1997 Mercury Sable leaking coolant from under radiator cap

I posted earlier this week about coolant coming out from under the cap of the coolant recovery tank and the mechanic that answered said he was almost certain it was a bad thermostat. Well I replaced the thermostat and it is still doing the same thing. The fans come on as usual its just that after i've driven for a bit it starts running hot and when I lift hood theres coolant coming from under the cap. I've changed the coolant recovery tank as well in case it was cracked. A friend suggested it may be head gasket but the mechanic who answered said if I took the cap off and cranked it coolant would gush out if it was a bad gasket but it didn't. Any more ideas? Thanks.