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Today I was changing the air filter and I noticed that the previous mechanic had bodged the outpipe coming from the air filter cover.

However, that pipe goes into a box of some kind. I noticed some carbon build up in that box when I removed the pipe.

Can anyone tell me what that box is for?

Below is the bodged pipe and under that is a picture of the inlet for the pipe which leads to some kind of box in the back of the engine.

Thats the crankcase breather. The bit in the hose with the plug is the heater for the crankcase breather

and mine is getting blown off, that's the issue I have.

If it’s being blown off, you’ve got excessive positive crankcase pressure. Possibly need a new oil separator on the back of the block

I take it that's the expensive plastic thing??
https://www.volkswagen-classic-parts.co ... 99014&ct=2

Depends on your engine code. There’s a few versions

Do you know what yours is? (And what year)

It's AUA 2001 which matches info on the page for the pipe at least.
I found the plastic thing on ebay.

That should be the one. Usually available in a variety of places. Bear in mind anything used will almost certainly be going soft, or will have hardened up due to age and heat so new is ideally the best way

If however your hose is ok, you’ll need the oil seperator itself which is an inexpensive part - febi still make them

The pipe is going soft. I'll probably bite the bullet on this and get it done.

By the looks of it your hose/pipe has had its best years, probably for the same reasons Alex mentioned previously. As far as I can see, it has already been repaired twice. If I were you, I'd replace both. See illustration below for your specific engine code. You will need partnumbers 19 and 28a.

19 oil separator: oem partnumber 036 103 464 G
28a vent hose: oem partnumber 036 103 474 L