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Old 01-08-2020, 07:45 AM   #1
BrianSmith
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City: Wherever Smartini is
Vessel Name: Smartini
Vessel Model: 2002 Kristen 52' Flybridge Trawler
Join Date: Mar 2014
Posts: 487
Main engine coolant temp drop

For several months, I've had temperature sensors on various parts of the main engine (each alternator case, the thermostat housing, and the junction where the raw water is injected into the exhaust), which I monitor while underway. Over the last few trips, I've noticed that the temp at the thermostat housing has dropped about 5 - 7 degrees Fahrenheit from it's normal cruising temp. I don't mean that it gets to temp then drops - I mean it just doesn't get to the same temp as before. Instead of the normal 185, now it's reaching only about 178 - 180.

The coolant is keel-cooled, and we've been in the Eastern Caribbean for a year, so no change in water temps. We run at the same engine RPM all the time, so nothing there.

My only thought is that possibly the thermostat is not closing completely, or is opening at a lower-than-normal temperature, but could it be something else? I have a spare thermostat, but I don't want to change it without needing to.

How low should I let that temp get before I need to try to fix the issue? I think most diesels run about 180 - 190, but what happens if they run cooler? How cool is too cool?

The temperature at the junction where the raw water is injected into the exhaust has also dropped by about the same amount, during this same period. It's probably related, but I don't know how, as the raw water that dumps into the exhaust isn't used to cool the engine - only the transmission oil, and the hydraulic fluid.

Any brilliant ideas, TF?
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