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Engine not cooling: weak or no cooling water

If little or no water comes out of the exhaust, the flow is blocked somewhere between the intake and the heat exchanger. Check the intake, raw water strainer, hoses, impeller, belt and thermostat in that order.

Check step by step

Select your engine on its model page to see the relevant steps and part numbers.

  1. DIY

    Is the seacock open and the intake clear of seaweed and plastic?

    A closed or blocked intake stops all cooling water.

    Action: With the engine stopped, open the seacock fully and feel or look at the intake on the hull or drive. Clear it where you can reach safely. Don't start until the water has a clear path.

  2. DIY

    Unscrew the lid on the raw water strainer, if the boat has one, and look at the basket.

    A clogged filter restricts the flow even though the impeller is intact.

    Action: Close the seacock, rinse the strainer basket and refit the cover and seal as described in the owner's manual. Open the seacock and check that the cover seals tightly. If the cover is stuck, don't force it off.

  3. DIY

    Go along the cooling water hoses. Look for loose clamps, cracks and moisture.

    Before the pump, a leak draws in air. After the pump, the water leaks out.

    Action: Close the seacock before you loosen anything. Tighten clamps and replace cracked hoses as described in the manual. If you can't get it to seal, the workshop takes over.

  4. Workshop

    Open the raw water pump and inspect the impeller.

    A damaged impeller gives poor cooling. Water reaching the pump does not show that the pump is pumping.

    Action: Replace the impeller and seal if the vanes are cracked, bent or missing. Missing vane pieces must be found further along the system. Only open the pump if you have the tools and spare parts. On some drives the drive has to be taken apart, and that is a job for a workshop.

    impellerseal kit
  5. DIY

    Is the circulation pump belt-driven? Press on the belt and check whether it slips.

    A slipping belt is on the manufacturer's list of causes of high coolant temperature.

    Action: Do this with the engine stopped. Tension or replace the belt according to the owner's manual. Electric and gear-driven pumps have no belt to tension.

    V-belt
  6. Workshop

    Flow is good but the engine still runs hot? Then that leaves the thermostat, circulation pump and heat exchanger.

    A faulty thermostat, a broken circulation pump or debris in the heat exchanger causes overheating even when the raw water pump is doing its job.

    Action: The workshop measures the flow and chooses a thermostat to suit your cooling system variant. A thermostat that fits the housing is not necessarily the right one.

    thermostatseal kit

When to call a workshop

If the cooling water stops or the temperature rises, shut down. Call a workshop if the flow does not return after checking the intake, filter and impeller, if a leak cannot be sealed, or if the pump or drive has to be taken apart. Never test run without cooling water.

Frequently asked questions

The impeller is intact and water reaches the pump. So why is the engine still not cooling?

Check the intake, the seawater filter, air leaks on the suction side, the hoses, the circulation pump belt and the thermostat. If the engine has a heat exchanger, it may be clogged. An intact impeller tells you nothing about the other parts.

How do I loosen a stuck raw water strainer lid?

Stop the engine and close the seacock first. The D1/D2 manual describes the cleaning but gives no method for a cover that is stuck. Follow the filter manufacturer's instructions or get help. If the cover cracks, water will get into the boat.

Related troubleshooting

Sources

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