I thought I had read or heard that the plastic impeller can fail just because of heat cycles, even if the water pump bearing is fine. Flat 6 Innovations says this on their website -
"Replace the water pump, water pumps can fail silently and lead to cracked heads! Replace the water pump no matter how many miles are on your car, as time in service kills the plastic impeller blades of the pump, not miles!! Water pumps are cheap insurance, if a water pump creates a cracked head and intermixed coolant and oil exist the costs can be over 15,000.00 for repair"
So just because it looked great when it came out doesn't mean it had a lot of life left in it - you've re-set the clock on time in service and have that peace of mind.
What I thought that I had read, or heard, was that the thermal heat cycles eventually create cracks in the plastic impeller, and then bits of the impeller break off and wander through the cooling system and then can get caught in small places in the cylinder heads leading to hot spots. And that this could happen even if the water pump bearing was not failing.
When I had my water pump replaced proactively about 2 years ago at around 42,000 miles, on my 2003 S that I bought new, my independent shop thought it was a good idea to go with a metal impeller water pump. I'm now at over 55,000 miles and so far so good. Since I had my water pump replaced, I've found this forum and the general consensus seems to be stick with OEM plastic impeller water pumps. I will keep an eye on the bearing and proactively replace it again down the road. I'll revisit the plastic vs metal impeller again then, but for this time I felt the risk of plastic outweighed the risk of metal.
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