Monday, 5 December 2011

Winterising 2 - changing the antifreeze

As it must be at least three years since the engine antifreeze was changed, we decided to replace it. Antifreeze not only protects the engine against freezing, but is important in preventing internal corrosion. It is degradation of the corrosion inhibitor that limits its useful life and it is usually recommended that it is replaced every couple of years.

There are various types and colours of antifreeze. The original antifreeze in the cooling system was green - probably 'HOAT' (hybrid organic acid technology) in an ethylene glycol base. We have replaced it with Unipart 'Cool Blue' 2 year antifreeze, which is stated as being suitable for all types of diesel and petrol engines. This is a traditional ethylene glycol based mix.
One of the problems with a swim-tank-cooled marine engine is the large volume of coolant in the system compared to a vehicle engine. The volume of the swim tank in Patience was calculated at about 25 litres, which together with the engine block and connecting hoses would account for an estimated total coolant volume of between 30 and 35 litres. We therefore bought 15 litres of antifreeze to give a 43 to 50% mix. The recommended mix is 50% to cover the very lowest expected UK temperatures.

If you use a hydrometer for measuring the concentration, don't forget that the specific gravity of the mix depends on its temperature as well as its concentration. A 50/50 mix at 70 deg C has a similar specific gravity to a 30/70 mix at 20 deg C. Therefore, according to the hydrometer, the mix will seem to be weaker if measured at engine running temperature compared to the same mixture measured when cold.

Another problem is draining the system, as the bottom of the swim tank is right at the bottom of the engine compartment, so draining it into a bucket just doesn't work! We decided to drain it into the engine compartment bilges and then use the bilge pump to pump it into empty containers for safe disposal. This works up to a point, but it is difficult for the bilge pump to extract the last couple of centimetres, which had to be sponged out into a bucket!

We extracted a total of 30 litres of coolant, which, allowing for some coolant left behind at the bottom of the swim tank and engine block, confirmed our original estimate of 30 to 35 litres in total.

Having drained the system and replaced a couple of the flexible hoses that looked slightly worn, we put the 15 litres of new antifreeze into the system and topped it up with water. Running the engine for a few minutes made sure that the water and antifreeze were well mixed and that any air pockets had been eliminated. A final topping up with water was then carried out.
On the way home we disposed of the old coolant mix at the local recycling centre. Job done!

PS Based on our experience above, do this before it gets so cold that you're the one who needs anti-freeze! Make sure you have enough containers for the old liquid; they should be used only for anti-freeze so label them boldly! Finally, don't forget to wear disposable gloves, as antifreeze is toxic, and dispose of the old mixture responsibly - don't just tip it into the river!
See also the earlier post: Winterising

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