Marinpedia.
My boat
EN
SvenskaEnglishDanskNorsk (bokmål)SuomiÍslenskaDeutschFrançaisEspañolItalianoNederlandsPolskiPortuguêsУкраїнськаРусский
Menu

Engine not cooling – weak or no cooling water

Troubleshoot weak cooling water flow, from the intake and raw water strainer to the impeller and thermostat. Checks from Volvo Penta and Mercury MerCruiser.

Check step by step

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

  1. DIY

    Check that the seawater intake is open and free of seaweed or plastic.

    A closed or blocked intake stops the cooling water flow.

    Action: With the engine stopped: check the seacock or the drive's water intake. Only clear what you can reach safely. Don't run the engine without cooling water.

  2. DIY

    Check the raw water strainer, if fitted.

    A clogged strainer reduces flow even when the impeller is intact.

    Action: Close the seacock, clean the strainer basket and refit the cover and seal according to the owner's manual. Reopen the seacock and check for leaks. Don't force a stuck cover.

  3. DIY

    Look for loose hose clamps and damaged cooling water hoses.

    Air can be drawn in before the pump and water can leak out after it.

    Action: Close the seacock before disassembly. Fix visible faults as described in the model's manual; leave it to a workshop if you can't get the system watertight.

  4. Workshop

    Inspect the raw water pump impeller as described in the model's manual.

    A damaged impeller means poor cooling. Water reaching the pump doesn't prove the pump is working.

    Action: Replace a damaged impeller and seal. Look for missing vane fragments as described in the manual. Only open the pump if you have the right tools and spare parts; on some drives it requires disassembly best left to a workshop.

    impellerseal kit
  5. DIY

    Check the circulation pump belt, if the pump is belt-driven.

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

    Action: Check with the engine stopped. Tension or replace the belt as described in the correct manual; an electric or gear-driven pump doesn't have the same check.

    V-belt
  6. Workshop

    Have the thermostat, circulation pump and any heat exchanger checked.

    A faulty thermostat, pump failure or debris in the heat exchanger can cause overheating even with a working raw-water pump.

    Action: The workshop checks the flow and the correct thermostat for your specific cooling system variant. Don't choose a thermostat just because it physically fits.

    thermostatseal kit

When to call a workshop

Stop running the engine if cooling water stops flowing or the temperature rises. Call a workshop if simple checks don't restore the flow, if you can't seal a leak, or if the pump or drive has to be taken apart. Ask for help rather than test-running 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 and strainer, air leaks on the suction side, hoses, the belt-driven circulation pump and the thermostat. On engines with a heat exchanger, it may also be clogged. An intact impeller doesn't rule out the other causes.

How do I loosen a stuck raw water strainer lid?

Stop the engine and close the seacock before opening the filter. The D1/D2 manual describes cleaning but not any method for freeing a stuck lid. Get help if the lid cannot be loosened according to the filter manufacturer's instructions; a crack can let water in.

Related troubleshooting

Sources

Report an error on this page