If the gfci trips immediately when you plug in the pump then the pump is likely the problem. I would replace the gfci anyway as I have ZERO confidence in Leviton. I had a troubleshoot not to long ago where the gfci was tripping on a fountain pump exactly like you describe and I replaced the Leviton gfci on principle before going to get a new pump and voila the problem was solved! I'm unclear on whether or not you replaced the pump though. If not 4 years of daily use is not a bad lifespan in my experience, and the pump may still work just fine plugged into a regular outlet, but the wear on the motor is causing enough leakage current to trip the gfci. Good luck.

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1984 300TD 1981 240D