2009 F150 4x4, 4.6 2v motor, 155k w/no problems ...................until cyl #3 had a sudden loss of compression. All cylinders testes at 125 psi except #3 which suddenly developed a misfire. Compression is down to 40 cold and 35 psi hot. What would cause this? The local mechanic doesn't want to do anything except change out the motor, to the tune of between $3-$4k. Any suggestions would be appreciated. thanks

 

Since it happened suddenly I'd venture to guess a bent valve or possible broken piston ring.