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I have to change my transmission oil (2008 Nissan Sentra CVT). It has a 7 liter total capacity, but due to the torque converter, I can only drain 4 liters at a time. Thus, I have to make several partial changes. The question is: How many partial changes do I have to do in order to have a clean transmission fluid (5% [or less] old oil)?

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I'm not sure this is a simple as # number fluid changes to get to x dilution factor. If so M1V1 = M2V2 is what you would be looking for which looks to be around 140L worth of oil changes.

It's my understanding that this transmission is expecting an actual flush procedure where either the transmission pump is forcing the old fluid out or there's a flushing machine pumping the fluid out.

This forum post describes a DIY procedure that seems to somewhat mimic the dealer process.
http://my6thgen.org/f107/how-change-your-cvt-transmission-fluid-flush-2822/

If it's true that it takes about 7 seconds to pump 1L of oil out while idling then I would suggest doing it in one shot instead of the start/stop procedure. Do your drain and fill, then unhook the return line. Have your 6L of "flush" ready, start the car and add them while it's dumping out (in to a catch can of course). This should pump the dirty fluid out of the converter while preventing the pump/transmission from going dry.

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