In Chicago I change mine [oberdofer] every 3 seasons, about 60 -75 hrs total on early model A4 I always [slowly and carefully] take the impellor and shaft out over winter layup and find this extends its' useful life. [rounded off - deburred the shaft edges so not o damage bushings] Always order a new spare and keep recent old one as emergency backup. Inspect, lightly lube and carefully reinstall in spring. Never had a impellor problem in 40+ yrs and am on only my second water pump. I have fairly decent access so job is not too difficult. A couple of times my backup has gone into someone else boat when there fins have cracked off.
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