by Scott Huber
What do I do if my engine is overheating?
There are three common reasons why marine engines overheat.
- Low flow or no flow of raw water (either lake water or sea water);
- Lack of circulation of engine coolant or anti-freeze; and
- Compression gases or air in the cooling system.
Low or No Flow
The symptoms of low flow on your raw water cooling circuit are generally characterized by a slow overheat. The engine will usually maintain temperature at low RPMs, but will begin to slowly overheat at higher RPMs. A good place to start is with your raw water or impeller pump. Check for broken or missing fins on the rubber impeller. The second place to look for restrictions is on the intake side of the pump. Any coolers (such as power steering coolers, transmission coolers) are suspect. These coolers are a good place for debris, grasses, etc., to restrict water flow into the engine. If the intake side is found free and clear, make sure that all hoses and hose clamps are tight and intact so that no air can be introduced into the system through a loose fitting or failing hose.
A restriction on the outlet side of the raw water pump will cause low flow and overheating because the raw water cannot exit the engine fast enough to remove the heat. Therefore the quantity of water moving through the engine is reduced. A common place for this to happen is on the exhaust risers or elbows where deposits in the water passages are restricting flow.
A “no flow” of raw water is generally caused by a failed raw water pump, a bad pump impeller or a major suction leak on the intake side of the impeller pump that is preventing the raw water pump from picking up any water. The symptom of this type of overheating problem is generally a quick spike in temperature where you can actually watch the temperature gauge go up.
Lack of Circulation
If you have a closed-circuit cooling system in your engine (your engine requires anti-freeze) then your cooling problem could be caused by lack of flow or no flow in the engine cooling system. Check to make sure that the belts driving the circulation pump are intact and that the circulation pump is not leaking water from the “tattle-tale” hole located on the underside of the pulley shaft assembly. If these items are found to be o.k., then the next most logical place to check is the thermostat. If a thermostat is stuck closed, it will cause a spike in temperature from normal operating range. The engine will act normally until it reaches the proper operating temperature. The gauge will continue to rise quickly beyond that temperature.
Dirty heat exchangers are a common problem on closed circuit cooling systems. Symptoms associated with a dirty heat exchanger are generally a slow overheat that increases with engine RPM. Example: your engine will maintain normal temperature at 2500 RPMs but will begin to slowly overheat at 2800 RPMs. The reason for this is the build up of “sludge” on the heat exchanger which impedes its ability to dissipate heat to the raw water system. Heat exchangers can generally be cleaned by your local radiator shop or by a light solution of muriatic acid, (like that used to clean concrete) available at most hardware stores. Please note that you may not use this method if your heat exchanger is made of aluminum! Use extreme caution with this acid: it is harmful to your eyes, skin and produces strong vapors.
Gas or Air in the Cooling System
Air or compression gases in your cooling system will cause overheating due to the fact that air is a poor conductor of heat. Air in your cooling system can also cause your circulation pump to quit. (Circulating pumps circulate antifreeze.) Centrifugal pumps cannot pump air. Air comes into the cooling system through leaking circulation pumps, bad hoses, loose hose clamps on the suction side of the pump, or even by the engine itself in the form of compression gases. Most people would refer to this as a bad head gasket. Example: a bad head gasket allows compression pressure from the piston cylinder to find its way into the cooling system. These gases are extremely hot and allow air into the system which then causes the engine to overheat. There are various places where engine gases or compression gases can enter the system, the most common being the head gasket. The other places that compression gases can come from are cracked cylinder heads, cracked blocks, or exhaust manifolds. There is a chemical test available to check for compression gases in the cooling system. Contact your local auto parts store for a chemical testing kit.
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For all your marine engine parts needs, call us toll free at 877.621.2628, or outside the U.S. at 207.882.6165.
December 1, 2009 / mpartsexpress / 0
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