original owner of 03 cobra, I performed the first coolant flush at 36k miles and used the over the counter flush with only distilled water, also only used distilled water when I refilled with new antifreeze/distilled water. I did not see any way to drain the block, just used the radiator drain. I had some seditment in the overfill tank (that I ended up replacing). Anyhow just did a second flush, now at 78k, also sediment in overfill tank again, hoping there is no seditiment in the water jackets of the motor, still runs cool. I have only used distilled water and antifreeze so not sure what the residue is.