Do water pumps generally fail all of a sudden, or can they slowly stop working effectively?

My 99 Subaru Legacy recently started overheating on long-ish trips (> 25 min or so). I already replaced the thermostat, so I doubt that's (still) the problem. I also tried just taking out the thermostat and running water through it to see if the pump was working (and if the radiator was clogged). I put a hose in the top to keep it full, took off the top hose so I could see if water was pumping through, and started it up. To my surprise, the radiator filled up, and water started coming out of the top hose (from the engine)! Do I still need to replace the water pump, or should I check something else?

  • Update: Turns out it was a head gasket issue. I currently have the engine in the garage going back together.
    – Cullub
    Dec 15, 2017 at 4:35

2 Answers 2


When you replaced the thermostat, did you use a Subaru Thermostat? That's essential; the replacements available at car spares stores are NOT the same, and are often the cause of cooling system failures shortly after replacement. You can see in the attached image that they are physically different, even if the specs are the same.

enter image description here

Water pumps can fail over time - how many miles on your Legacy? If it's at, or near, a multiple of 105,000 then the timing belt needs doing, and as that has to come off to get the water pump, it's a good time to do both. Before you put money into that, though, check that the dreaded Subaru Head Gasket issue hasn't hit you.

  • Mine looks like a cross between those two, so it might not be Subaru. However, the car overheated the same day I changed the thermostat, so I doubt that is the problem. About 130,000 miles on the car.
    – Cullub
    Oct 1, 2017 at 22:14
  • How might water pumps fail? Could you explain how they work maybe? Thanks
    – Cullub
    Oct 1, 2017 at 22:15
  • @Cullub a water pump may fail because of rust, at 130K that could be a reason, other reasons could be mechanical failure. Did you see small metal particles when flushing coolant?
    – Nilabja
    Oct 3, 2017 at 5:49

The water pump can fail intermittently. The impeller can slip on its shaft but on occasion seize and pump again. If so, it needs to be replaced.

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