"The impeller in the waterpump
"The impeller in the waterpump on an outboard is a maintenance item, and must be replaced every few years, or earlier depending on conditions. I use a water pressure gauge on my engines. With new impellers, the gauges read very high, even at idle. After 6 months, you can see a difference in performance. After a year, you can see even more degradation of performance. After about two years--I usually have the warning horn also "waking me up." Incidentally, when I pull out the old impeller, and it looks perfect, I always feel kind of foolish. Yet, after imstalling a new one, the pressure more than doubles. Looks don't mean anything with an impeller, unless it looks bad!"