Vehicle would require long starts and would suffer symptoms of vapor lock except on the coldest days of the year. Ford mechanic unable to identify issue. Replaced both fuel pumps and several solenoids/ sensors before Ford Technical Support recommended pressuring fuel rail with fuel injectors intact. Technician monitored for and observed a rapid bleed off of pressure in the fuel system and noticed a leak on two fuel injectors. Difficult problem to trace took 24 hours of service to identify and correct issue. Lincoln covered $2200 in warranty labor and parts.