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Heat Exchanger Coolant Loss

Posted: 13 May 2023
We often get told by owners that they find headertank water levels low when they do routine checks before going out, and despite then filling up, they again find the level low again the next day.
This is quite normal, as when the water in the engine heats up in running, it expands more than the metal of the engine components does and so excess water is forced out of the overflow pipe and lost, and so when the engine cools down again and coolant contracts, air is sucked in to replace it
Lancing Marine offer a specific header tank expansion tank kit. Please select the link or see the links at the bottom of this page, including for spare parts such as O-rings. We recomend this 0.75L tank as its designed for the task.

To always keep the headertank coolant at a good working level a set-up is needed to recover and re-use the expelled. A simple set-up would be something like this:


  1. Take a length of hose that is a good push fit onto the headertank overflow connection and run it down through the lid of a bottle or other container that will take 1-2 litres (a Coke bottle will do for trials).
  2. Ensure the hose reaches the close to the bottom of the container.The container may be above or below the headertank, position in a convient location. 
  3. Approximately fill the container one quarter full with water.
  4. Check the heat exchanger cap rubber seals. Ensure that both seal rubbers of the cap (one on the end of the spring and one in the cap that you grasp to remove it) are in good condition.
  5. Top up the Heat Exchanger tank.
  6. When the engine is hot observe the amount of coolant in the container and have another look when the engine has cooled right down.
  7. Do not remove the filler cap in this time! If you are happy that the system works, tidy it up with a nice, well secured container and carefully positioned hose. ..



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