Coolant Flush ?????

I have a '98 Ranger 4.0 4x4. I recently got the engine replaced w/ a used one because the old one had a crack in the block. My mechanic told me to bring it back after 3k miles to flush the coolant w/ their vacuum device. Is that neccessary or can I just by the stuff from the auto store and run it through my coolant system for draining and refilling?
thanks.

i dont see the need to even flush it. but if it must be done, i would do it my self. unless it were a warranty issue.
if you want to flush it just pull the thermostat.out of the housing and replace the housing.
drain the radiator get a water hose and refill the radiator
turn over the engine and let it run for a few minutes. turn it off then drain.
repeat the process then drain it one last time and replace the thermostat in the housing and refill the radiator with 50/50 mix antifreeze and distilled water.

i dont see the need to even flush it. but if it must be done, i would do it my self. unless it were a warranty issue.
if you want to flush it just pull the thermostat.out of the housing and replace the housing.
drain the radiator get a water hose and refill the radiator
turn over the engine and let it run for a few minutes. turn it off then drain.
repeat the process then drain it one last time and replace the thermostat in the housing and refill the radiator with 50/50 mix antifreeze and distilled water.
thanks. He told me I need to flush it becuz it is a used engine and there might be stuff left in there...