How to tell if the water pump is bad?
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  1. #1
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    How to tell if the water pump is bad?

    I was having over heating problem that is similar to some of those described here, that many suggested to be water pump problem. My question is whether there is a more direct way to tell if the water pump is bad? I think it would be a good idea to be more certain that the water pump goes bad before take all the pains to replace it.

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  3. #2
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    The only definitive test is to take it out and look at it - same as for the thermostat. Not what you wanted to hear, but everything else is just inference.

  4. #3
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    If the car overheats on the highway but not on low speed driving, 99% chance the water pump is bad (plastic impeller slipping on shaft). If the water pump has over 70K, 50% chance it is leaking. If the coolant is leaking down from the lower pulley onto the oil pan, 100% chance the water pump is bad.

    The only other way to be more certain is to remove the lower timing belt cover and inspect the pump for leaks. To see if the impeller is slipping, you need to remove it and put it in boiling water and then see if the impeller can be turned while the pulley is still.

    Either way, that is 40-50% of a timing belt change.

  5. #4
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    If the WP is totally shot, but the thermostat is good:
    The cold engine starts and warms up normally, but the lower rad hose doesn't get hot for a very long time. Reason: the only fluid circulation is by convection.

    You will not sleep well until you know, so go ahead and change it out ! Sorry, that is so easy to type and so much harder to do :o(

  6. #5
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    Thanks for all these suggestions! Helped me to make up my mind to replace it anyway. Especially from PZ's suggestion, it seems indeed that my water pump is failing.

    The first overheating occurred on the highway on a hot day, preceded by intermittent stopping of cool air from AC. After that, the overheating usually shows up after a few miles drive, especially after an acceleration. There seems to be no leaking of coolant. No clear drop of the level in the tank.
    But the wired thing is that the water pump is a new one with metal impeller. I replaced it together with my TB change early last year, when it was at ~76000 miles. Unless it was a defective one. I bought the kit from mjmautohaus . I didn't replace the thermostat though, because I didn't know how on my AUG engine.

    Anyway I think I can't escape doing all the pains again.

  7. #6
    PZ
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    That info would be good to know beforehand.

    Defective metal water pumps are very rare. You may have an air pocket in the coolant. I would try bleeding the system via the heater hoses. I would also change the thermosat.

    If that does not work, I would run a compression and leak down test on the engine to rule out a bad head gasket. Only then would I replace the water pump.

  8. #7
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    I don't know, it sounds like a thermostat to me, especially since you said that was the one part you didn't replace. The first time you overheated on the highway, were you cruising at 70 mph, or stuck in traffic?

  9. #8
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    hmm... so the water pump is less likely to be bad...
    I feel that it's not very likely to be a big bubble in the coolant. Because I flushed the coolant following the TB change over 1 year ago. There was a big bubble back then, and I think I got it out and topped off the tank. Everything had been fine in the past year.

    Doesn't changing thermostat require similar steps or amount of work as change water pump?

    Quote Originally Posted by PZ View Post
    That info would be good to know beforehand.

    Defective metal water pumps are very rare. You may have an air pocket in the coolant. I would try bleeding the system via the heater hoses. I would also change the thermosat.

    If that does not work, I would run a compression and leak down test on the engine to rule out a bad head gasket. Only then would I replace the water pump.

  10. #9
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    The first time overheating was when driving in a highway maybe 50~60 mph, but not when idling.
    Later it usually started overheating after several miles highway drive. I had to pull over and wait for a few minutes. Then it came back much sooner, and usually after an acceleration.

    Would all these seem to be consistent with a bad thermostat?


    Quote Originally Posted by VT Variant View Post
    I don't know, it sounds like a thermostat to me, especially since you said that was the one part you didn't replace. The first time you overheated on the highway, were you cruising at 70 mph, or stuck in traffic?

  11. #10
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    Quote Originally Posted by ning View Post
    The first time overheating was when driving in a highway maybe 50~60 mph, but not when idling.
    Later it usually started overheating after several miles highway drive. I had to pull over and wait for a few minutes. Then it came back much sooner, and usually after an acceleration.

