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Message: <blockquote data-quote="kofkorn" data-source="post: 390506" data-attributes="member: 7076">Sounds like you already did that investigation pretty well.  If you yanked the thermostat and the temp plummeted when running the motor, then the t-stat was causing a restriction.  It's POSSIBLE that you may have a weak water pump, but I'd guess you're on target with the t-stat.  The price you found seems awfully expensive for a t-stat.  I did a quick check on MarineEngine.com and the t-stat for a 30 hp mariner is $25.  Cheaper if you go for a Sierra brand.  And I'm sure you'd be in the teens if you go to Ebay.Honestly, I'd swap out both the impeller and t-stat and not worry about it from there.  Good luck!</blockquote>