It could be minor or serious.
1: Bad thermostat (minor) To test, pull the thermostat, put housing back together, and run it to see if it pees.
2: Bad water pump. (Minor-medium). Only way to test is pull lower unit off and look at it. This is the most likely cause.
3: Plugged or corroded water channels. (RUN away, this is a mess and expensive to fix). To test make sure 1 and 2 aren't the problem, and when you are pulling the thermostat, look into the hole. If there's a thick white chalky coating stuff in there, then the motor has rusted up internally. Bad motor.

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