Reply To: 1957 Merc Mark 25 not warming up

[JACQUES]

no thermostat expert but in general water always go right back out via the tell tale and the thermostat  only allows new  cooling water to be circulated around the cyl jackets by releasing the jacket  hot water  when the temp exceeds around 140 deg.

in the attached diag. on a OMC  engine tell tale water would get warmer after a while.  since your’s does not I suspect Merc’s  dump  it down the internal exhaust  housing…to also  provide cooling

if your engine ever gets burning hot then  there is a problem. A cheap IR gun will tell you  the temp on the PH  and a   hot spot  would indicate blockage needing attention

If not a stuck impeller blade  it  could be just  accumulated sedidements which  you can flush out with commercial products.

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