Why is my main engine jacket cooling water temperature high?

High jacket cooling water temperature is fundamentally a heat balance problem: more heat is being put into the water than is being carried away, so fault finding should start on the removal side rather than assuming a faulty sensor. Typical HFO fired two-stroke and four-stroke jacket outlet temperatures run roughly in the 70 to 85 C band depending on engine type and manufacturer setting, with a high temperature alarm set a little above normal running and a slow down or shutdown trip set higher again. A genuine, sustained rise toward those set points needs to be worked through methodically rather than silenced by nudging the alarm point.

Start with flow. Confirm the jacket water pump, or the duty pump if a standby exists, is actually turning at rated speed and delivering full pressure, since a worn impeller, a cavitating suction, a partly shut discharge valve or a blocked strainer can starve the whole circuit even while the pump sounds and looks normal. Check for air trapped at the highest point of the system, typically at the cylinder head outlets or the top of the expansion tank line, because a pocket of air blocks circulation just as effectively as a closed valve and is a very common, easily missed cause; vent it and watch the temperature respond.

Move on to the heat rejection side. The jacket water cooler, whether a plate type or a shell and tube type, loses effectiveness when it fouls, on the fresh water side with scale and biological growth, or on the low temperature and seawater side with silt and marine growth. On a central cooling ship, check the low temperature circuit pump, the seawater pump, and the sea chest and strainers for fouling, mud or a jellyfish blockage that starves the whole cooling train. A temperature control three way valve or thermostat stuck toward recirculation instead of routing water through the cooler is another frequent culprit, and is easy to check by feel or by comparing cooler inlet and outlet temperatures.

If flow and cooling both check out, look at the heat input side: genuine overload, late fuel injection timing, poor combustion or a fouled turbocharger raising exhaust backpressure all increase the heat rejected into the jacket water for a given load. Check the expansion tank level and watch for combustion gas leaking into the jacket through a cracked liner or a leaking cylinder head gasket, which pressurizes the system and can show up as a rising expansion tank level, bubbling at the header, or a positive result on a compression leak test kit. Reduce load if the temperature keeps climbing, restore flow and cooling before anything else, and log and escalate any trend that will not settle once the obvious causes are cleared, since jacket water problems that resist the standard checks often point to something happening inside the cylinder itself.