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Troubleshooting for Fresh Water Marine Cooling System

Feb 15th, 2012 by admin Shop the PPT Online Store to Buy Mercruiser Closed Cooling Systems....Read More
marine heat exchangers, fresh water cooling marine heat exchangers, fresh water cooling Fresh Water Cooling Trouble Shooting, Fresh Water Cooling
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Trouble Shooting Table Leakage Problems Temperature Problems Overcooling Overheating Heat Exchanger Defects
Temperature problems  Normal temperatures

Most marine FWC thermostats open at 160°F and will be fully open at approximately 180°F.  A new clean system under moderate load should operate at the lower end of this range and a dirty system under full load might operate in the upper range.  Any temperature that reads outside of this range is abnormal and should be investigated.

Temperature gauge problems

Before trying to diagnose a temperature problem, check the temperature gauge.  Temperature gauges, especially electric, are often inaccurate and have temperature scales that are difficult to interpret.  Before investigating a suspected temperature problem, check your temperature gauge against a thermometer of known accuracy.

Trouble Cause Remedy
OVERCOOLING Thermostat not functioning properly.
  • Check to make sure that the thermostat is of the right type.
  • Check to make sure that the thermostat is properly installed.
  • Make sure thermostat functions properly.
OVERHEATING Lack of raw-water flow
  • Restrictions on the inlet side of the raw-water pump
  • Restrictions on the outlet side of the raw-water pump
  • Raw-water pump problems
  • Exhaust elbow clog
  • Restrictions on suction side of water pump
 Lack of freshwater flow
  • Jacket water pump problems
  • Restrictions on pressure side of jacket water pump
  • Engine internal blockage
  • Thermostat not functioning properly
  • Restriction in the heat exchanger inlet
  • Dirt build-up

Over-cooling Though the most common problem is overheating, the opposite may also happen and will create long-term problems.  Any good Fresh Water Cooling System should have excess cooling capacity.  This compensates for the inevitable decrease in cooling capacity that results from the normal buildup of dirt on the heat transfer surfaces.  In order to make sure that such a system, when new and clean, does not over-cool, a thermostat is used to control the flow of coolant to the heat exchanger.  Over-cooling can only result from the thermostat not functioning properly.

Thermostat NOT functioning properly. Check to make sure that the thermostat is of the right type.  Do not assume that just because it fits it will function properly.  There is a lot more to thermostat design than most people realize.


Check to make sure that the thermostat is properly installed.

If the right type and correctly installed, make sure thermostat functions properly by immersing it in hot water of known temperature.  The easiest way is to use a pot of water on a stove with an accurate thermometer.  Hold thermostat by the flange with a pair of pliers.  Do not let either thermostat or thermometer rest against bottom of pan.  Thermostat should open at temperature marked on it and be fully open approximately 20°F higher.  It should close again when immersed in colder water.  Malfunctioning thermostats cannot be repaired, they must be replaced: make sure you get the right type.

Overheating Overheating problems can be categorized into three basic problems, which either alone or in combination with one another will create overheating.  They are lack of raw-water flow, lack of freshwater flow, and heat exchanger defects.

Lack of raw-water flow Lack of raw-water flow will show up as an excessive increase of the raw-water temperature as the raw-water passes through the cooling system.  Normal temperature increase varies between different engine models but is usually in the range of 40-60°F.  In other words, if incoming raw-water temperature is 70°F, the outgoing water passing through the exhaust elbows will be in the range of 110-130°F.  This will create surface temperatures on the elbow that will be warm but not excessively hot.  Therefore, the easiest way to identify a raw-water problem is to check whether the engine overheating is combined with excessive temperatures on the outlet side of the raw-water system.  If the raw-water side is to blame, there could be three basic reasons.

Restrictions on the inlet side of the raw-water pump Of the pump could be design problems such as undersized plumbing.  In addition, it could be maintenance problems such as debris buildup in seacocks, strainers or other components located on the suction side of the raw-water system.  Check and clean.

Raw-water pump problems The most common pump in use today is the rubber impeller pump.  The impeller in this pump must never be run dry or it will be ruined.  Eventually this impeller will also lose some of its flexibility due to old age and lose capacity.

In order to be on the safe side, we recommend that you replace the impeller annually especially if it is located in the sterndrive and difficult to service during the season.  Keep the old impeller as a spare.  If the impeller is damaged with blades missing, make sure that you find the missing blades.  They could be stuck downstream from the pump interfering with proper flow.  If raw-water pump is belt driven, make sure that belt has correct tension.

Restrictions on the outlet side of the raw-water pump These restrictions are often in the form of raw-water debris accumulating on the inlet side of oil coolers and heat exchangers.  Always check the units closest to the pump first and work yourself downstream.

Exhaust elbow clog Over a few years, a problem with rust buildup in the exhaust elbows may develop.  Many exhaust elbows have several small holes in the area where the raw-water enters the exhaust pipe.  These orifices are designed to ensure proper water distribution at this point.  Unfortunately, because of their small diameter they tend to be clogged with the rust particles that a raw-water-cooled elbow gives off.  Eventually, an exhaust elbow may be completely plugged up preventing raw-water from entering the exhaust pipe and thereby creating a fire hazard.

