Coolant airlock? dribbles from overflow

[SimonA]

I have noticed for a while that the coolant in my header tank disappears after a run but the engine never overheats. It takes about a litre or two to top it back up. On investigation I found it drips out of the overflow (pours out at high engine revs).

The engine is a BMC 1.5 diesel, totally rebuilt 2 years ago by Calcutt boats. I have no issues with the engine, it starts first time and runs nicely.

The only thing I have done recently was to change the thermostat, the original had 3 holes drilled through it and this stopped the engine reaching operating temperature so I replaced it for a new one without holes.

I think I must have an airlock which is causing the coolant to drip from the overflow. Question is how exactly do you bleed the cooling system?

 

[RobWales]

Check that the spring on the header tank cap is not shot allowing coolant to blow past the mating sealing faces and out the overflow!

 

[scottie]

Check each run and of its the same each time don't top up next time and see what happens .
It's a common scenario

 

[VicS]

As scottie says you do have to let it find its working level rather than expect it to stay full to the top.
Losing all the water from the head tank is not right though.

Check the pressure cap is sealing properly.

Id agree that it is probably an air lock.

Modern cooling systems do tend to have vents that automatically allow air back to the header tank or even have little bleed nipples on them but I doubt if that applies to a BMC 1.5.

You could try catching the coolant that is ejected and returning it to the head tank as it cools. I've done that and after a while it settled down and was Ok.

You could put a bottle in to catch the coolant and arrange it so that it sucks it back in as it cools. That's what I did eventually but I had to put a secondary seal on the cap to ensure an air tight seal with the top of the neck.

Id try one small hole in the thermostat, just big enough to allow any trapped air to escape.

 

[SimonA]

Check that the spring on the header tank cap is not shot allowing coolant to blow past the mating sealing faces and out the overflow!

Thank you I tried looking on ASAP supplies for a replacement spring loaded cap but they only do a blanking cap. Is there a way of checking my cap to see if that is the problem?