• Too Hot or Too Cold, Troubleshooting Marine Cooling Systems

    It should go without saying to use caution when troubleshooting an engine suspected of overheating. On freshwater cooled systems the coolant is under pressure and if hot may cause severe burns.

    If you suspect your engine is running either too hot or too cold, verify that the problem actually exists. Temperature gauges are prone to fail or provide inaccurate information especially with age. If your gauge indicates the engine is running hot, shut the engine down immediately to avoid serious damage. One of the best tools for troubleshooting cooling system problems is a handheld laser thermometer. They are relatively inexpensive and widely available on the internet and local parts stores. If you suspect your temperature gauge may be faulty, see the Troubleshooting Those Pesky Gauges article.


    Most marine engines operate at cooler temperatures than automobile engines. Early model marine engines utilized thermostats designed to open at appx 143 degrees, fully open at appx 160 degrees and maintain an engine operating temperature range of appx 145 ? 165 degrees. As the engine aged it tended to operate toward the upper temperature limit. Most late model marine engines are designed to operate warmer with thermostats designed to open at appx 160 degrees, fully open at 180 degrees and maintain an operating temperature roughly within that range depending on the age and efficiency of the engine. The thermostats main purpose is to bring the engine quickly up to operating temperature and maintain a minimum temperature. It will fluctuate from full closed to full open as necessary to increase temperatures. Cooling the engine is not the primary function of the thermostat. That job belongs to heat exchangers and the pumps delivering cool water.

    There are many types of thermostats. The temperature rating is normally stamped on the flange of the thermostat. The manufacturers? service manual should be consulted to aid in the troubleshooting process. It will indicate the correct thermostat and proper installation steps. Additionally the service manual will provide cooling system troubleshooting steps along with guidance on rebuilding and replacing faulty components.

    If you have a used boat, do not assume it has the correct thermostat or that the thermostat is installed correctly. In the process of troubleshooting a crusader engine running too cool, I discovered it had the wrong thermostat, installed with the gasket on the wrong side. Once that was corrected the engine operated within the correct temperature parameters.

    Thermostat Test Procedure

    Begin heating a pot of water on a stove. Suspend the thermostat along with a thermometer in the slowly heating water with a string (do not allow the thermostat or thermometer to touch the bottom or sides). The thermostat should begin to open at the temperature that is marked on it and should be fully open at 20 degrees F higher. Allow the water to cool, the thermostat should be fully closed no later than 10 degrees F below the temperature marked on it. If the thermostat does not meet these specs, replace.


    Overcooling is not a common problem but can occur. Simply put overcooling is a result of excess cooling water flowing through the engine. This results in too much heat being removed preventing the engine from reaching proper operating temperature. If this occurs the thermostat is not performing as expected. After removing the thermostat housing cover and before removing the thermostat, note how the thermostat is installed. Is it installed correctly? Are the gaskets in the proper place and sequence? Now gently remove the thermostat. Is it stuck open? If not does it have any leakage? Leakage indicates the thermostat is not closing completely (a small amount of leakage at 1 or 2 points along the perimeter are acceptable). Is the thermostat the correct one? If all of the above steps don?t reveal the problem, either test the thermostat as explained in the thermostat test procedure or replace with a new one.
    Attachment 89


    Overheating problems can be caused by an individual component or a combination of components. For a seawater cooled engine overheating is caused by a lack of sea water flow. For fresh water cooled engines, overheating may be caused by: lack of sea water flow, lack of fresh water flow, or heat exchanger problems.

    Sea Water (Raw Water) Cooled

    Water flow for most seawater cooled engines is relatively simple to follow. Consult the manufacturer?s service manual for the flow pattern specific to your engine. An example flow pattern is discussed in the following paragraphs.

    On Inboard/Outboard boats seawater is normally pumped into the system via an impeller pump in the sterndrive. The seawater then is directed to the thermostat housing and may pass through a power steering fluid cooler along the way.

    On Inboards the seawater is pumped into the system via a through hull in the bottom of the boat. It then passes through a shutoff valve and strainer on its way to the seawater pump. From this point the seawater flow proceeds to the thermostat housing and may pass through a power steering fluid cooler along the way. The thermostat housing then mixes some of the cool seawater with the hot seawater from the engine where it is sent to the engine circulating pump.

