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The Normal Temperature of a Boat Engine (Prevent Overheating)

There is nothing quite like piloting a fast-speed boat across a stretch of water; bouncing across other boats wakes to the sound of a powerful engine propelling you forward. Keeping the engine in good order requires the boat captain to be aware of the state of the engine and the temperatures at which it is operating.

The temperatures that a boat engine should be run at vary depending on the type of motor.

  1. 2-stroke motors operate between 118F – 160F (47C – 70C).
  2. 4- stroke motors operate between 115F – 125F (46C – 52C).
  3. Inboard engines operate between 160F – 180F (70C – 82C).

Modern engines are very reliable and are designed to operate at relatively high temperatures, which is where they are most efficient. However, if the system experiences a fault unless the boat captain responds immediately, significant damage may be caused.

boat engine

Outboard And Inboard Engine Operating Temperatures

Internal combustion engines require heat to work. However, they operate most efficiently if the temperatures are managed to fall within a specific range.

To control this, marine engines are equipped with cooling systems that draw water from outside the boat through the engine’s water jacket and ports by way of a mechanical impeller. While the water is flowing through the system, it absorbs the heat from the engine. At the end of the cycle, the water is expelled through the exhaust by the propellor.

Inboard and Outboard Engines

A thermostat monitors the engine temperature. As it rises, the thermostat opens and allows water in. When the engine temperature decreases, the thermostat closes (either completely or partially) and prevents cod water from flowing and cooling the engine too much.

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Newer marine engines use an enclosed cooling system that has two water systems.

  1. The first is a small tank filled with coolant and freshwater. This freshwater is circulated through the engine and a heat exchanger.
  2. The second pulls water from outside the boat and through the heat exchanger.
  3. The water from outside the boat is cooler and absorbs the heat from the freshwater system, and it is then pumped out the exhaust.

When everything is working correctly, the optimal temperatures at which a boat engine runs are:

Two-Stroke Outboard Engines

The thermostat opens at 118F – 160F (47C – 70C). The overheat warning activates at 160F (70C).

Four-Stroke Engines

The normal operating range is 115F – 125F (46C – 52C) and can increase  to 150F (66C).

Inboard Four-Stroke Engines

The normal operating temperature is 160F – 180F (70C – 82C).

How To Trouble Shoot A Heat Running Engine

mechanic boat engine

While both modern outboard and inboard engines are very reliable, an overheating event can cause serious damage, most of which is easy to detect and rectify if dealt with timeously.

The most common reasons for both an inboard and an outboard engine overheating are.

  1. A damaged or inoperable thermostat may cause the engine to overheat.
  2. If the impeller is damaged or inoperative, the engine will overheat.
  3. Dirty engine oil may cause the engine to overheat.
  4. Operating the boat in polluted water or heavy algae growth may cause the engine to overheat.
  5. Saltwater builds up a corrosive scale, which can block the cooling system when the engine operates above 140°.
  6. If an outboard engine is incorrectly mounted too high, it will cause overheating.
  7. Starting the engine without muffs delivering water will cause overheating when on dry land.
  8. The engine will overheat if water does not constantly flow out of the pilot hole.
  9. It will overheat
  10.  if the engine is tilted too far out of the water while running.
  11. If the water intake for the water cooling passage is blocked, the engine will overheat.
  12. If you spray silicone protectant over a running engine, it may overheat.
  13. Using the incorrect grade of engine oil may cause the engine to overheat.
  14. A damaged cylinder head gasket will cause an engine to overheat.
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Just because the temperature gauge doesn’t seem to read that the engine is overheating does not mean there isn’t a problem.

Particularly after the engine’s systems correctly identify an overheating event, the actual water temperature may not be displayed on the temperature gauge if you continue to operate the motor.

The temperature gauge records the water temperature in the engine’s cooling system. If the water does not get into the cooling system, the temperature gauge will record the temperature of the air in the system rather than the water.

It will cause the temperature gauge to underrecord the temperature, and even if there is a serious overheating event, it may not register on the indicator.

You mustn’t write off an overheating warning from the system.

What Happens If The Boat Engine Runs Too Hot?

installing new engine boat

If the engine overheating alarm sounds or the warning lights turn on, immediately switch off the engine and check the following.

  1. Turn the engine off and tilt it to check that the inlet pipe for the cooling system is not blocked.
  2. An inboard engine has a sea filter, which is a transparent bowl that you can easily check for a blockage.
  3. If it is a four-stroke engine, use the dipstick to check the oil level.
  4. If there is oil, check that it is not a milky color (if it is, it means that water has entered the system).
  5. If it is a two-stroke engine with a separate oil tank, check that the oil tank has sufficient oil in it.
  6. While the engine is idling, check that water flows out of the pilot hole.
  7. Listen carefully for any unevenness in the engine idling speed.
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Remember, just because the overheating alarm does not sound a second time does not mean there is no damage.

If you are away from the land, you have three options.

  1. Use the ores to row the boat back to land (hard, slow work).
  2. Ask another boat for a tow (best option)
  3. Run your boat engine at idle speed (no higher) and slowly return to the shore.

If the system detects an unacceptably low oil level, most engines have a low oil level alarm that will sound, put the engine into limp mode, and not allow the engine speed to exceed 2,000-3,000 rpm.

The Consequences Of An Overheating Engine

If the engine overheats, there are several possible consequences. While there are many possible reasons an engine overheats, the most likely is.

If You Reacted Correctly To An Overheating Event

If you followed the actions listed above and did not continue running the engine after an overheating event, you probably only need to service the cooling system.

The easiest way to check if the cooling system is operating correctly is to check that the tell-tale water is squirting out of the pilot tube when the engine is idling.

If water is not flowing through the cooling system, three potential causes are.

  1. Check that cooling inlet pipes (grids) are not blocked.
  2. The thermostat is damaged or worn and is not opening to let water in. 
  3. The impeller, which forces the water through the system, is damaged and needs replacing.

The Engine Needs To Be Serviced

Two components may be damaged if you continue to operate the engine despite the overheating warning.

  1. Cylinder head gasket.
  2. The piston rings (and even the pistons).
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Each of these will require an engine rebuild, which is costly.

The Engine May Seize

In the worst-case scenario, if the overheating is allowed to continue, the pistons and the piston rings will eventually expand to a point where they cannot move up and down in the cylinder, which will bring everything to a grinding halt.

The fix is either a complete engine rebuild or possibly a new engine.

Conclusion

The boat operator must stay aware of the temperatures at which the engine is operating and be mindful of any sound alarms. In particular, the captain should respond immediately to the overheating and low oil level alarms. 

If there is an overheating event, action must be taken immediately, and where possible, the boat engine should be switched off until the cause of the alarm can be identified.