The top disc part has a spring loaded diaphragm in it that is designed to close on excessive engine vacuum to keep the engine from drawing oil up the dipstick tube and into the intake. Under normal operation it draws air from the valve cover, oil vapor laden, and the bottom part condenses the vapor to drain down to the pan while the air gets pulled into the intake.
For what they cost £30odd plus any pipes you may want, total kit been around £145 plus dip stick tube £60 ish. But some pipes and dip stick can be cleaned out, so total price of parts will be cheaper.
Will cost around an hr and half labour, or if your handy at DIY do it your self.
I just had my X5 done at the dealers, £280. I did for peace of mind as I had the same yellow gunk, after they had done it I asked if it was worth it and he said yes.
In total the dealer said it tuck about 4.5 hrs, but most of that was cleaning pipes that could be cleaned. 2 pipes was replaced as they went brittle, I did flush engine and serviced it 3 days before it went to the dealers, this did not fix the problem and cost me £150 for the service.
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EDIT I did try and offer my help
2000 323ci e46, think the headgasket is gone.
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