    The remainder of the cool seawater may be sent to the exhaust risers and pumped overboard. From the engine circulating pump the seawater is pumped through the engine cooling passages then back to the thermostat housing. Some of the water is directed back to the engine circulating pump mixing with the cool seawater, while the remainder passes through the thermostat and is sent to the exhaust manifolds to be discharged overboard. See the Cooling System Flow Example article for an interactive example.

    Attachment 87
    Troubleshooting Seawater (Raw Water) Cooled

    A key to troubleshooting this problem is to keep in mind the possible issues that could cause a lack of seawater flow. A malfunctioning thermostat, faulty seawater pump impeller or restrictions in hoses, engine block, exhaust risers, or manifolds are just a few of the possible problem areas. A good handheld infrared thermometer is a valuable tool for troubleshooting engine overheating problems. It is helpful to use the thermometer to find various temperatures at key points along the seawater flow pattern. I use a pencil and write these temperatures on the water flow schematic in the service manual. This helps me to develop an overall mental picture of what the water flow is doing temperature wise. Since thermostats are normally easy to replace and relatively inexpensive, consider replacing them. Even if the thermostat ends up not being the source of the problem it does not hurt to replace them every few years. If this does not solve the problem, start at the seawater source.

    I/O Engines

    1. Ensure the intake water passages on the foot of the sterndrive are not blocked with debris.

    2. As you trace the water flow from the sterndrive, check for leaks, collapsed or kinked hoses.

    3. If a power steering fluid cooler is installed, in many cases you can remove the seawater hoses and pass water through it with a water hose to verify that it is not clogged.

    Inboard Engines

    1. Check for debris blocking the thru-hull or a clogged strainer.

    2. Ensure the intake hose for the seawater pump is not leaking, collapsed or kinked. Unless these hoses are very short, they should be wire reinforced to prevent collapse. While the engine is running the seawater pump may be collapsing the hoses enough to restrict water flow. Then when the engine is shut down the vacuum is removed and the hoses appear normal. Also since this hose is on the suction side of the pump, ensure the hose clamps are tight. If not the seawater pump could be sucking air into the system.

    3. Check the power steering fluid cooler as described above, if installed.

    4. Check the hose from the seawater pump to the thermostat housing.

    Common To Both Inboard and I/O

    1. From the thermostat housing check the intake hose to the engine circulating pump. Ensure there are no leaks and the hose is not collapsed.

    2. Make sure the engine circulating pump is not leaking. In some cases you can check the condition of the pump by grasping the belt pulley and see if it has any side to side play. If it does it means the bearings are going bad and it needs replaced.

    3. Check the hoses from the thermostat housing to the exhaust manifolds and risers for leaks, loose clamps etc.

    If none of the above steps indicate the problem, more detailed troubleshooting is required.

    1. Thermostat: test or replace thermostat if not already accomplished (procedure described above)

    2. Seawater Pump: if the seawater pump is belt driven ensure the belt is in good condition and has the correct tension. The manufacturer?s service manual will have a detailed procedure to test the output of the seawater pump. In most cases it is easier to just replace the rubber impeller and should be replaced on a yearly basis. A common indication of a faulty impeller pump is an engine running hot at idle but cools down with increased rpm. As the rubber impeller ages the vanes begin to retain their curled shape and lose their flex. As the engine is idling the curled impeller is not very efficient but as the rotational speed increases the vanes tend to return to their original shape. This along with some positive pressure on the sterndrive water inlets can provide enough additional flow to cool the engine down. If you discover parts of the impeller missing during the replacement process, you will need to look for the missing blades, they could be stuck downstream impairing the water flow.

    3. Engine Blockage: engine blockage can be a factor especially with an engine that has been operated in saltwater and not flushed on a regular basis. Scale and rust buildup within the engine are more likely to pass from the engine and block items downstream. If the engine is suspect consult the service manual for the flushing procedure.

