2002 Mercruiser 6.2 temp fluctuates

by Joe Strazzante
(Bradenton, Fl )

Hi, I have a 2002 31’ doral and my temp fluctuates from 120 (the lowest temp), to 165, then maybe 120, then to 170’s, then drops to maybe 130, and keeps rising and dropping until it eventually overheats. I have changed 4 t stats, circ pump, temp sender, swapped t stat housings, 3 impellers and housings, to no avail. The kicker is that it seems the if the system is opened, i.e. change the impeller, the temp is stable the next time I run it. The next time I try to take it out, it will start fluctuating again and eventually overheat. I am at a loss here. The only thing I can think of is collapsed hose somewhere. Please help if possible.