Remember, there is coolant inside the engine block, intake manifold, and your heater core. So unless you flushed and removed all the coolant from these, there will still be quite a bit of coolant still in the system.

Flushing the radiator alone only removes maybe half of the total coolant, and is not good enough if you have dirty fluid.

88 4R, 350 V8, R150 5 speed Sold <...uot; />
97 4R, stock, daily driver
98 Sienna, kid and dog hauler, wife's ride