Running engine when changing coolant?

[KompetentKrew]

I need to flush the coolant in my Nanni 4150HE.

A number of guides say to run the engine before doing so, to ensure the thermostat opens. E.g. this YouTube video says to run it up to about 200°F, 90°C.

The 2014 PBO article, How to service your marine diesel engine, says the opposite - "Only change the coolant when the engine is cold".

Which is correct, please?

I think the YouTube video says to leave the engine to cool for a few minutes after running it up to temperature, because obviously one doesn't want to bleed the coolant whilst it is still scalding hot. Had already started the engine before doing more reading, so am now confused and conflicted with doubt.

 

[lw395]

I wouldn't want to pour cold water into a hot engine.

 

[KompetentKrew]

Good point, but it needs draining first anyway.

What about a warm engine?

 

[Graham376]

Fine to do it cold.

 

[vyv_cox]

I cannot see any reason to do it hot. Coolant can be drained or filled with the thermostat closed. Worth checking the level after heating up in case of an airlock but topping up may not be needed.

Boat engines are a little different as a calorifier may be fitted, sometimes quite some way from the engine. Airlocks can be a problem, especially with directly cooled ones.

 

[RichardS]

It depends whether you are replacing the coolant or flushing the system.

If replacing the coolant, then just drain it out cold and then pour in new. If you're flushing you'll have to run it up to temperature between each flush so will then be draining whilst it's still hot .... but not scalding hot.

Richard

 

[KompetentKrew]

The old coolant is / was pretty brown and gunky, so a knowledgable acquaintance suggested draining it, refilling with water and circulating that for an hour or two, before refreshing.

At what temperature is it safe to drain please, Richard? I ran the engine for an hour or 90 minutes earlier but, having seen these replies I stopped it about 30 minutes ago and was going to leave it until tomorrow.

 

[VicS]

The old coolant is / was pretty brown and gunky, so a knowledgeable acquaintance suggested draining it, refilling with water and circulating that for an hour or two, before refreshing.

At what temperature is it safe to drain please, Richard? I ran the engine for an hour or 90 minutes earlier but, having seen these replies I stopped it about 30 minutes ago and was going to leave it until tomorrow.

The coolant should remain reasonably clean even if not changed for some time. In view of the condition It might be advisable to flush the system with a radiator flushing agent such as Holts Speedflush . In which case follow the instructions on the bottle.
It would also be a good idea to check the heat exchanger for leaks between the coolant and the seawater because the gunky state might be the result contamination of the coolant with seawater

Water must be below 60 C to avoid the the risk of almost immediate scalding, but judge when is safe by feel.

 

[RichardS]

The old coolant is / was pretty brown and gunky, so a knowledgable acquaintance suggested draining it, refilling with water and circulating that for an hour or two, before refreshing.

At what temperature is it safe to drain please, Richard? I ran the engine for an hour or 90 minutes earlier but, having seen these replies I stopped it about 30 minutes ago and was going to leave it until tomorrow.

Circulating the flushing water/cleaner for an hour or two is excessive. I would circulate for 15 - 20 mins as that should be sufficient time for the thermostat to open which is all you need, but flush it several times to wash all the crud out.

You should only need to leave the engine for 15 minutes to cool down sufficiently before you drain out the flushing water. After the 15 minutes, carefully loosen the filler cap on the heat exchanger header tank before you start to drain and remove it fully and start flushing when there is no excessive boiling and hissing as you start to crack open the cap.

Richard

 

[KompetentKrew]

Circulating the flushing water/cleaner for an hour or two is excessive. I would circulate for 15 - 20 mins as that should be sufficient time for the thermostat to open which is all you need, but flush it several times to wash all the crud out.

You should only need to leave the engine for 15 minutes to cool down sufficiently before you drain out the flushing water. After the 15 minutes, carefully loosen the filler cap on the heat exchanger header tank before you start to drain and remove it fully and start flushing when there is no excessive boiling and hissing as you start to crack open the cap.

Thanks very much - that was exactly the level of detail I needed. Thank you.

 

[Refueler]

Coolant change ? As long as you have a lower hose from engine that can be disconnected as well as open up heat exchanger to block ... the coolant should drain from both sides of thermostat. If not - fill with clean water ... run engine again as a flush ... drain again.
Fill up with required coolant. Job done. No need for hot engine ... but of course like the car - you will need to run engine while topping up to make sure all air locks are removed ..

Only time I can think of needing reasonably warm engine is when changing oil ... to have oil warm and easy flowing. Especially if you use a Pela to vacuum it out.