Anatomy of Water Pump Failure

Since my Bardstown, KY brother made a hobby of fixing up Gen 3 Priuses he's accumulated a bucket of dead water pumps (engine-water pump, not inverter pump) removed from vehicles showing P261 error codes.

I analyzed a couple of these bad pumps and noticed a common failure mode. The impeller is a mostly plastic device filled with permanent magnets. The impeller rides on a fixed steel spindle without the benefit of bearings. The interface between the plastic impeller and the steel spindle is the bearing. Antifreeze serves as lubricant.

After 100k+ miles the impeller side of the bearing begins to wear. Plastic shards accumulate in a necked-down portion of the spindle shaft. The run-out of the impeller increases to the point where it begins to rub the sidewalls of the cavity it sits in, loading down the motor. Eventually the engine computer detects the loading.

There is a 1mm wide water channel molded into the impeller at the interface with the steel spindle. This water channel gets packed with plastic wear shards on failed units.

FWIW, you can purchase for less than $25 a new impeller to replace worn ones. These impellers are sold in "water pump rebuild kits".

However, using one of these rebuild kits requires that you pull the impeller from the pump fairly early in its wear cycle. By the time impeller wear begins to drag on the pump's RPM the plastic shards in the necked portion of the spindle prevent the impeller from being pulled from the pump.

At the point where the impeller is heavily worn and "draggy" you can (usually) spin it with your fingers but not pull it from the pump. Plastic shards are acting as an impeller retaining clip on the shoulder of the necked spindle.

Intuition tells you that corrosion on the steel spindle would play some role in the demise of the bearing surface but I see no corrosion in these failed units, only accumulated plastic shards.

 

BTW, it doesn't take much wear-caused run-out of the impeller to allow it to rub the sidewalls. Aisin has the air (water actually) gap between the impeller and stationary cavity sidewall down to a gnat's fingernail, probably 0.3mm or less.

I'll bet the narrow gap around the spinning impeller is to encourage laminar flow of antifreeze in the gap to lessen the drag. And they're also trying to get the stator and rotor magnets as close to each other as they can to improve motor efficiency.

 

There are several on Amazon in the under $30 category. He and I are separately about 5k miles deep into those with good luck so far.

The Amazon vendor "Dasbecan" offers an impeller whose design differs somewhat from Aisin's but it seems to work.

If you assume that Aisin's huge engineering staff and well-equipped labs generated the best impeller design with the highest efficiency and reliability, any knock-off is likely to be worse. But I suspect that there's huge safety margin in the overall pump design that allows lesser but non-infringing impeller designs to do the job.

The riskiest park of any knock off design would seem to be the choice of molding resin. The steel/plastic interface of the water bearing is the Achilles heel, together with the extremely tight tolerances at the perimeter of the magnet housing.

I have no mech training so if I were an enterprising Asian parts manufacturer tinkering with Aisin's design I might be tempted to widen the gap between the magnet housing and the cavity it sits in to permit a bit of wobble. Aisin's design tolerates near-zero run out at the magnet's perimeter--maybe 0.2mm.

But I suspect widening that gap increases drag on the motor if it encourages vortices to form in the water.

 

Tombutkt2, I spent most of my career gnawing on the perimeter of markets dominated by large established companies. Everybody gotta eat. You use your cleverness to make a cheaper product that gets the customer's job done.

I can easily afford a new $200 Aisin water pump but I get a hobbyist's thrill out of driving a vehicle held together (barely) with aftermarket parts made by little shops around the world.

I smile and wave (with one finger) every time I pass the Indianapolis dealership where I bought my last new Prius, my wallet still firmly in my jeans pocket. It's a fun game to see how long I can stay away and assist others in doing the same.