So this vehicle is leaking coolant. And I couldn't see where the leak was coming from when looking over the hood or under the car, but it looked like it came from the water pump. I got the water pump out of the vehicle and I was about to replace it with a new one that I got along with a new thermostat but then I started thinking about how the vehicle only seemed to leak while the engine was running.

Looking online, I read that the water pump will even when the engine is off, which I don't think happened with my vehicle.

So what parts of the vehicle will leak only if the engine is running? Is it likely that the water pump is not the problem?

  • It leaks much more under pressure, bearing seal failures, perhaps 3 rubber O rings Sep 27, 2020 at 16:25

1 Answer 1


The water pump tends to leak (more in some cases, or only in some cases) when the engine is running due to a few reasons:

  1. the pump causes a pressure difference in the system

  2. the cooling system becomes pressurised due to temperature to above atmospheric pressure

  3. the rotation of the pump shaft acts on the seals and if the bearings have failed the shaft wobbles

Any of these or all will make the leak at the pump apparent and it can be severe in some cases or just a small dribble in others.

  • That makes sense. It's been more than a day since I replaced the water pump and refilled the cooling system and I haven't found any leaks nor has it over-heated when driving around. So it looks like the water pump was the culprit. Although it could of just have been that the gaskets went bad but oh well. Oct 1, 2020 at 18:41

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