I'm assuming that's a pressurized reservoir like on my BMW, if there is steam at operating temperature you only have a couple of choices. Either the reservoir is cracked or the cap is defective.
But if the temperature is rising above normal then there's something else going on. All the pressurization allows is a higher operating temperature.
Originally Posted by AshleyQuick
I turned the new cap until it clicked and steam is still escaping, causing the temp to rise.
The reservoir isn't that old...options?
2001 Ford Taurus
Check the temp of your engine with one of those cheap infrared temp guns. It may be you have a bad thermostat or defective fan and your temp is climbing past 210 F.
Not all Caps are the same but here is mine off a 2005 Taurus. It will vent at pressures above 16 psi. Look at the underside. It has a spring just like an old traditional radiator cap. I'm thinking your system might be at over over 210 F and it's building pressure because if it.
Run it without the cap, it it shoots coolant like a geyser it may be a bad HG. If the cap is the correct one it sounds like it may be over pressurizing the system.
Time for a pressure test.
I used the pressure testing tool at AutoZone for free. It's a very nice tool set to check pressure in the cooling system and basically "buy" it for a few hundred dollars and get your money refunded when you return it.