Cooling system not just a water pump

Last month, a reader emailed to say he has a 1985 model 40-horsepower motor, and is having trouble with the water system. He replaced the impeller and the gaskets with an impeller kit, but the engine tell tale is still not as strong as it is supposed to be. He checked the lower unit, and nothing seemed to be clogged. The tell tale is fairly strong when he runs the engine on the garden hose, but when in the water it sputters or blows steam at idle. At wide open, it blows steam or does nothing at all.

There is a lot more to an outboard motor cooling system than just an impeller and a couple of gaskets. Let’s start with the water pump itself and work our way through the entire cooling system.

The water pump consists of a rubber impeller that is keyed to the engine’s drive shaft. This impeller is inside of an off-centric housing. Most pump housings are plastic with a stainless steel cup liner for the impeller to ride in.

As the impeller turns in this off-centric cup, the blades of the impeller are compressed as they pass the short side of the housing, and they open back up on the long side. This compression and expansion action is what gives the pump its ability to send pressurized water through the engine’s cooling chambers.

The reader’s e-mail said he changed the impeller and gaskets. He made no mention of the water-pump housing, the stainless steel cup, the stainless steel plate that is under the impeller or the water pump’s base that the plate and gaskets mount to.

If any of these other components are damaged even slightly, they can cause a disruption in the impeller’s ability to create water pressure. Even though the impeller is brand new, it cannot function if one of these other components is damaged or worn. The impeller actually begins to suck air into the system.

It is very similar to a propeller that is cavitating. When a prop cavitates, the RPMs go up but the boat speed decreases because the prop has lost its grip in the water.

This is why I never replace an impeller kit only. For just a few dollars more, I can get a complete water pump repair kit that includes the impeller, gaskets, plate, upper housing and cup and any other necessary parts to successfully repair my engine’s water pump.

As I stated previously, the outboard motor cooling system is not just a water pump. Most outboard motors have one or two thermostats that are located in the power head of the engine. No matter how good the water pump may be, if the thermostats are stuck closed that engine is going to overheat.

You may have to do the unthinkable and refer to your service manual for the location of your engine’s thermostat. Many people will tell you that there is no need to have a thermostat in your outboard motor because our winters are not that terribly cold and your engine does not need a thermostat.

Unfortunately they do not realize that the prime function of the thermostat is to properly warm up your engine before taking off down the bayou. If your engine is not properly warmed up and you give it max power, you could have a piston seize up, and then you are faced with a major repair bill.

Another side effect of removing the thermostat is that the opening you create by doing this allows too much water to flow through the engine. The larger opening in the water passageways decreases the water pressure. The larger volume flows through the engine too quickly, and loses its ability to properly dissipate the heat from the engine.

Along with the thermostat there is another vital component in many engine cooling systems called a poppet valve or pressure relief valve. The thermostat’s main function is to regulate the engine’s temperature at slow speeds.

Once the boat is up on plane running, the demand for cooling increases and the poppet valve is designed to satisfy that demand. As engine speed increases and the pressure of the water from the pump also increases, the poppet valve will open and allow some of the cooling water to go around the thermostat for a more efficient cooling system.

Many times with age, corrosion will build up inside the water chambers of your engine, and this corrosion will bind the pressure relief valve and not allow it to open at the proper time to cool your engine.

The last component of your engine’s cooling system is the actual passageway inside the engine block and cylinder head(s). I have seen many engines that have new water pumps, new thermostats and new poppet valves, but they still have overheating troubles.

Inside the engine block and especially inside the cylinder head(s), corrosion can build up and create a coating over the metal surfaces. Usually this coating is not thick enough to stop the water from flowing through the system, but it will insulate the metal from the water. When this happens, the engine will usually overheat at high speeds and cool down at slower speeds.

If you suspect this is happening to your engine, you will need to remove the cylinder heads and water jacket covers off the heads. If you find a film or coating, you should thoroughly clean the metal.

I like to use a bead blast machine to clean off the surface. I then spray a thin coating of primer paint to help fight against future corrosion and reassemble the engine.

You may be wondering why our reader’s engine worked better at home on the garden hose than it did in the bayou. It is pretty simple: The garden hose is supplying water under pressure. It is simply helping the pump with that pressure. The only way to properly test your cooling system is to run the engine in the lake or bayou.

If you have any questions or problems with your boat, motor or trailer, send an e-mail to: