M54 Water Pump Failure

This is from my E53, but posting here for the benefit of pre-LCI E83 owners. Based on my online research, this is not a widespread problem, so I don't think one needs to draw negative conclusions about Saleri WP's or composite impellers. However, it's just something to consider as a possibility if you have a strange overheating problem. A sheared WP impeller just doesn't jump to to the top of the troubleshooting list and I've read reports of multiple professionals failing to figure this out. With that said,... for your viewing pleasure:



This WP only had 45K miles on it. The first symptom was inconsistent cabin heating, which I assumed to be a failing thermostat, so no urgency to fix. The second symptom was an upper radiator hose failure resulting in loss of coolant and brief overheating, so still no reason to suspect I had any other issues. Since I had to replace the upper hose, I decided it was a good time to replace that suspect thermostat and I put in a new ET for good measure. No more apparent cooling system issues for all the short trips I used it for (it's a backup vehicle, so doesn't see any long distance driving), except... the cabin heater still didn't work quite right which puzzled me.

I should have used Torque or similar to check actual coolant temps rather than relying on the heavily damped dash temp gauge, but again... I didn't suspect I had anything wrong. Out of nowhere, I got the ominous "ding" notifying me of high coolant temps. I had to abandon the vehicle a few miles from home to let it cool overnight. I was able to get it home the next morning just before overheating again. Thinking through all the symptoms, I suspected I had perhaps a blockage of some sort in the cooling system preventing flow. Turned out I was ALMOST right!

AM.