Engine overheating.

[Graham_Wright]

I currently take the calorifier input from below the thermostat housing and the return is teed into the return from the heat exchange to the pump. Others have referred to a tapping in the head but I can't see one (either on the spare engine or the installed one).
Before leaving the boat, I closed the inlet and outlet to the calorifier and ran the engine. After ten minutes, the temperature stayed normal (a bit low as I have left the thermostat out.). I am now suspecting the header tank. It feeds into the pump inlet (teed into the calorifier outlet) and I noticed that the cap was unsealed. As it is supposed to take the normal expansion pressure in the engine, the absence of back pressure would lower the boiling point (I think).
I have read that the heatexchanger is supposed to act as a header tank but this means that at start up, the top of the stack is uncovered. What to do? I have never had this problem in the past four years of running so finding the cause has become a mission.

 

[Graham_Wright]

I believe it might be this one (from another forum), indicate by a brass plug dropped in.

 

[Boater Sam]

That is where the tapping for the flow to the calorifier should be. It vents the back of the head. Take the flow to the bottom coil connection on the calorifier, it then vents easily.
Calcutt can supply the hose adaptor to the UNC threaded hole that the 1.5D engine needs, the 1.8D head is threaded 1/2" BSP, don't ask why they are different but they are.

Where you have the calorifier hose connected is wrong, it can suck air into the pump from the calorifier hoses, both are connected to a low pressure area.

 

[Graham_Wright]

Yes - I understand now. Thanks for helping my comprehension.
But;- curiouser and curiouser
I have found another tapping at the front of the head (on top) that appears to be connected to the water ways.
I tried what I thought was a UNF plug gauge (wrong drawer) and it gauged nicely. Checking, it turned out to be M16 fine!
However, it is almost a size for size match with 5/8" UNF.
Is this an alternative connection for the return (or maybe the source)?

 

[Boater Sam]

I think that it is just another plug put in when the head is machined, there are several brass blanking plugs in these heads.
These engines were fitted in cars and vans and there were some industrial units as well so extra tapping could have been for all sorts of things.
The take off at the back of the head was to even the temperature across the head as I understood it but as the van heater was connected there and it could be turned off, it seems nonsensical.

 

[Graham_Wright]

I don't believe it is a blanking screw. It connects directly to the water ways and there seems no reason to associate it with the casting core.