Most likely the water pump.
stock mustang 5.0L... 15/16" socket
socket and force bar on bolt brace against frame and use starter to loosen
Put a long breaker bar on socket that fits onto the harmonic balancer bolt then disconnect the main coil wire so the engine will not start. Now gently turn the long breaker bar on the bolt like you are trying to take it out. When the breaker bar hits the floor get into the car and turn the starter motor and like majic the bolt will come loose.
There are numbers stamped into the harmonic balancer, if you can't see them, take some sand paper and clean the balancer off, you should see the pointer that points at what the timing is when you look down from the top of the engine bay. The best way I've found to see your mark, is to use a white or yellow grease marker and put a line on the balancer where the particular degrees you want to set it on.
The coolant system capacity of the 2004 Ford Mustang GT is : 14.1 quarts ( 13.3 liters ) 50 / 50 mixture of antifreeze and preferably distilled water ( so just over 7 quarts / 6.6 liters of antifreeze )
Weld the place where it is leaking
According to the 2005 Ford Mustang Owner Guide : The cooling system capacity is 13.6 quarts ( 12.9 liters ) So , a 50 / 50 mix of antifreeze and preferably distilled water would require : ( 6.8 quarts / 6.45 liters of antifreeze ) * Ford shows not to use less than 40 % antifreeze and not to exceed 60 % antifreeze in the cooling system mixture *
Usually 50 / 50 - antifreeze and preferably distilled water ( Ford says not to exceed 60 % antifreeze )
Here is a link from a repair site i subscribe too. Hope it helps. It is an "Exploded" View of all the components for the timing stuff. http://www.ondemand5.com/mric/common/asp/ImageViewer.aspx?MOD=REPTIFF&MODULE_ID=1&SESSION_ID=%7BE80FA39E-8189-4681-B82C-6C58934EA88E%7D&IMAGE_GUID=90C10183&PRINT_BW=0&ARTICLE_TITLE=TIMING%20BELT%20REPLACEMENT%20-%202.0L%20&%202.3L%20-%201974-94%20EXCEPT%20PROBE&YEAR=1993&MAKE=Ford&MS_MODEL=Mustang%20LX&IMAGE_CAPTION=Fig.%201:%20Exploded%20View%20Of%20Timing%20Belt%20&%20Components%20(Typical)&COURTESYTAG=Courtesy%20of%20FORD%20MOTOR%20CO.& Most all timing marks are located on the harmonic balancer. A timing tab is located on the sheet metal that overhangs the balancer. Harmonic balancer is the lowest round device on an engine that has the belts running around it.
cos i broke it cos i broke it cos i broke it
Intake manifold is leaking, or worse, cracked cylinder most likely tho, the intake manifold.
Motorcraft Premium Gold Engine Coolant (yellow coolant)