Originally Posted by Efthreeoh
Not to throw cold water on this whole statistics lesson, but I think the better data to make a statistical analysis from is time (rather than mileage) and type of use (i.e. percent split between city and hwy driving). The water pump fails due to the heat load it sees based on the environment it operates in. A water pump in car that sees more stop and go driving in Dallas, Texas will fail at a different mileage than a water pump in a car that sees no heavy traffic in Anchorage, Alaska. Tying failure rates to mileage, while you can make a statistical evaluation, is not a true measurement of predictability when a pump will fail as a result of mileage.
Also, throwing in N52s into the data set is going to skew the results because the water pumps for each engine are different parts. You should not be trying to correlate failure rates for two different models of water pumps.
Considering the high cost of replacement, it would be a disservice to members of this Forum to have them use this thread to decide when a preemptive replacement of the water pump should be made (i.e at 70,000 miles). Being that the cost for replacement is anywhere between $800 - $1,200 (non-DIY), it is probably better for an owner to wait until the pump fails and have roadside assistance insurance to handle the tow bill. A much better investment would be for an owner to buy a BMW-reading scan tool and periodically scan the car for the shadow codes that indicate the pump is starting to fail. Having been through this whole ordeal, this is just my opinion.
N54 water pumps and N52 water pumps are identical. They're the same in the N55, 335iS and 1M as well. Only the M3 and 335d pumps are different in the 3 series.
How do you suggest calculating the number of hours run by each pump? As you said in a heat-sink situation mileage isn't the best data point but it is the only usable one really. From there you could split it into highway mileage or city mileage to account for the load times, but... too may people in here with cars they bought used and not knowing whether it was highway or city driven. Too many people who've had their cars since new and have driven a good mix of highway and city. Too many whose cars are FBO and are run harder, hotter, longer than a stock car. Too many people who let their cars idle for 30 minutes every morning under which circumstance lots of heat-sink and no mileage occurs.
Mileage is the only data point which is going to be standard across all of the pumps. Amount of heat sink is going to be different for every single car.