i don't have a wiring diagram, i'm guessing-dangerous.

if it's a 4 or 5 pin relay, you will see numbers on it, 30, 85, 86 and 87/87a. constant power, maybe key off, probably key on is on 30.

87 should be to the pump

85 might be power or ground from a switch-or ecu.

86 is the opposite of 85. if 85 is 12v, 86 is ground, etc.

next time it does it, pull the relay out enough to expose the terminals but not enough to unplug it. poke all the terminals with a test light. if the pump is off and the key is on, only 30 should be hot. if the pump is on with the key off, only 30 and 87 will be hot, because the relay is stuck closed. if, by chance 85 or 86 is also hot-OR-this is why guessing is dangerous-if the relay always has power on 2 terminals and relies on a ground to close the relay, then that needs to be looked at as the problem when the key is off. good luck, chuck.