The DIY "Coolant Flush" by trustdestruction shows how to change out your coolant using repeated drain and fill of the radiator with distilled water. The block drain plug is not used. With undiluted coolant he adds half the volume of the overhaul capacity of the cooling system and tops off with distilled water to obtain a final 50:50 mix. I have several follow up questions (in bold).

Using trustdestruction's procedure the drain and fill flushes are done with a relatively cool engine. How does the fresh distilled water in the radiator mix with the old coolant in the engine and get past the closed thermostat? Is that what is accomplished by sliding the temperature control lever to hot? Does it make a difference if the thermostat is present or absent? [Note: I plan to change the thermostat at the same time I change out the coolant.]

Honda now sells coolant (Type II) as a 50:50 premix, thus this procedure needs modification to achieve a final 50:50 concentration. That said, I live in southern CA where temperatures rarely reach freezing. Heat is more the concern. I am considering using 30:70, coolant:water, for better cooling. At this lower ratio is there any problem with corrosion protection?

Thanks,

Scott
'96 Civic DX, D16Y7 AT

 

Excellent question. The coolant is water-soluble so, in theory, it should eventually mix with the new water. However, I'd pull the thermostat, then let it cycle through the system that way. In the past, I usually remove the thermostat anyway, then manually flow distilled water through the cooling system until it runs clear. In most Honda engines, I'd imagine you'd flush it clean with less than a gallon of water.

No - that helps open up the flow to the heater core, which contains a small amount of coolant.

With the system (and engine) being aluminum, copper and plastic, I think you'd be fine to run that ratio.

 

Thanks for the reply, Christian. In other write ups I've read they ran the engine long enough for the thermostat to open. But then they say to wait for the water/coolant to cool down before repeating the flush procedure. It seems easier to do what you suggest, that is, remove the thermostat before the flush since I plan to change the thermostat anyway.