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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.

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

WARNING:  IF A TEMPERATURE PROBLEM IS SUSPECTED, BE CAREFUL.  THE COOLING SYSTEM CONTAINS HOT LIQUID UNDER PRESSURE AND A CARELESS OR THOUGHTLESS ACTION COULD RESULT IN SEVERE INJURY.

TEMPERATURE GAUGE PROBLEMS

Before spending any time trying to diagnose a temperature problem, make sure that you really have a problem.  Temperature gauges, especially the electric type, which are the most common in boats, are often very inaccurate and have temperature scales that are difficult to interpret.  Therefore, before worrying about a suspected temperature problem, check your temperature gauge against a thermometer of known accuracy.

NORMAL TEMPERATURES

Most marine FWC thermostats open at 160 degrees F and will be fully open at approximately 180 degrees 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.  Anything outside of this is abnormal and should be investigated.

OVERCOOLING

The most common problem is a temperature above the thermostat range--overheating.  The opposite may also happen and will create long term problems.  Any good FWC system should have excess cooling capacity.  This compensates for the inevitable decrease in cooling capacity that results from the normal gradual buildup of dirt on the heat transfer surfaces.  In order to make sure that such a system, when new and clean, does not overcool, a thermostat is used to control the flow of coolant to the heat exchanger.  An overcooling problem therefore, has a single simple explanation.  The thermostat is not functioning properly.  Check to make sure that it is of the right type and properly installed.  Do not assume that just because it fits it will function right.  There is a lot more to thermostat design than most people realize.  Do not experiment with unapproved thermostats.  If the right type and correctly installed make sure thermostat functions properly by immersing it in hot water of known temperature.  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 degrees 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 that either alone or in combination with one another, will create overheating. They are: lack of raw water flow, lack of fresh water 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 temp increase varies between different engine models but is usually in the range of 40 - 60 degrees F. In other words, if incoming raw water temperature is 70 degrees F, the outgoing water passing through the exhaust elbows will be in the range of 110 - 130 degrees F. This will create surface temperatures on the elbow that will be warm but not excessively hot.  So the easiest way to identify a raw water problem is to check whether or not 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 INLET SIDE OF RAW WATER PUMP

They could either be design problems such as undersized plumbing or 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

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 OUTLET SIDE OF 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 get clogged with the rust particles that a raw water-cooled elbow gives off.  Eventually, an exhaust elbow may get 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 and matching 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 fresh water will help minimize these problems.

LACK OF FRESH WATER FLOW

Lack of fresh water flow will show up as an increase in the temperature difference between in and outlet of heat exchanger.  Most modern engines 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 degrees F. If manifolds are included in fresh water system, add another 10-20 degrees.  Most people find 140 degrees F to be the approximate max temperature that they can leave their hand on without discomfort.  Since fresh water temperatures normally are above 160 degrees 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 fresh water 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 either be wire-reinforced or have a loose spring inside to prevent collapsing.
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.  Also, 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 engines 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 though 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

In this category will belong internal blockage in the engine, malfunctioning thermostats, and restrictions in the inlet side of the heat exchanger.

ENGINE INTERNAL BLOCKAGE

It is very unusual that a fresh water-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 get struck in the heat exchanger.


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Boatfix.com specializes in marine engine parts and boating accessories; including risers, manifolds, alternators, starters, water pumps, bilge pumps, pistons, gaskets, lifejackets ( PFD ), hoses for Mercruiser, Crusader, OMC, Volvo, Sea Maxx, Evinrude, etc. (Gas and Diesel)- you name it.  If it goes on a boat, it's in our catalog!