In an in-line engine with a single exhaust elbow, this complete blockage will automatically cause engine overheating before the exhaust overheats.  This will signal a problem before a fire hazard develops.

In a V-type engine however, the situation is more dangerous since one elbow could become plugged and the other one not.  In this case, sufficient raw-water may be able to exit through the open elbow to keep enough raw-water flowing through the engine heat exchanger.  The engine may not overheat but the plugged elbow, exhaust manifold, and exhaust pipe could burn and be destroyed.

We recommend that you periodically during the season feel the exhaust elbows to make sure that they stay at a normal and even temperature.  Clean or replace these elbows before they cause further damage.  Periodic flushing of the engine with freshwater will help minimize these problems.

Lack of freshwater flow Lack of freshwater flow will show up as an increase in the temperature difference between in and outlet of heat exchanger.  Most modern engine(s) have a flow rate at a level where the temperature difference between in and out on a block only system, will be in the range of 10-20°F.  If manifolds are included in freshwater system, add another 10-20°F.  Most people find 140°F to be the approximate max temperature that they can leave their hand on without discomfort.  Since freshwater temperatures normally are above 160°F, it is not practical to check this difference without special equipment.  If the engine is cool enough to be touched, it is probably running too cold. If lack of freshwater flow is the problem, these are the basic causes of it.

Restrictions on suction side of jacket water pump Besides design problems such as an undersized heat exchanger outlet connection and/or hose, the only thing that can go wrong would be a hose being sucked closed.  That is why hoses on the suction side, unless they are very short, should be either wire reinforced or have a loose spring inside to prevent collapsing.

Jacket water pump problems Some older designs may have a rubber impeller pump or even gear pumps.  If so, the rules relating to raw-water pumps apply.

Since rubber swells at increasing temperatures, it may be a good idea to give the pump impeller more space by using a thicker gasket under the pump cover.  In addition, it is a good idea to use a full 50/50 antifreeze solution since the antifreeze will help lubricate the pump.

The vast majority of modern marine engine(s) use the standard automotive centrifugal jacket water pump.  If these pumps have been operated in a raw-water system, they may have corrosion damage and may need to be repaired or replaced.  Otherwise, this pump is very trouble free.  The only service necessary should be to make sure that it operates at proper speed, by checking that the drive belt is not slipping.  In the long run, it may develop a leak or bad bearing, just like in a car, but will continue pumping and is seldom causing overheat problems.

Be aware that if the engine is opposite rotation from the automotive standard, the pump may have a somewhat lower flow rate, which may result in slightly higher operating temperatures.

Restrictions on pressure side of jacket water pump In this category belongs: engine internal blockage, malfunctioning thermostats, and restrictions in the inlet side of the heat exchanger.

Engine internal blockage It is very unusual that a freshwater-cooled engine would have this problem.  If an engine that has been operated on raw-water is converted to FWC, it is possible that old rust and scale deposits will create restrictions.  That is why it is important to try to remove as much of this rust and scale as possible, as part of the installation process.  Some of the debris may not come loose until normal engine operating conditions with higher jacket water flow, heat, and vibration.  It is unusual that this debris will create blockage within the engine.  More likely, it will be flushed along and are struck in the heat exchanger.

Malfunctioning thermostats 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.

Restrictions in the inlet side of the heat exchanger 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.

Heat exchanger defects Since heat exchangers contain no moving parts there is very little that can go wrong with them except that they can be plugged with debris and that they will become dirty.  They can also develop internal and external leaks.  Leaks 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.

Debris blockage 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.

Dirt build-up 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, "fouling factor" can only go so far and eventually a thorough cleaning of the heat exchanger may be necessary.

Proper cleaning is a two-step operation.  First, clean the unit with a strong alkaline solution to remove organic dirt such as oil.  Second, clean the unit in an acid solution to remove scale.  The most drastic solution is to remove the heat exchanger and take it to a radiator shop for a complete cleaning.  Make sure that the radiator shop has experience with marine heat exchangers.

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.  The small tubes on the raw-water side can be cleaned by using a small diameter long handle brush, similar to what is used for cleaning a rifle barrel.  It is possible to acid clean the raw-water side 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.

Leakage problems 

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.  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 External leaks on the raw-water side seldom create major problems until they reach a high level.  They should be fixed since a small leak easily could develop into a major one.  Raw-water can do major damage if allowed to encounter 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.

Freshwater leaks

External leaks External leaks o n the freshwater side 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 be 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 Internal leaks o n the freshwater side can be internal to the heat exchanger.  The freshwater will leak out into the raw-water side 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 freshwater side will help locate the leak. 

Engine leaks A more serious form of internal leak would be one within the engine where the coolant, 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 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 de-aeration.  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 coolant contaminating the oil.  This is why we recommend running an engine initially on plain water to confirm 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 to create a permanent solution by bringing the leaking components into the proper condition.  If major components such as the heat exchanger leaks, make sure they are repaired or replaced by competent professionals, preferably by referring them to manufacturers.  Use only 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 under way.

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 freshwater side.  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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