New water pump after 20K miles?

I'm sitting at the Toyota dealer at the moment having a regular service done. They just came by to tell me my water pump is leaking, at 60,300 miles on a 5/60. They are calling Toyota to see if they will assist in covering the repair.

This seems like a weakness in the Prius. The service guy said he's been working here for 5 months and has seen that as his top repair item on both Prius and Camry.

I have a 1997 Honda Civic with 80K miles, and last year sold a 10 year old Odyssey with 140K miles. Neither has had a water pump issue.

Update: Toyota is going to cover the water pump replacement. Nice of them. They are also going to keep the car overnight to do the gas petal safety recall and are providing me a free rental.

 

Good to know to watch for water pump leakage on this car. If there is pink coming out of the weep hole the seal is failing, and the bearings are next. I will get a new American made lifetime water pump from Kragen if mine goes. It is really all about the seal quality and modern water pumps have advanced in seal technology. On Fords I have never replaced one, but have on a 92 Honda Accord, which was a nightmare job as it is driven by the timing belt. At least on the Prius engine the wp is where it should be for servicing. It is hard to believe this simple little water pump on it's own little belt would ever fail.

 

One thing that can cause premature pump failure is an over-tensioned belt. My 2008 had one that I considered way too tight even for a normal belt system!

The Prius only has one belt used solely for it's water pump, and has no other engine driven accessories. Leaving this belt loose will still transmit plenty of energy while reducing radial load on the pump bearing. The Prius water pump is an insignificant load on the belt, which is designed to handle much greater loads that normal cars typically have. If you tried this in a normal car, you would experience belt slippage/squealing as the other accessories such as the Alternator, AC compressor, Power Steering pump, etc. try to pull many thousands of watts of mechanical energy from the belt. The Prius water pump is probably less than a few hundred watts at worst.

I recommend to loosen the belt to the point where you can manually just turn the impeller pulley by hand with a bit of a struggle. The belt at this point will deflect an inch without much effort from a single finger.

This adjustment also slightly reduces belt friction and heating which (very slightly) improves mileage and increases belt life.

I suspect with this technique, I will never have to replace the belt or the pump as long as I own the car. YMMV!

 

If the serpentine belt is loose enough so that you can move the coolant pump pulley by hand while the belt is stationary, then as the belt is moving, it would tend to slip more (not less) which would cause more (not less) heating of the belt. That would reduce (not increase) belt life.

I recall that the spec is to set the new belt tension at 0.4" when 22 lb of pressure is applied midway between the pulleys. That spec is pretty normal.

Although this belt carries less of a load, it also is much narrower and a bit thinner than a normal serpentine belt that powers the AC compressor, alternator, power steering pump, etc.

Anyway, the belt only costs $20 so this is not a big deal either way, as long as the belt doesn't come off the pulleys.

 

It is the seal that causes the failure of the bearings, as well as an over tightened belt, but mainly the seal. The Aisin pump is supposed to be about the best, but it is possible they let out some lesser quality batches, and we got them. If one has ever used machinery that runs in a water spray, it becomes quickly known how hard it is to spin a shaft IN water without the water getting into the bearings. They use labyrinths, ceramic discs etc but still it is a difficult problem to solve. I am going to probably stock an extra wp in the car, with the wrenches to change it on the road.

 

It is interesting to note looking at the G1 and G2 belt drive pictures that the pumps on the different generations run in opposite directions although they are the same engine and pump.

 

My belt is not loose enough to slip in normal use. It's tight enough to transmit the torque needed by the water pump, without loading the bearings. When I checked the tension, (factory tension as far as I can assume) it seemed way too tight. .4" with 22lbs is pretty tight for a belt spanning that distance! I get about an inch of deflection with the pressure of one finger on it. (hard to assess what 22 lbs is, but I doubt it's that much)

My "yank the pulley test" seems more accurate, as it's slipping but only when I really tug on the pulley, by my estimation that's probably at least 10 foot-pounds or so, which is over 4kw of energy @ 3000 RPM. Even allowing for gross over-estimation, there is no way that water pump needs that much energy.

Regardless, if it isn't excessive belt tension causing premature water pump failure (as I suspect it is), then are the pumps defective?

I am going to keep my low-tension setup. I'll let the forum know if/when I need to replace a pump or belt.

 

The seal fails usually, ruining the bearing with coolant. If there is coolant leaking, it is the seal. Most of the time that is what to look for as a sign of wp failure, coolant leaking. Nothing wrong with having the belt a little less tight than spec, except I have found on family members cars I am back in tightening more when they hear belts squealing later. The engine pulley moves faster and a whole lot stronger than a human hand. Put a 20 lb sack of potatoes on a finger and try to hold it up. The factory knows what the tension should be, whether the assemblers did it right is another story. They probably did it right with a tension gauge. The best way is to use a belt gauge IMO.

 

Obviously the engine is stronger than a human hand, but the pump doesn't need much energy, so this is irrelevant. The engineers probably spec'd the tension from the belt manufacturer's spec sheet, which in turn was derived from calculations to transmit the maximum torque for that belt/pulley design. At "stock" tension, That belt is likely sufficient to transmit over 7kw (~10hp) of energy with no problems, and the water pump is no where near that.

Yes, it's also obvious that the pump's failure mode is a blown seal. One of the more common failure reasons for a seal to go is excessive radial run-out in the bearing. Once the bearing wears to a point, the shaft tilts toward the many pounds of (factory) tension, and eventually the seal is not able to handle to gap and proceeds to wear fast. Once the seal begins leaking, it allows water into the bearing greatly compounding bearing failure.

I'm not saying Toyota is wrong for spec'ing the tension that high, but you have to realize, they are only human, and good engineering is simply learning from your mistakes. Obviously they learned well, for on the 2010 they finally got rid of the crappy belt drive once and for all. For the rest of us still in older cars, it may pay to question the engineering when pumps are dying at 20k miles. You don't have to heed my advice, but when I see people say "the factory knows best" or similar, that kind of thinking can be the downfall of good engineering. A good engineer must question everything, and not necessarily rely on "it's always been done that way", or what suppliers feed them.

Here's an interesting funny anecdote on this way of thinking: Monkey Lessons

 

I greatly prefer the belt G2 water pump system. The world was surely waiting for my opinion on that. I have to take the short way out on this, and say I disagree 100% with everything you said. I've heeded your advice, fifty years ago, being a looser belt is better'n too tight, armchair engineer guy in general.

 

Interesting. I just had a leaking water pump replaced on my 2006 Prius with just shy of 65K miles, and the dealer didn't offer to contact Toyota on my behalf. I may call the service advisor tomorrow and raise a fuss.