Overheating can be caused by several problems so if your boat has some water pump ISSUE we will help you find the problem. The easy way to find what is overheating your boat is to divide and conquer. The first thing to do is connect a flushing device to your unit and run your engine in the driveway. First notice after a while if the exhaust water from the prop is hot. If the engine is overheating and the exhaust water is cold you either are bypassing your cooling system because you removed the thermostat or you are fresh water cooled and the water in the heat exchanger is low, or for some reason the water is not circulating.
If the water is hot another test must be tried. While observing the excess water leaking around the flushing device, have someone accelerate the motor. The excess water should be sucked in and the flushing device goes dry. If this does not happen either the water pump is bad or the exhaust system is faulty and clogged.
To check for clogged manifold or elbow place a water hose into the far end of the hose that supplies the elbow. If the elbow is fed through the manifold, put the water into the front of the manifold. If two hoses feed the manifold and none on the elbow then pinch one closed to avoid escape of pressure. The result of this test should be plenty water from the prop from the exhaust system. A low water flow or high pressure in the elbow hose indicates back pressure and a clogged elbow
If there is no back pressure and the water excess does not dry up during your test the water pump in the drive is bad, that is presuming the passage through the transom is clear.
If you have a fresh water cooled Mercruiser like a 170 with the old 3" heat exchanger the problem could be more complicated. Before checking your engine remove the water tank on the front of the manifold, Remove the thermostat and plug the 1" hole below it with a boat plug, and replace tank.
Remove the rear hose from the heat exchanger to the elbow at the top, place a garden hose into it and clamp Turn the hose on full and check for exhaust water If the water is sparse and the hose develops pressure the elbow must be replaces or unclogged.. Elbows over 4 years old in salt water should be replaced. If you have the 170 low profile elbow and have 2" clearance on the top, the 140 elbow can be used instead.
With clear elbow and bypassed thermostat check for full water tank and take the boat for a ride full throttle. The boat should run between 180 and 210 degrees with a 3" heat exchanger steady state. a rise that never stops indicates a bad impeller pump OR CLOGGED ELBOW.
If the boat runs hot immediately feel the temp of the exchanger, a cold exchanger indicates water on fresh side is not circulating. Either something is clogged or the water on the fresh side is low or the aluminum impeller is broken.
If the temp comes up suddenly or the water is low you have a leak, most occur in three places, the elbow gasket, the heat exchanger or the front cam seal.
The elbow gasket will show up as a green puddle below the drive, presuming you put the boat up hot and the water is green. Green water indicates anti freeze which should be 50/50%. A 100% anti freeze mixture will overheat.
A leaking cam seal will emit water out of the drain below the damper.
The heat exchanger will be harder to check. Remove the two inner hoses on the top of the exchanger, they will drain down from the hoses but the fittings on the top will stay full. Insert and clamp your garden hose into the top of the hose from the rear of the exchanger and clamp off the front hose. With the hose turned on slowly the pressure in the rear hose will stiffen.
If the water in the open fittings on the top of the exchanger run over the unit is leaking and should be taken to a radiator shop to seal the leaking tubes. More than two tubes in any third of the unit will render it useless.
Gas detected in the cooling water indicates leak in head gasket.
Good luck
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