Your Outboard’s Cooling System

Your Outboard’s Cooling System
New thermostat on left. Salt and mineral buildup is a threat to proper engine temperature regulation.

The cooling system is of primary importance on any outboard motor. Most seasoned boatowners have likely experienced a water pump impeller failure, or at least understand the need for water pump annual maintenance.

Here on the Texas coast we experience salt and sand running through our engines every time we hit the water. Salt and other minerals build up inside the cooling system each time we use the outboard. Flushing with fresh water after each outing is the first line of defense.

The cooling system components that should be inspected every year (or 100 hours of operation) are; intake screens on the sides of the lower unit gearcase, the water pump, the water pump pick up tube, the thermostat(s), and relief valve, when equipped.

The thermostat, or water pressure regulator, is a key component often overlooked during service. As engine temperature increases and decreases, the thermostat opens and closes to regulate the flow of cooling water. Salt and other minerals can build up over time, coating the thermostat with a dense crust that inhibits the ability of the metals to conduct heat, thereby delaying the opening and closing at the engine manufacturer’s prescribed temperatures.

While the thermostat is out, inspect the cavity for evidence of white powder or scale; this is a sign of internal mineral build-up on the internal cooling passages of the cylinder block and head. An acid bath would be the standard remedy for heavy buildup. 

Outboards are designed to run their best when thermally efficient, and this is the purpose of the thermostat(s). Running an outboard without thermostats should be considered only in emergency situations.

Have a great fall fishing season,

Chris Mapp
Coastal Bend Marine | Port O’Connor, TX | 361-983-4841