, unless it touches the block, nothing will affect it.  Unless the seal is gapped, then you will get a leak, the pump will still be rotating.  The impeller will still be doing its job.

I have changed 2 VW water pumps recently, and both broke up and crumbled in my hands, the plastic has become very brittle due to H/C cycles, nothing to do with differing flows around it.

you are right, the fluid will induce a resistance to rotation, this is a shear force trying to twist the impeller off,  but I think we are talking about the same thing.  Tomatoes and red things!

in the pic below you can see the cracks and how aged, brittle and crumbly the plastic has become.

Didn't the same thing happen on the MKIV? Wasn't the mod to change the water pump to one with metal impeller blades?

[QD MBE]

I believe the failure mode is that the impellers shear off the spindle, and there is a theory going about that the stress of detensioning and retensioning with a new belt is actually the prime culprit in causing this.  Thats why you should get a new pump with a new belt.

....That's what I was independently told and later confirmed by my friendly VW dealer.

Not sure about that.

I think the prime culprit is the hot and cold incessant cycle the plastic has to endure, which makes the plastic age harden and  brittle.  The Bearings would take the brunt of the De tensioning/Tensioning.

The impeller is on the shaft and unless the shaft has the capability to force the impeller to be stressed against the orifice it sits in, the the only forces the impeller is subject to is the rotational shear force the coolant can induce, and the afore mentioned H/C cycling.

Regarding the H/C cycle; there is some truth in what you say there, it contributes to the failure but is greatly accelerated by the new belt.  This isn't unique to VAG, its a common problem with most water pumps / engines.  The most common time for pump failure is shortly after the belt is changed. 

To be pedantic its the impeller that induces a force on the fluid, the fluid merely provides resistance.   

I suspect that the tensioing / retensioning causes the shaft and impeller to move slightly off centre which adversely changes the flow around the impeller, concentrating stresses where they are not designed to be and ultimately resulting in failure. 

So the impeller is rotating as you as fit the belt?  It doesn't matter where the impeller sits within the void within the engine block, unless it touches the block, nothing will affect it.  Unless the seal is gapped, then you will get a leak, the pump will still be rotating.  The impeller will still be doing its job.

I have changed 2 VW water pumps recently, and both broke up and crumbled in my hands, the plastic has become very brittle due to H/C cycles, nothing to do with differing flows around it.

you are right, the fluid will induce a resistance to rotation, this is a shear force trying to twist the impeller off,  but I think we are talking about the same thing.  Tomatoes and red things!

in the pic below you can see the cracks and how aged, brittle and crumbly the plastic has become.

Didn't the same thing happen on the MKIV? Wasn't the mod to change the water pump to one with metal impeller blades?

Yes it did.

1.8T pump.





Both changed at the same time as the cambelt, yet found in the condition above.  More threads.................

http://uk-mkivs.net/photos/chrismanc/picture2056761.aspx

http://uk-mkivs.net/forums/p/106809/731603.aspx#731603

http://uk-mkivs.net/forums/p/27510/240333.aspx#240333

All the above cars had pumps changed at the same time as their first timing belt, and all pump impellers were in very bad condition.  All the cars were on their first belt change, and mileage was between 20 and 50 (ish)K.  IMO noithing to do with a new belt, tensioning and de-tensioning as none of them had ever been given one.

Just my opinion, and happy to leave it there.

Cheers
 

[Tarmac_Terrorist]

Interesting debate guys. At 7k miles, I'm still not conviced that I need a new pump. It's unlikely my pump will have suffered too much from the H/C cycle yet. I really can't see what a new pump would offer. I think if I had done 20k miles then I would go for it.

Still time to decide 

[RedRobin]

Interesting debate guys. At 7k miles, I'm still not conviced that I need a new pump. It's unlikely my pump will have suffered too much from the H/C cycle yet. I really can't see what a new pump would offer. I think if I had done 20k miles then I would go for it.

Still time to decide 

....Putting it into that context, I can see how changing the water pump might be unecessary. Pretty unusual to have only done 7k miles in the timeframe though.

[candy turbo]

cambelt at 4 years regardless of mileage , if you dont do the pump you have the same costs as cam belt all over again when it does !

[QD MBE]

Irrespective of how many miles the pump has done, when you get the belt done, do the pump.  As CT alludes to, you will have to take the belt off again, and incur the same costs all over again.  Pump is small money compared to paying it all out gain.

[RedRobin]

Irrespective of how many miles the pump has done, when you get the belt done, do the pump.  As CT alludes to, you will have to take the belt off again, and incur the same costs all over again.  Pump is small money compared to paying it all out gain.

....Good point 

[Tarmac_Terrorist]

Yeah, I guess it's if my pump which has done 7k miles will do say another 8k - 10k miles over the next four years. I think VW guarantee new parts for 2 years right? Might get it done at the same time - the car is 4years old in May so will consider.

Thanks for the help / advice guys 

[Robert74]

My car is 4 years old in April and has done only 25k. My local dealer said the cam. belt needs to done because of the age but did not mention the water pump. The local Seat garage and a couple of VW/Audi specialist said the same but also said they recommend the water pump be changed at the same time. I am getting the cam. belt and pump done next week for £300 at AMD.

[Tarmac_Terrorist]

Well, I decided to get the water pump changed as well as the cambelt & tensioner. The car is now 4 years old and just covered 7,338 miles from new. Nearly run in eh?!

I signed up to a 2 year servicing package (time & distance) a couple of months ago for £299. This also includes 2 MOT's too.

The work undertaken yesterday was:

Year 4 Time & Distance service (major service)
Brake fluid change
MOT
Cambelt, tensioner and water pump.

Total bill to me - £385 (basically just the belt & pump costs)

I supplied my own Castrol Longlife oil as my local dealer still uses the semi-synthetic crap for Tiime & Distance.

I only managed to drive her for 1130 miles in the last year  Must try harder 

[RedRobin]

I only managed to drive her for 1130 miles in the last year   Must try harder  

....I just drove 1200 miles in 4 days last week.





Changing the water pump when cambelt service is highly recommended  

Hope to see you at Castle Combe sometime this summer  

[Poverty]

can water pumps be intermittent in operation?

[bacillus]

can water pumps be intermittent in operation?

Hmm, the impeller is either spinning or not when the engine is running so no, the water pump shouldn't be intermittent in operation.

[RedRobin]

can water pumps be intermittent in operation?

Hmm, the impeller is either spinning or not when the engine is running so no, the water pump shouldn't be intermittent in operation.

....But there is a good reason which I've forgotten, for changing the water pump whenever the cambelt is changed. Perhaps Poverty was indirectly asking about that.