It sounds reasonable to do the water pump every time but still there's the question of why the factory isn't on board. Is it because they know that water pumps outlast timing belts by a factor or 3 or 4 or 5 as apparently with the million mile Tundra.
I'm sure the 'expectation' is that a water pump (usually fairly reliable) will have a greater service life than the Timing Belt which is subject to different stresses and made from material that literally has a shelf life. But I wouldn't hold the 'million mile Tundra' up as an example. We all know that is quite the anomaly and NOT the expected design life of either the engine or its external components.
It makes sense to replace it to avoid having to "go in again" before the next timing belt service. But does it make sense to replace 4 or 5 water pumps unnecessarily and prematurely to avoid having to "go in again"once?
The trouble I have with this argument is that it assumes the Water Pump is designed and expected to last 400K-500K miles (if belts are changed every 100K). But I concede IF timing belts are changed at 90K intervals...then also replacing the Water Pump (from a perspective of 'need' VS. 'prevention' due to the possibility of early failure) is likely an imbalance. I don't intend to follow those intervals.
Personally, I will be replacing all components (TB, Water Pump, Pulleys, Tensioner, checking cam seals, etc) at 150K mile intervals, but that is due the the environment in which I live and the relatively light service asked of my vehicle.
IF the Water Pump were right out in the Bald Ass Open as with my 80 Series ...then yes...I'd leave it for sure.
But it isn't. Nor is the Starter (up under the intake manifold). To me....it just makes sense to NOT wait until they fail to replace/repair IF you are already in there for other service AND the part has appreciable use/wear already.
Water pump failures tend to be very benign events - a little noise, a little seepage, no harm to anything.
Agreed. Outright failures (no warning or symptoms) would be a rare event. Typically Water Pump failures manifest as a leak. However...owing to the location of the water pump and the several points at which it 'could' leak...it might allow coolant to drip down on the timing belt or infiltrate bearings that are no longer well sealed.
You won't see 'leakage' until it has made its way from the Water Pump all the way down the front of the engine and out the weep hole in the cover just behind the Balancer. Same thing with engine oil leaking from Cam Seals.
It's understandable that dealers like it but I'm still wondering why Toyota isn't on board?
Couldn't say. But admittedly I am confounded by some of the things 'Toyota' does and completely in agreement with others.
Thank You for your thoughts on the subject. It gives us all something to chew on with respect to vehicle maintenance.