5.7 was running hot, new pump, now runs cool

[GregR]

2000 5.7 MPI, twin engines, Bravo IIIs. Starboard engine was running around 190 degrees at cruise and sometimes spiked to 200, port around 175. Replaced the engine water pump on starboard engine. It's now running under 150 degrees with now spikes, even a little less. I haven't put the lazer heat gun on the thermostat housing yet to confirm.

Is this too cool? Should I replace the thermostat? If yes, what temp thermostat does it take?

 

[Sandy]

Might the stbd engine thermostat have been removed when it was running hot? Otherwise it sounds like it may be stuck open or the gauge or temp sender is failing . You can use your IR gun at the t-stat housing to compare with the port engine , and or swap parts between engines.
Are these RWC-only engines?

My Crusader 5.7L MPI's take a 170*F t-sat and run within a few degrees +/- of T-stat rating.
I ...think.... Merc MPI's use a slightly cooler T-stat, likely 160* , (or less likely 165*)?
MPI's need higher temps than non-injected engines to provide the ECM with proper input from it's sensors to get out of default mode and attain more efficient combustion. But I don't know what the critical min. temp would be for your engines.

As seawater temp declines the engines may run a few degrees cooler, but 10 degrees cooler when fully warmed seems excessive.

Lastly, it could be worth noting, with American standard (not Euro) a loose or corroded sender-gauge wiring connection can add resistance which will lower gauge reading.

 

[GregR]

I hope the thermostat was not removed, and then not replaced, but it's possible. I did not do the work, shop/mechanic did. I will check next time I'm on the boat.

Yes RWC engines.