Purpose: To de-scale and remove build up from the radiator and cooling system of an early mustang in order to help lower coolant temperature.  Test case was at 215 degrees before and 185 degrees after.

Supplies:
Powdered Citric Acid (not sodium citrate) like this: http://www.amazon.com/Dafna-DFNCA2-C...ds=citric+acid in a formula of about 1 lb per 5 quarts of water. The typical Mustang holds 14.5-16 qts so 2-3 lbs of citric acid would be correct.
New Thermostat gasket at intake
New Thermostat
Flush Tee


Drain the cooling system thoroughly. Also locate and remove the block drains. May need to spray them with some penetrant (my favorite is 50/50 acetone & ATF). Remove the thermostat and reinstall the thermostat housing. I'd also get a "flush tee": http://www.summitracing.com/parts/rnb-47152/overview/, and install it using a scrap piece of heater hose at the intake manifold heater nipple.

Make sure the heater controls are "ON" for "MAX" temperature so the heater core will be cleaned as well.

Flush the cooling system several times with clean water from a hose until it runs clear. Be sure to spray to at lower radiator inlet, upper radiator inlet, hose at lower block inlet, hose at intake manifold and hose at heater and intake. 

Mix a solution of a hot water and 1/2 cup Tide HE laundry detergent. Note must be HE or you will never get all the suds out! (Alternate use a half bottle of Shout gel) Add to the radiator and top off with water. Start the engine and run for 30 minutes. Drain and flush several times at all hoses and plugs until water runs clear. This will remove any residues of oil and allow the citric acid to work.

To be safe, fill system with water, run for 30 minutes, then drain and flush again.

Next, wear safety glasses, gloves and work in a well ventilated environment!

Mix citric acid powder with boiling hot water and fill up the cooling system, then start and run the engine at fast idle for 20 minutes AFTER it reaches operating temperature. Don't overheat the engine!

At the end of the period, immediately drain the system and flush several times with fresh water until it runs clear. Take precautions as the cooling system will be very hot! You should also remove the block drains (one on each side) to let any sediment trapped in the water jackets out. Depending on the level of junk in the system it may be necessary to repeat the process.

When finished, reinstall all plugs and refill the cooling system with a 50/50 mix of coolant and distilled water (if using a antifreeze concentrate).