Dealing with Engine Temperature Problems

Temperature Gauge Problems Initially it is important to establish whether you have an overheating problem or a temperature gauge fault.  If you suspect a problem then it is a good idea to check the gauge against a thermometer that is known to be accurate.  The majority of marine thermostats open at 71°C and will be fully open at 82°C.  As a rule, a clean system, under moderate load should operate at the lower end of this range whilst a dirty system under a full load might operate in the upper range.  Anything outside this is abnormal and should be investigated.

Overcooling Whilst the most common problem is the engine overheating, the opposite can also happen and lead to long term problems.  A thermostat should have excess cooling capacity.  This compensates for the inevitable decrease in cooling capacity that results from the normal gradual build-up of dirt on the heat transfer surfaces.  A thermostat is fitted to avoid an over cooling problem in a clean, new system.  If there is a problem, quite simply, the thermostat is not working properly.  Ensure that it is of the right type and properly installed.  Do not assume that just because it fits it will function correctly.  There is a lot more to thermostat design than is often realised, so make sure that the thermostat functions properly by immersing it in hot water of known temperature.  The easiest way is to use a pan of water on a stove with an accurate thermometer.  Hold the thermostat by the flange with a pair of pliers and do not let either thermostat or thermometer rest against the bottom of the pan. The thermostat should open at the temperature marked on it and be fully open approximately 20°C higher.  Malfunctioning thermostats cannot be repaired, they must be replaced.

Overheating Overheating problems can be categorised into two basic problems that either alone or in combination with one another, will create overheating:-

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 110°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.  So the easiest wasy 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:-

1) Pump Problems The most common pump used 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 it’s flexibility due to old age and capacity.  To be on the safe side,  it is recommended that the impeller is replaced annually.  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 the raw water pump is belt driven, make sure that the belt has the correct tension.

2) 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 downstream.

3) Exhaust Elbow Restriction Over time, problems with rust build up in the exhaust elbows may develop.  Many exhaust elbows have several small holes in the area where the raw water enters the exhaust pipe.  The 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 if one elbow becomes blocked,  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 blocked elbow and matching exhaust manifold and exhaust pipe could burn and be destroyed.  It is recommended that you periodically during the season feel the exhaust elbows to make sure that they stay at a normal temperature. Clean or replace these elbows before they cause further damage.  Periodic flushing of the engine with fresh water will help minimise 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 in and out on a block only system, will be in the range of 10-20°F.   If manifolds are included in the fresh water system adds another 10-20°F.  Most people find 60°C to be the approximate maximum temperature that they can leave their hand on without discomfort.  Since fresh water temperatures normally are above 71°C, it is not practical to check this difference without special equipment.

Be careful when dealing with a suspected temperature problem, the cooling system contains hot liquid held under pressure


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