2001 Merc 150hp mid-range RPM overheat

I have a 150hp carbureted Merc with about 600 hrs on it. Runs great at all RPMs but starting in August, overheat alarm started going off at trolling speed (2500RPM) but not at high end or idle. Water pump was replaced in September, but did not fix problem (cheap after mkt pump?), just changed out t-stats, and put new diaphragm/seals on poppet valve but still overheating after a couple of minutes at 2500 RPM. Next thought is to flush with salt terminator and perhaps adjust spring on poppet. Any thoughts as to whether it is still bad water pump or if poppet valve spring should be stretched or trim short section out of it, or is there anything else you might try??? Any thoughts on the matter would be appreciated.

Re: 2001 Merc 150hp mid-range RPM overheat

If my motor had your symptoms and I already did the repairs you describe, I would next install a water pressure gauge on the motor to monitor the PSI readings during motor operation. These readings would provide additional good clues as to what the possible problem is.

Of course, others on this forum may be able to identify the likely cause of your overheat problem based upon your description of the symptoms, which may save your the trouble of installing a pressure gauge.

Re: 2001 Merc 150hp mid-range RPM overheat

Hi have you tried the least thought of part, the temp sender/switch, also there was a model round that age that merc changed the heat range of the thermostats to fix the problem

Cheers