Drain the block, otherwise you will either leave old coolant or flush in there, not desirable.

Use the citric acid, Preston isn't anywhere near as good at removing deposits. Not necessary unless you think you have considerable scale, though.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!