Just a note on this ongoing problem. Vac leaks seem to affect these X350 a lot. I inspected and secured all of the plastic tubes, and just by chance wiggled the rubber boot between the MAF sensor and the TB. It was very loose, most likely off the TB lip on the bottom side!! Thing is, I secured this boot 2 days ago and tightened the clamp.. It had come loose again!!! This time I wrapped one layer of black electrical tape to the outer lip of the TB so the rubber boot has a snug fit and now has a non mirror surface to grip to. If you have rough idle problems, re check this connection on the TB just to be sure. An easy check/fix for free! Brandon. 2004 X350

 

Modern cars with OBD II need a non-leaking engine vacuum.

 

I just figured out why the rubber boot pulls off !!! The boot connects to the MAF sensor housing which connects to the air filter plastic lid which snaps on to the air filter housing box which is secured by a bolt to the inner fender structure... If the engine moves even slightly under torque when driving, the boot will pull off of the TB!!! The slightest wear of the motor mounts would allow this! Brandon