Since you're going to go with ELC, stop by the supermarket and pick up a few gallons of DISTILLED water. Flush out the block a few times with tap water, then drain it and flush the tap water out with distilled a couple of times. Save a gallon or so to top off the ELC after you've filled it.

Be sure the ELC is the full-strength stuff, NOT 50/50. The cooling system's capacity is eight gallons total, so putting four gallons full strength and filling the remaining with distilled should give you a 50/50 at the end.

If you don't get all of the old coolant out, you lose the benefit of the ELC, and have to chemically treat it as the old-style coolant that requires SCA's. See:
https://www.ford-trucks.com/forums/9...ml#post8935201 for a good description by Stewart and Gooch on the subject.

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