Below  you will find information on marine engine cooling system troubleshooting.  We do not claim to be experts in all fields.  We can not promise to be correct on all issues.  We can not address the content of these pages.  (Please read our legal disclaimer page)

But, I hope you will find this page useful.  We want to thank Monitor Products Inc., for their text, on which a large portion of this page is based.  Monitor is a supplier of high quality fresh water cooling system kits to BoatFix Service.

This is page two of two.  We suggest you read both pages before considering specific actions.



The thermostat has a very important function in any cooling system.  If stuck in an open position it will cause overcooling and if stuck closed, overheating.  Its function is more complicated than most people realize.  Even a fully open thermostat creates a restriction at the outlet of the engine.  This restriction is designed into the system in order to build up pressure in the engine block to help suppress localized internal boiling.  Removing a thermostat that is not opening may work as a temporary solution to an overheating problem.  If so it should strictly be used in an emergency and the engine should only be operated under minimal load.  As to how a thermostat should be tested see previous discussion about overcooling problems.


Any debris that may work itself loose in the engine block may be flushed along and get stuck at the inlet of the heat exchanger a form a restriction.  If this happens the heat exchanger should be back-flushed, that is flushed with water going in the opposite direction from normal.  Depending on heat exchanger design and location this may or may not be possible to do with the heat exchanger installed.  The best way is to do it with the heat exchanger removed so that it can be turned with the inlet down and let both water flow and the force of gravity help remove debris.

Since heat exchangers contain no moving parts there is very little that can go wrong with them except that they can get plugged with debris and that they will become dirty.  They can also develop leaks internal as well as external.  These problems will be covered in the next category of troubleshooting.  There is also the possibility that a heat exchanger is insufficient in capacity and/or incorrectly manufactured.  Those problems, however, will show up immediately upon start-up and the manufacturer should be contacted.  In this discussion we are concerned with the problems that could develop during normal usage.


The most common problem is where enough debris accumulates in the inlet chamber of the heat exchanger to prevent the raw water from passing through the tubes and pick up the heat.  Since all good heat exchangers should have an existing de-mountable end cover at the inlet and it should be a simple operation to remove this debris.  If the problem persists due to local water conditions install a good capacity raw water strainer in the raw water inlet hose.  Even better is a good hull mounted strainer that will prevent debris from entering the system in the first place.  Obviously there is a different solution depending on whether the raw water intake is through a seacock or a sterndrive.


The more long-term problem with heat exchangers is the gradual slow build-up of dirt on both the inside and outside of the small tubes that form the heat transfer area.  This will gradually build up as a layer of insulation and is compensated for in every good heat exchanger design by so called "fouling factor".  This simply means the heat exchanger has excess capacity when new and clean so it still performs acceptably when old and dirty.  However, this "fouling factor" can only go so far and eventually a thorough cleaning of the heat exchanger may be necessary.  The most drastic solution is to remove the heat exchanger and bring it to a radiator shop for a complete cleaning.

Make sure that the radiator shop has experience with marine heat exchangers.  Proper cleaning should be a two step operation.  First clean the unit with a strong alkaline solution to remove organic dirt such as oil.  Second, in order to remove scale, the unit should be cleaned in an acid solution.

If a less thorough cleaning is desirable the heat exchanger can be left in the system.  The jacket waterside can be cleaned in the same way as an automobile cooling system using any of the better radiator cleaning solutions on the market.  Using a small diameter long handle brush similar to what is used for cleaning a rifle barrel can clean the small tubes on the raw waterside.  It is possible to acid clean the raw waterside of the heat exchanger without removing it from the boat, but it is messy and dangerous.  The ports of the heat exchanger will have to be plugged and the unit filled with an inhibited muriatic acid.  Since an operation like that necessitates special safety equipment and creates a disposal problem we do not recommend that you try it unless you really know what you are doing.

External & Internal Leaks

External leaks are the ones where the liquid leaks to the outside of the cooling system and can be seen or felt.  Internal leaks are impossible to see or feel directly since they allow liquid to leak into internal area of the cooling system or the engine itself.  Since they are more difficult to find and therefore may go on longer, they are often the most serious ones.

