my 305tbi has 141,000 miles on it and i have never taken the valve covers off. it does smoke when i first fire it up, its either valve stem seals or my head gasket is leaking and coolant is burning off when i fire it up, other than that i roast sh*tstangs every chance i get.replaced: radiator,rebuilt water pump myself,alternator after 136,000,plugs,wires,etc...