As I've said before it vents excess fuel it was mainly to meet European standards does nothing better off deleting it off!
The only MK2 Golf it was manufactured with is the Rallye
It doesnt vent fuel, as Danny says, it dumps the fuel fumes from the pressurised fuel tank into the inlet rather than to atmosphere. The fuel fumes are extremely bad for the environment (much more so than a cars exhaust) and the dump only happens at times when you wont notice it anyway so actually it does do something useful. Yes it is emissions kit but of the sort thats actually worthwhile and effective with minimal impact on your cars running and power

. Its the same reason modern cars have those irritating trapdoors on the fuel filler, to stop evaporative emissions.
Oh and yes, I know its useless on a mk2 as you dont have the vent pipe that comes from the tank

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