Most of the time when one smokes only at start up and then quits its the valve seals. When you run it and shut it off , try cranking it back up pretty quick, bet you theres no smoke. Only does it when the car has set for a little while right? If so then oil is leaking past your seals and making it smoke for a sec when you crank it up. A turbo seal usually makes one just keep on smoking from what I have seen. I would put my money on your valve seals myself Later Daniel Ray