Considered this aspect.

Problem is that the plastic parts in general are much bigger than the metal parts. OK, thin metal parts could affect the bearings of a new water pump. Big plastic parts can block thin water lines. And as we all know there is not only clear fluid in this lines. Over the years we get some corrosion and other dirt - even it's an aluminium block (and there is also aluminium corrosion).

Still on the metal impeller side. ;)

Regards, Markus

You are overlooking the fact that even very small metal bits can agglomerate into passage blocking as well. We have seen it happen. And the case damage from the pump hitting is also permanent, which reduces the overall coolant flow in even completely rebuilt engines, so you have a fresh engine starting off with one hand tied behind its back.

Hi,

understand. But there are still some downsides on plastic impellers. And if you check the pump and play regularly i see more downsides for the plastic impeller, because you can't see inside if the impeller is deoriantating - which doesn't mean that the bearing has to be gone bad.

For a normal guy who doesn't check anything on his car or isn't willing to replace the pump preventively every 10 years or 100 TKm a plastic impeller pump will be the better choice.

For those who are interested in a metal impeller pump. GEBA is an after market manufacturer who is / was also OEM fpr Porsche. They offer a water pump with a metal molding impeller (not a cheap sheet metal impeller) for 986 / 996 cars. Part number is GEBA 15007. This pump is Made in Germany (not Turkey, Czech, Poland or elswhere in Europe) and uses high quality bearings from a well known german manufacturer.

Regards
Markus

If you are dealing with a replacement of a water pump that looks like post #21 above - you need to find every broken part. Here is a little practical experience to go on but others may have more to contribute.
Remove the heat exchanger/oil cooler. Examine the coolant lines and passages there.Some pieces will lodge there.
My M96 engine had a new water pump fitted just before I bought it. When I dismantled the engine(another story) I eventually retrieved all the broken pieces from the previous pump(engine has 89,000 miles). I would guess that coolant circulation was reduced by at least 50% with the broken pieces.
If you can't find all the broken pieces from the old impeller -you are taking a big risk if you just slap in a new pump - regardless of the impeller material.

Great point.

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Oh Gawd, this topic is starting to get as bad as the IMS debate.

Other than mileage, what technique can I use to determine the health of a water pump? My temps are fine. I don't hear a noise from the engine. I have 56k miles.

Is this purely proactive maintenance or can you determine when to replace?

I think it is a proactive maintenance item. If your car has its original water pump and antifreeze, I would change the pump and the antifreeze. The antifreeze is supposed to last forever but 13-14+ years is a long time. And the composite impellers eventually may start throwing off chunks of blade due to failure from repeated heating and cooling cycles.

Never ever put a metal impeller pump in an engine block no matter what engine it is. If that bearing fails then you have thousand of metal bits now making your way round your coolant system as well as reshaped ports around the pump.

As for when to replace a good rule of thumb is every 40000 miles or 4 years. That's what VW recommends on their pumps and i reckon that's not a bad shout.

I've done a lot of research the last few days on this as I'm in the middle of replacing mine.. And apparently it's a subjective opinion really. Pros and cons of both. Metal impeller versions are less likely to fail but if they do you'll chew up ur block. And plastic impeller pumps can break and block water ways causing hot-spots. I opted for a composite one

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Well, I just replaced mine because it started leaking (appeared to be the original, and impellers looked really gnarly). But if you make it a proactive maintenance every couple years, you probably don't have to worry much about this debate.

I always recommend and use factory (Pierburg) plastic impeller pumps and replace every 10 years or 60K miles. Jeff (as usual) is correct on the why. I only add, don't wait for it to go bad, just put on a schedule and replace proactively.