My car started misfiring on my way back from work today but it wasn't bad enough to stop me. I eventually got home, lifted the bonnet and found an air leak from a vacuum port in the intake manifold, just behind the fuel pressure regulator. Is this normally capped and I just lost a cap or is there a line missing there that should go somewhere? I couldn't find any and just taped it for now, idle is back to normal.

 

The short line goes from the intake manifold to the fuel pressure regulator . The fuel pressure on the fuel rail is at 43 psi at idle and increases up to about 47 on throttle up from the increased vacuum . You can find the line at the local auto parts store .

 

There is actually two vacuum ports there, the one on the left(closer to the front of the car) is indeed going to the fuel pressure regulator and the line is still there plugged in ok.
There is however a second one next to it(literally an inch away to the right) and this is the one I'm not sure about and that caused the air leak.

 

Will have to look at mine as I did not notice .

 

Yes mine is capped off at the nipple just fwd of the fuel pressure regulator intake manifold nipple . Looks like a factory cap . Might have something to do with the fuel EVAP system as there are 2 different versions on the X300 . Early version has 2 square canisters and other different devices and the later version has 1 minus the other devises . You can look under the car without jacking fwd of the rear wheel on the left and right side . Mine has only 1 on the left side , VIN # 755xxx