It started with a code P0304 (#4 Cylinder misfire) I found a bad #4 spark plug. I replaced all plugs, wires, cap & rotor (all Bosch & only 11K since last replaced). Easy fix & I cleared code. 40 miles later, I get a P0172. I think I've read all the P0172 posts here & everywhere. I've done all that's suggested & code returns. 13K ago, I had replaced all PCV: 7 hoses, cannister, solinoids, EVAP hoses (at tank), secondary air pump & hoses, SAS valve, etc. I have gone over each vacuum hose since, including tubing from air cleaner to turbo, turbo to intercooler, and intercooler to throttle body. I checked for intake leaks, spraying carb cleaner around intake & throttle body, no change in RPM. I replaced MAF (w/new Bosch unit )& I replaced both O2 sensors (W/Bosch). P0172 code returns after 30-50 miles. It doesn't smoke, fuel milage is ok, runs fine, but will not pass emmision test because of this fault code. Help!