Cooling Systems Guide
The pressure and temperature-controlled cooling system is composed of a water pump, thermostats, and pressure relief springs.
The water pump brings in water through intake screens on the gearcase and circulates it through the outboard to cool the powerhead.
The water pump is located at the top of the gearcase, and is driven by the driveshaft. It consists of the pump housing and a synthetic rubber impeller.
The pump housing is offset from the center with respect to the driveshaft, and the impeller is the keyed to the driveshaft. Because the housing is offset, the impeller blades flex as they rotate varying the space between them.
The pump inlet port is open to the blades when the space between them is increasing. The pump outlet port, in the impeller housing, is open to the blades when the space between them is decreasing.
Low Speed (left) High Speed ( right)
At low speeds, the impeller works as a displacement pump. At higher speeds the blades flex, thier outer edges no longer touch the impeller housing, and the pump becomes centrifugal.
Never turn the driveshaft or the flywheel counter-clockwise as it can damage these blades.
Thermostats and pressure relief springs
Thermostats and pressure relief springs are located on each cylinder head. The thermostats prevent cooling water from flowing freely until the engine warms up or is operating at high speed.
Water Bypassing the Thermostats
The thermostats open when the powerhead and cooling system warm to correct operating temperature. As engine speed increases, water pump pressure opens the pressure relief springs and water bypasses the thermostats.
On some models, separate pressure relief valves perform the same function. Let's review how water flows through the cooling system.
Water enters the geacase through screened inlets and is directed to the water pump.
The water is pumped to the powerhead, but the flow is restricted by the closed thermostats and pressure relief springs.
When the water temperature reaches the correct operating temperature, the thermostats open, allowing the water to circulate around the cylinder and cylinder heads.
The water then exits the outboard through gearcase and exhaust passages.
When the water temperature drops below the correct operating temperature, the thermostats close, restricting the water flow. This allows the engine to maintain normal temperature at low speeds.
As engine speed increases, water pump pressure overcomes the pressure relief spring or pressure relief valve.
Water bypasses the thermostats and flows through the cylinder heads, allowing the engine to run cooler at higher speeds.
Maintenence and Adjustments
The two most common cooling system problems are overheating and overcooling. Pyrometers are used to check powerhead temperature. Common types include probes and infrared non-contact pyrometers. When measuring powerhead temperature with a probe, use Wakefield thermal joint compound to ensure an accurate reading.
The service manual describes where to take a temperature reading. It will also specify a temperature range at idle, and a maximum temperature at high speed. There is no minimum temperature specified at high speed. On direct injection, E-TEC, and four-stroke models 40 horsepower and larger, you can use the diagnostic software for temperature readings.
DI models use a temperature sensor in the port bank and a temperature switch in the starboard bank. The switch closes when it reaches a predetermined temperature. It cannot report the temperature, only that it has passed a certain threshold.
E-TEC models use a temperature sensor in each cylinder head. Four stoke models with electronic fuel injection use temperature sensors in the block and exhaust manifold. Overheating can lead to severe powerhead damage and is usually caused by restricted intake screens or a damaged water pump.
The pump can be damaged if it is deprived of water while the outboard is running. Never start the outboard out of the water without supplying water with a flushing device or by attaching a garden hose.
Keep intake screens clear of debris. They can be obstructed by weeds, plastic bags, and other items in the water. Some models have additional intakes that must be submerged when the engine is running to prevent overheating. Many Evinrude and Johnson outboards are equipped with a warning system to alert the operator of overheating.
The powerpack - or the Engine Management Module- constantly monitors a temperature a switch or sensor, located on the cylinder head.
When the engine reaches a predetermined temperature, the S.L.O.W. ( Speed Limiting Operational Warning) or S.A.F.E. ( Speed Adjusting Failsafe Electronics) system is activated.
The engine speed will decrease and a horn will sound. If the boat is equipped with a System check gauge, the hot indicator will light. I- Command gauges will display a warning.
On some models, if the overheat is severe enough, or continues long enough, the EMM will shut down the engine.
To inspect the water pump, remove the gearcase. Remove the water pump housing. Inspect the water pump liner, and the wear plate, for scoring and distortion.
Examine the impeller for wear, crumbling, and other damage. Overcooling can cause poor idle characteristics, plug fouling, and excessive carbon buildup in the powerhead. Overcooling is most often caused by a thermostat or pressure relief component that is stuck open.
To service a thermostat, remove it and its related parts. Clean them, making sure all of the components are free of debris.
Proper temperature regulation is vital to engine life and important to proper emission control.
Evinrude and Johnson outboards use a pressure and temperature controlled cooling system
that is composed of:
• A water pump – consists of a pump housing and a synthetic rubber impeller.
• Thermostats – prevent cooling water from flowing freely until the engine warms up or is
operating at high speeds.
• Pressure relief springs – allow the engine to run cooler at higher speeds.
At low speeds, the water pump impeller’s blades contact the pump housing and the pump works
as a displacement pump. At higher speeds the blades flex, their outer edges no longer touch the
pump housing, and the pump works as a centrifugal pump.
The thermostats open and allow cooling water to enter the engine at a predetermined temperature.
As the engine speed increases, water pump pressure increases and overcomes the pressure
relief springs. Water then bypasses the thermostats, allowing additional water to flow through
the cooling passages.
The impeller blades can be damaged if you rotate the outboard’s driveshaft opposite of its normal
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