All of a sudden my engine block heater is popping the gfci and I’m not sure why. I just replaced the water pump because it was leaking (significantly) from the weep hole. I was only able to drain a little over a gallon of coolant prior to the replacement. The truck had been plugged in and I’m wondering if the coolant level got too low and caused the block heater to fail.

 

The cords will fail eventually as well, I would try a new cord before I went to the trouble of replacing the heater

 

It is unlikely that the GFI will trip because of an amperage issue on the hot line. You can test the element of the heater with an ohm meter. It should be 20-25 ohms. under 10 ohms is probably a short and much past 25 ohms means it is worn out.

To test the ground fault issue, put an ohm meter on the ground terminal of the plug and the engine block. If there is no continuity, that is why the GFI is popping. If there is no continuity, the cord is bad or the connector between the block heater and cord has a problem.

Also, check the ground wire continuity of the extension cord you are using just to rule it out.