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The Great Water Pump Debate

by Nick Jenkins

The title of this article may be a little misleading. There's actually not much debate over when to replace the water pump in your Miata. Almost everyone will tell you to do it when you change your timing belt. The reason is simple. The timing belt cover—which needs to be removed to replace the pump—is already off. The cost of the pump is around $100, and replacement takes less than an hour. A couple hundred bucks for some peace of mind sounds reasonable.

When I took my first Miata, a '91 Crystal White, to the dealer for its 60,000-mile service, way back in 1996, I was a little surprised when they suggested I replace the water pump. I didn't have any cooling problems, and I wasn't losing any coolant. I'd previously owned several British sports cars, and even those rolling wrecks went more than 60K miles on a single water pump. But the Mazda dealer said it would be cheaper in the long run to replace it, and in those days I knew very little about Miata mechanics, so I had them do it.

I've never replaced a Miata water pump since. My '91 ran for another 150,000 miles on that pump. My '97 and '01, at around 120K and 90K miles respectively, are doing great with their original pumps. I would really like to see a Miata water pump fail, just to get a clue about their longevity, but so far no luck. I haven't even heard a good story about a failed Miata water pump. Radiators, sure. Alternator, hoses, belts—my '91 gave up all of those at one time or another. But never the water pump.

Failure mode for a water pump would be one of four things. 1) The rotating seal could leak, allowing coolant to
Miata OEM water pump

escape. This would first be noticeable by an increase in water temperature, coolant on the floor of your garage, or possibly a dry catch tank if you're looking under the hood. But several more likely problems could cause this as well, like a broken hose or radiator leak. 2) The bearing could fail. This would be noisy, and likely to result in the same problems. 3) An impeller blade could break. British pumps with their cast-iron impellers were notorious for this. The result would be a loss in pumping volume and a resultant increase in temperature, along with possible clattering from inside the pump. 4) The gasket seal could leak, with the same results as 1) above.

Mazda does not design their water pumps to fail after 60,000 miles, or even 160,000 miles. They design them not to fail ever. If a water pump fails in service the result can be as serious as a blown engine, if it's ignored too long. Mazda is all about building a great water pump, and they know how to do it, unlike some of the British car makers of the '50s and '60s. Keep in mind that it’s the dealers and the shops who recommend replacement. Mazda doesn’t.

Consider another factor if you’re thinking of replacing the pump. When a water pump is installed in Hiroshima on a new Miata engine block, everything is spotless. Mating surfaces are freshly machined. The gasket makes a perfect seal with the block, and the bolts are torqued to exact specs. To me, that's peace of mind.

When a water pump is replaced at the dealer, the engine block has been through a few heat cycles. Hopefully the mechanic wipes a rag around the edge of the pump before taking it off, but conditions aren't exactly factory-fresh. Hopefully they’ll use a genuine Mazda OEM pump, and not one of the many cheaper replacements on the market. Hopefully the new gasket is installed without any dust or grime on the mating surfaces. Hopefully the bolts are torqued per the book, and not just to the mechanic's best recollection. I don't mean to disparage mechanics—they’re doing the best they can under shop conditions and against the clock—but the results will not duplicate a factory installation of a new pump on a new block.

In the end, the decision to replace or not replace your water pump with the timing belt may be more emotional than practical. No one wants to be stranded on the side of the road with a broken pump and steam pouring out from under the hood. And the cost of replacing a pump by itself may be $100-$200 more than replacing it along with the timing belt. But you have to weigh that against a factory versus dealer installation, Mazda's dedication to reliability, and the idea of tossing out a perfectly good water pump.