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Bruce, One good comment deserves another.
Gambling with your cooling system for all those years? Tsk! Tsk! Such heresy should not be permitted on this board. We all know you should never deviate from the green stone tablets passed down from Gothenburg, but then I've known people running just with water even during the winter -now that's gambling.
In my temperate coastal zone, rust, scale and water pump protection are equally, if not more important to me as temperature protection. For the interest of others, the normal 50/50 glycol mix gives freeze protection to about -34degF/-37degC and boilover protection to about 265degF/130degC. For harsh corners of the world, a 70/30 mix protects from about -82F/-63C to about 275F/135C. A 40/60 mix only protects down to about -10F/-23C, adequate perhaps in many climates, but most car and coolant manufacturers say anything less than a 50/50 mix gives less than ideal corrosion protection. For a simple drain and re-fill, like you said, the 40/60 fresh mix (50/50 coolant mix) achieved has little or no effect on temperature protection, but you are admittedly sacrificing some corrosion protection.
In my case, I'm often overdue by the time I get around to doing a flush. Long life coolants were made for people like me. So I do a fairly thorough job to appease my guilt. I'll use a pre-flush additive to loosen the scale if I think it's warranted. If I'm in the mood to attack the heater hose, I normally do a reverse flush with one of those adapters. I normally don't keep the adapter in place as I typically want it for the next car and don't have a spare plus I don't trust aging plastic in a hot engine compartment. The upshot of all that is I'm refilling a system with virtually no old antifreeze. My hypothetical retained fluid scenario (admittedly concocted to 6.6 quarts to give a simple 1/3rd result) isn't too far off my experience with my 740/940's, although I do vaguely recall my 140/240's not being so retentive. But as I said, it depends on how much effort I put into draining the system -like having the car level, blowing out the heater core and waiting for the last drop.
I no longer run the engine to push out the last bit of fluid as I don't want the water pump to run dry. I do rather dread flushing as I have flashbacks to a few previous efforts where the water pump failed within the following months. That's why I'm a bit more careful now. I'm not one for using sealant additives, but I have been known to dump in a little water pump lubricant at the first hint of a squeal.
So, I'd say over our mutual 19 years experience, we've each developed our own habits, they seem to work for us and the end result probably isn't significantly different.
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Dave -940's, prev 740/240/140/120 You'd think I'd have learned by now
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