Ok so I dont have a mustang but I am running a 1987 GT roller motor in my truck. I am having a problem with the motor it is not running really hot only about 180-195 so a little higher than my average temp i usually sit at 182 and never go higher. The problem is when I stop and turn my truck off antifreeze comes spewing out of the little hole in the top of the exansion bottle. I am not loosing any antifreeze going down the road and the truck is not smoking at all it just dumps it when I turn my truck off. Like when this first happened I was on a 200 mile trip and I never overheated. So for the ride home I just filled the radiator back up and drove home again it did not overheat just dumped when I stopped. If you guys could help that would be great.

 

I would look at your radiator cap. Pressure test it and see if its holding its pressure.
Every 1lb of pressure the cap holds, it will decrease the boiling point by 3 deg

Thats where i would start looking

 

New cap is cheap if you do not have access to pressure tester.

 

+1 for changing the cap. if its more then a year or two old, your over due.

 

Even if your cap is ****ed or your truck is overheating. The overflow should not dump out unless the bottle is totally overfull.

Check to see if you have the hose on the right connection on the bottle. It should feed/suck through the bottom.