Not all the coolant will drain. Probably 1/2 a gallon to an entire gallon will remain within the engine block (I'm not too familiar with the M119 but I know its predecessor the M117 holds a lot in the block). You really should remove both of the block drains if you're doing a coolant flush. Anything less is only flushing half the system!

If you have the owner's manual it will tell you within it. I'm going to guess about 3.5 to 4 gallons.

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Current:
2021 Charger Scat Pack Widebody "Sinabee"
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1972 280SE 4.5
2018 Durango R/T, 2014 Jeep Grand Cherokee Limited "Hefe", 1992 Jeep Cherokee Laredo "Jeepy", 2006 Charger R/T "Hemi"
1999 Chrysler 300M - RIP @ 221k