1976 Mercruiser 302, 188hp intermittent overheating problem
replaced the impeller last year and thermostat at start of this year. first trip out ran fine for over an hour, we went back to marina and parked for few minutes then went back out and it overheated. got it home, took into shop and tech couldn't get it to overheat -he put in tank and ran for about 30 min. shifted gears, ran rpms up but it wouldn't overheat- he said thermostat opened and cooled motor properly. took to lake last weekend, ran for hour and half no problems, sat in cove for hour, then headed back out of cove and it overheated, shut down and let cool for hour, started and temp was fine (150) so ran around for another hour or more and went in. turned boat off at dock while i got trailer and when wife was loading on trailer about 10 minutes later she said it was getting hot again. Note, i know it's getting hot and not the guage, could smell it and saw a little smoke. i was thinking my thermostat might be faulty since this only started happening this year after i had the thermo replaced but in looking through the forum i'm concerned i might have a corrosion obstruction issue.... it's an old boat that sat for years before i bought it from the orig owner last year, was well cared for though. is there a way to flush the cooling system? i'm a little handy but prefer to take it to the shop, just looking for direction to give them since they've already replaced the impeller and thermo. thanks, i value any input anyone can give.