I can categorically declare that it is a 'feature' of the FDV valve - S1 and 1.5 Xms also make the hiss sound - much louder too!My "sinker" Xantia hisses Graeme. I wonder if it's inherent in the use of an FDV in these vehicles.
Mine has always hissed, much to the amusement of others Laughing but the hydraulics otherwise work absolutely perfectly...
Something to be lived with I guess and all part of the "Character" Wink
It was actually one of the first things I noticed about Citroens - I've always secretly been quite fascinated by the Hiss.. My dad had a 1994 Xantia D LX that delighted me with its hissing noise. I noticed that his Xant (which was bought as a 3 year old from a Cit dealer) only really made short hiss intervals until the accumulator was changed where the hiss was just as frequent but longer. I'm still trying to work out what it's doing when it makes the loud hiss...
I can't remember if the xant has the same but the BX has a rigid return pipe from the pressure regulator that then changes to a rubber, flexable pipe the loop back to feed back into the FDV. I notice that while the hiss occurs this pipe surges! so it has something to do with the pressure coming out of the PRV. I also noticed that it quite often hisses when the steering is stopped moving while the car is in motion such as in a carpark you can hear it with the window open.
The FDV valve splits the flow from the pump 1/4 straight to the PAS and the remaining 3/4 to the regulator/accumulator. When the PRV cuts out the return goes back to the FDV and is made available to the PAS IF the pressure of the PAS is less than the return from the PRV, if it's not, it then returns to the LHM tank from the FDV. I -think- this might be 'hiss phase' but not absolutely certain.
I worked out some aspects of the FDV from the Citroen training manual scans on one of the forums (is it one here somewhere? or maybe it's on FCF)
Anyway, the early sinking Xants are my favourites! I like the odd hissing and I like the old skool suspension sinking!
What sort of things have others said about the noise? I'm curious!