AOS external or internal

[Ronnie38]

I have a 2001 C4, serial number 654451 that is showing signs of AOS problems, my Bentley manual only shows two internal separators, All the write-ups show a single external unit. What is correct?

[Ahsai]

There are two internal oil seperators inside the oil sump. There's also one external AOS, which is the one people always refer to.

Edited May 13, 2014 by Ahsai

[Ahsai]

Btw, their functions are different. The internal ones are to remove air from the oil to prevent aeration and has nothing to do with crankcase ventilation. The external one is to remove the oil from the blow by and allow the crankcase gas to be burned via the throttle body.

Edited May 13, 2014 by Ahsai

[Ronnie38]

Thanks for all the help, Now on to replace the external AOS, glad for AC in the garage.

[Y2K911]

Good point about the fact that the external AOS receives little coverage in Bentley if I recall correctly.

[Ahsai]

Good point about the fact that the external AOS receives little coverage in Bentley if I recall correctly.

Yea, the Bentley manual is good at a few things (e.g., extensive electrical diagrams and per model DTC code definition) but unfortunately AOS is not one of it albeit it's a common repair.