I have 2 WhisperFlo pumps for waterfalls that weren't used all summer (they were last used when the anti-freeze cycle activated, likely in March or so). Last week I tried to use them, and neither turned over -- they just hummed. I disassembled one of them, verified no blockage, and that the impeller turns freely (although with some resistance -- I'm not sure if it's a normal amount or not). If I turn on the pump without the diffuser assembly attached, it runs fine. I thought the diffuser assembly may have warped or something, so I bought a brand new one, but the pump still won't run with it on (still runs fine without it). What are the likely culprits? It appears that the starter capacitor is OK based on my testing (but I have found that test to be inaccurate in other situations).