    Would all these seem to be consistent with a bad thermostat?
    It's consistent with a bad water pump. It's very rare (by design) for a thermostat to fail closed; normally, they fail open. If it DID fail closed, the car would overheat under all circumstances.

    Hold in mind that at highway speed, your heat load is somewhat greater but your cooling airflow and water flow are significantly higher. This is why when my thermostat failed open, it reached operating temp around town but cooled off on the highway.

    On the other hand, when my water pump failed, it exhibited the exact same symptoms as yours - overheated on the highway, OK around town. This was because the impeller was loose on the shaft. At low RPM, the impeller moved with the shaft. On the highway, the increased water pressure made it slip, and cooling lagged. All clear?

  12. #11
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    Thanks! This is a really clear explanation and great suggestions.
    I think I'll go ahead and order a new water pump, and find a good time to go over the pains again ...

    Quote Originally Posted by scotts13 View Post
    It's consistent with a bad water pump. It's very rare (by design) for a thermostat to fail closed; normally, they fail open. If it DID fail closed, the car would overheat under all circumstances.

    Hold in mind that at highway speed, your heat load is somewhat greater but your cooling airflow and water flow are significantly higher. This is why when my thermostat failed open, it reached operating temp around town but cooled off on the highway.

    On the other hand, when my water pump failed, it exhibited the exact same symptoms as yours - overheated on the highway, OK around town. This was because the impeller was loose on the shaft. At low RPM, the impeller moved with the shaft. On the highway, the increased water pressure made it slip, and cooling lagged. All clear?

  13. #12
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    You mentioned that the WP might be slipping? Then it should not be a broken impeller. Would a metal impeller ever more likely to slip, due to more mass on it?

    Quote Originally Posted by scotts13 View Post
    It's consistent with a bad water pump. It's very rare (by design) for a thermostat to fail closed; normally, they fail open. If it DID fail closed, the car would overheat under all circumstances.

    Hold in mind that at highway speed, your heat load is somewhat greater but your cooling airflow and water flow are significantly higher. This is why when my thermostat failed open, it reached operating temp around town but cooled off on the highway.

    On the other hand, when my water pump failed, it exhibited the exact same symptoms as yours - overheated on the highway, OK around town. This was because the impeller was loose on the shaft. At low RPM, the impeller moved with the shaft. On the highway, the increased water pressure made it slip, and cooling lagged. All clear?

  14. #13
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    Quote Originally Posted by ning View Post
    You mentioned that the WP might be slipping? Then it should not be a broken impeller. Would a metal impeller ever more likely to slip, due to more mass on it?
    That, I can't answer, as I've never examined a VW water pump with a metal impeller. My gut feeling is it would be less likely to slip, as it's presumably a metal impeller pressed onto a metal shaft. It shouldn't crumble and become loose as the older plastic ones did.

    Frankly, your water pump should not have failed. If you had an AEB or ATW I'd say the belt might be slipping, but not on an AUG. On some older cars, a deteriorated hose might be collapsing under the draw of the water pump, but I'm told that can't happen on these cars. An air bubble usually blocks flow through the heater core, not the engine. There really isn't a good diagnosis - I'll be interested in hearing what you find when you get it apart.

  15. #14
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    Btw, with regards to your earlier question of how hard it is to change the thermostat, it is VERY easy! Takes about 20 minutes. Remove the throttle body for easier access, the t-stat housing is at the end of the coolant hose. Dust off your Allen wrenches
    when my wp went, I tried changing the t-stat first just to confirm.

  16. #15
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    That simple to replace t-stat on an AUG?

  17. #16
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    On AUG? Not sure.... I can only speak for AWM's

  18. #17
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    Yeah, AUG.... Everything seems to be less straight forward on it.
    But it's still good news that thermostat is could be easier. I ordered both water pump and thermostat from ecstuning. I'll definitely try TS first before taking everything apart...

  19. #18
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    Quote Originally Posted by Jordon View Post
    On AUG? Not sure.... I can only speak for AWM's
    AUG and AWM are the same block.

  20. #19
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    Thanks! That's encouraging!

    Quote Originally Posted by akhristov View Post
    AUG and AWM are the same block.

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