Raw Water Leaks

External leaks

On the raw waterside seldom in themselves create major problems until they reach a high level.  They should be fixed since a small leak easily could develop into a major one.  Also raw water can do major damage if allowed to come in contact with other components especially electric ones.  Besides the direct damage that a raw water leakage can do, it may also be a warning signal for other problems.  Leaks may start due to pressure build-up in the system resulting from raw water blockage in either heat exchangers or exhaust system.

Internal leaks

The result of an internal leak will depend on the pressure existing on either side of the leak.  On a FWC equipped engine, raw water can still enter internally into the engine or transmission through raw water-cooled oil coolers.  Raw water can also get into the engine jacket through the main heat exchanger, as well as enter the engine cylinders through leaky raw water-cooled exhaust manifolds and exhaust elbows.  Any of the above fluids may also go the other way and enter the raw water system.  Which way the leak will go will depend on which fluid is under the most pressure and usually will depend on whether or not the engine is running.  The leak might go one way when the engine is not running and the other way when it starts up.  Either way these leaks are very serious and must be caught early or serious engine damage may result.  Check engine and transmission fluid levels frequently and be alert to any abnormal changes in levels as well as condition of fluid.

Fresh Water Leaks

External leaks

On fresh waterside may eventually create enough loss of coolant to create an overheating situation.  Once the safety margin in the expansion tank is used up, air will enter the closed system.  Since a mixture of air and coolant is insufficient as a heat carrier overheating will result.  Some leaks may be difficult to find unless system is under pressure.  This pressure is created by normal engine temperature.  Since trouble shooting for safety reasons should be done on a cold engine, the system may have to artificially pressurized to show a leak.  A solution to this problem is a pressurizing pump for trouble shooting automobile cooling systems available from most auto supply stores.  This type of pump usually includes a pressure gauge that will confirm the presence of a leak.

Internal leaks

On the fresh waterside can be in the heat exchanger internally where the fresh water will leak out into the raw waterside and escape.  To check for this type of leakage, remove end covers from heat exchanger and drain raw water.  Most antifreeze solutions have a color to them and are easy to see if they leak out into the raw water part of the heat exchanger.  If operating on plain water, it may be desirable to dye water with food coloring to be able to see.  Pressurizing the fresh waterside will help locate the leak.  Another more serious form of internal leak that would be one within the engine where the fresh water, in most cases actually an ethylene glycol solution, leaks into the cylinders through a leaky head gasket.  This type of leak will usually show up when combustion gases are pushed into the jacket water when the engine is running and under load.  It may not show up at any speed unless the engine is under load.  It will show up as a steady stream of bubbles being pushed through the clear tubing leading from the heat exchanger to the expansion tank.  Do not confuse with normal start up deaeration.  A device called "Bloc Check" is available from the auto supply stores which will analyze the gases being pushed out from the cooling system and show whether they are air or combustion gases.  A leaky head gasket problem is very serious and should be attended to immediately by a professional mechanic.  It may result in Ethylene Glycol contaminating the oil.  This is why we recommend running an engine initially on plain water to confirm too that no leaks exist.  At worst, the leaking coolant could accumulate in a cylinder and upon start-up bend a connecting rod due to hydraulic lock.

Fixing Leaks

Permanent Repairs

The best way is obviously to create a permanent solution by bringing the leaking components into the proper condition.  If major components such as heat exchanger leaks make sure they are repaired or replaced by competent professionals, preferably by referring them to manufacturers.  Use only the material suitable for marine use.  Make sure that you get to the root of the problem so that you don't put a "patch on a patch".  If a problem appears persistent or suspicious make sure that you carry enough tools and spare parts aboard so that you have a chance to fix it again if the problem would reappear while underway.

Temporary Field Fixes

They should strictly be used to bring you back from a trip to proper repair facilities.  Auto supply stores normally have hose repair kits that could come in handy.  Also available are cooling system sealers.  These products can be tried as temporary solutions to leaks on the fresh waterside.  Hardening putty of an epoxy type if also available and may prove useful.  Always carry extra hose and hose clamps aboard for emergency repair.

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