i have replaced the raidiator,water pump, themastate, and cap. car over heats when u put on cap but not when on off hoses swell up and they r new too does anyone know what the problem is?
Believe it or not I had a 68 with a windsor motor that the head gasket between a water port and cylinder blew out and the compression was going into the coolant, and it also might be a cracked head or vacuumleak on the intake, get a pressure checker for the radiator and do a pressure test on it, also check compression on each cylinder.
good luck.
__________________
HI-PO Xcharger (#76),S.Demon coil, JBA hdrs, Mgna flow duals w/ Pypes X pipe, S.masters DS, che K mem with trq stabilizers, Che DS loop, Eibach Pro kit, 8.8 3.73, GT brakes, p.slot rotors, hawk hps pads, Roush strut brace, MSD wires, Moroso alum expan tank & p.steering resevoir, billet caps, MWG rad extention's, hurst shifter, Xenon frt Fascia, GT grill, GT rear bumper, Bullit wheels, c.fiber inter, alum pedals, PLX A/F Gauge, roush vent pod & hood scoop and custom paint.
i had this problem in a 95 3.8 cougar it was head gaskets it got hot after the coolant got pumped out the exhaust if you keep driving might crack the heads fix while its just gaskets you hope
i checked the heads for copression the were all the same the car was sitting for 8 years or longer. to move it i had to change the wheel cylinders, a new master cylinders they had gunk in them and where clogged could any thing in the heads do the same after sitting for a long time
Leaving the cap off ...Have you watched the coolant in the rad to see what it is doing ?
If the coolant bubbles then you have a major combustion chamber to coolant system leak (head gasket,water jacket)
How are you determining it is actually Over-heating ?
don't go by the temp gauge ..use a candy thermometer in the coolant to what the actual temp sis.. should be about 210*F
__________________
"Ford Mustang.....The Hoofbeat Of America and...The Heartbreak of Today's Chevrolet"
First feel the upper radiator hose, you should feel coolant moving when you squeeze if you don't everyone one here has missed the most obvious, you have a air bubble in the system, take radiator cap off and squeeze upper hose and let the bubbles continue to come out, as the fluid level goes down in radiator top off with water, continue this procedure until all bubbles are gone, you shouldn't have to use more than 2 jugs of water, if you do, pull your dip stick to check for water drops, if engine is on it will look like a chocolate milk shake, that is a blown or incorrectly installed head gasket. No need to jump the gun, start with the cheapest and easiest method. Do you have your heater core hooked up, if so try the same thing with removing airpocket from there, this too can cause your problem. 9 out of 10 this should resolve your problem, if not for a cheap test, try taking out the thermostate and turn it on, if it runs cool, well then we are in the right area, if it still runs hot then you may need to check on what type water pump you installed, some are reverse flow and some aren't, are you running a clutch fan or electric fans. If you've checked all these things it could be in the clutch fan not engaging your water pump. Start with the airpockets first and let me know what happens.
1964 Mustang Coupe over heating 
IF HOSES SWELL AND YOU OVER HEAT, YOU ARE BUILDING PRESSURE, WHAT ABOUT HEAD GASKETS??
Is it possible that the thermostat was installed backwards? That would certainly account for overheating accompanied by a build-up of pressure. 
