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Changing the Coolant on our Yanmar 3GM30F Engine

It’s been 5 years since we last did a coolant change on our Yanmar engine, so it was probably overdue. Coolant helps to keep the engine cool and prevent overheating plus it contains additives which aids the prevention of internal corrosion.

First up we needed to remove the existing coolant. We fitted a drain tube to the petcock (drain cock) on the port side of the engine block which led down to a 5 litre container lower down in the bilge – this would be used to collect the old coolant. There are a further two petcocks on the heat exchanger, one for draining the coolant from the heat exchanger only and one for draining the seawater from the heat exchanger. Neither are relevant to this task as the seawater side is undisturbed during this operation and as we are changing the entire contents of the cooling system, the lower petcock on the engine block does this job. (The coolant petcock on the heat exchanger is only used for working on the heat exchanger itself, where it would be unnecessary to drain the entire engine block of coolant.)

We lowered the expansion bottle to below the radiator cap height to stop any overflow, then undid the radiator filler cap (ensure engine is cool) before opening the petcock to start the drain down.

Our system holds approximately 5.4 litres of coolant though some will remain in the hot water circuit (calorifier) so don’t be surprised if you don’t extract this much. We decided against removing the hoses that feed the hot water heater circuit as we were concerned of introducing a lot of air into the system which in turn may have been difficult to bleed, electing to undertake more flushes to flush out this part of the system.

Once you are satisfied that the existing coolant has drained the process of flushing through really begins!

Close the drain cock, re-elevate the expansion bottle and fill the coolant system with distilled water to the top of the filler cap and burp (squeeze) the coolant pipes to expel any air. Replace the filler cap and top up the expansion bottle to the lower mark, also with distilled water. Burp the system again by squeezing the coolant pipes to help bleed any air out of system. Run the engine ensuring that the thermostat opens. You can tell that the thermostat has opened by feeling the cooling pipe on the forward face of the heat exchanger – if it gets hot then its working 😃. Run the engine for approximately 10-15 minutes, burp the pipes once again, then let the engine cool for least 20 minutes until it is safe to undo the radiator cap.

Lower the expansion bottle, undo radiator cap, undo petcock, drain distilled water, close petcock, re-elevate expansion bottle, fill system with distilled water, burp the system, replace radiator cap, top up expansion bottle with distilled water, burp the pipes again, run the engine, burp the pipes once more, ensure thermostat opens, cool engine.

And repeat.

And repeat.

And repeat.

In fact do as many as you feel is necessary! This is the difference in the colour of our after the first flush and the sixth flush:

Once you are happy that the system has been adequately flushed through drain down the last of the distilled water then top up with a 50/50 mixture of coolant and distilled water. Also top up the expansion bottle with the same mix. We used Granville Zerocol 48 Antifreeze & Coolant Concentrate which is a green coolant (our current existing coolant was also green). Many newer engines use pink coolant but we elected to stick to what we know. If you do intend to switch the green to pink then it is imperative that you remove all traces of the old coolant – the two should never mix.

Finally give the engine a good run, ideally for at least half an hour.

As always please consult a marine engineer if you have any doubts/concerns about undertaking this process. This post is for guidance only and is not intended to be a substitute for professional advice.


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