    4. Exhaust Elbows and Manifolds Clogged: some exhaust systems have small water passages that can be clogged over time with rust and scale from the engine. This may require their removal, inspection and cleanup. On an engine with a single exhaust manifold/elbow combination the engine will normally heat up prior to the manifold. On boats with two exhaust manifold/elbow combinations one side may be blocked and heat up considerably more than the engine. This can be a dangerous situation since the exhaust can approach extreme temperatures without warning.

    Troubleshooting A Closed Cooling (freshwater cooled) System

    Essentially the same troubleshooting process can be followed for a closed cooling system, with additional consideration given to the heat exchanger.

    A closed cooling system actually consists of two subsystems: seawater system providing cooling water to the heat exchanger, and a freshwater system that circulates through the engine and to the heat exchanger and back. Most closed cooling systems route the seawater from the heat exchanger to the exhaust riser and discharge it overboard.. Meanwhile the freshwater is circulated through the exhaust manifolds, then heat exchanger on its path back to the engine.

    There are a few systems that do not route freshwater through the manifolds. On those systems the seawater is generally routed to the manifold then through passages into the riser before being discharged overboard. See the Cooling System Flow Examples article for an interactive example.

    Lack of freshwater flow through the heat exchanger can be a source of overheating. If the closed cooling system does not circulate through the manifolds the temperature difference between the freshwater flow into the heat exchanger versus the freshwater flow out of the heat exchanger is normally in the 10-20 deg F range. If freshwater is circulated through the manifolds the temperature difference may by higher, as much as 20-40 deg F. If temperature differences between inlet and outlet are higher than this suspect a lack of freshwater flow.

    Lack of raw water flow could be caused by a faulty engine circulating pump, engine blockage, restrictions in the heat exchanger, faulty thermostat or manifold blockage if it is part of the closed freshwater system. All of these issues have already been covered in the previous sections except the heat exchanger which will be covered below.

    Heat Exchanger

    Most heat exchanger problems are caused by debris causing a blockage or the heat exchanger getting dirty as it ages. Depending on the amount and size of the debris, it may be possible to remove it by back flushing the heat exchanger. Remove the hoses from the heat exchanger and flush water through it in a direction opposite its normal in/out flow. Some exchangers have an end cover that is removable to make cleaning and debris removal easier. To take care of dirt and scale buildup the heat exchanger should be removed from the engine. Any of the more reputable radiator cleaning solutions may be used to clean the heat exchanger. A small diameter brush with a flexible handle may be needed to clean some of the passages. If the buildup is excessive the heat exchanger may be taken to a radiator shop for cleaning. Make sure the shop has experience with marine heat exchangers. Professional cleaning is normally a two step process using an alkaline solution to remove dirt and then an acidic solution to remove scale buildup.

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    Comments 5 Comments
    1. floater's Avatar
      floater -
      Excellent article, thanks for taking the time to put this together, it helped find the source of my problems.
    1. sdemino's Avatar
      sdemino -
      Great article, one other thing to check before moving on, This happened to me. If you have a Hot water heater integrated into the engine cooling system (such as an Atwood with heat exchanger) make sure it is working correctly. NO leaks, obvious, But I believe there is a flow restrictor in that heater to prevent to much coolant leaving the engine. If that restrictor rusts out your engine will run much hotter then specs. I replaced thermostats, hoses, impellers nothing worked until I took the heater out of the equation and the engine temp was perfect!!!!!!!!! Now I run heater strictly off generator. BTW the water temp in that heater would scald you!!
    1. TimG's Avatar
      TimG -
      Great info thanks Sdemino!
    1. captmike2's Avatar
      captmike2 -
      WOW! Great detail of water flow and explanations. I was motoring my 27' Concorde with single 318 FWC for 5 miles before it started heating up. Pulled the thermostat and it now runs around 190 degrees. I still think I have some blockage somewhere-thanks for advice!
    1. Roger's Avatar
      Roger -
      Quote Originally Posted by captmike2 View Post
      WOW! Great detail of water flow and explanations. I was motoring my 27' Concorde with single 318 FWC for 5 miles before it started heating up. Pulled the thermostat and it now runs around 190 degrees. I still think I have some blockage somewhere-thanks for advice!
      May be your impeller in the raw water pump is missing a few fins. You can easily check by pulling the hose at the 1st heat exchanger and look for rubber bits.
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