The XM came equipped with one of two possible power steering systems. The conventional one is practically equivalent to the BX one, with sizes of cylinders adjusted for proper assistance force. This is called DIRASS from the French Direction Asistée. The other is the self-centering, speed sensitive, fully amplified system to be found on the SM and the CX as well, called DIRAVI (Direction Appel Rasservi) or, sometimes, Varipower. This system is completely different from the conventional one, even the supporting hydraulic components are different.
Noise from the front
There are several possible causes. The connecting rod ball joints might be worn (there are two of them for each rod, they connect the strut to the anti-roll bar). There are two types of rods, adjustable and fixed length; the replacement rods will usually be fixed. If you find an adjustable one on your car, replace both of them.
The steering rod ball joints might be worn.
Changing the LHM and rinsing the system is a part of normal service and care for all hydraulic Citroëns. Don't neglect this if you want reliable and trouble-free operation from you car. Dirt in the system eventually wears out some of the components if it's not dealt with in time. Although the system is extremely robust in itself, this does not mean that you should tempt fate.
Failures in the Hydractive system (generally problems with sensors or their wiring) can force the suspension to be stuck in hard mode. To diagnose the fault, it would be convenient to know which mode the car is currently in. As H II systems use a 1 kHz chopper to form the current operating the valves, this leads to an interesting way to learn the current status of the suspension: this chopped current actually disturbs the radio reception. Tune your radio to a spot somewhere near 700 kHz where there is no incoming broadcast.
There is a product called Hydraurinçage (sometimes also called HydraFlush, the French name is pronounced as \id-ro-ra[n]-sa'zh\ or, more precisely, using the IPA phonetic symbols: ɪdrɔrɛŋ'sɑ:ʒ). It's a special cleaning liquid manufactured by Total, orange in color; your nearest Citroën dealer is sure to have it on stock. The price is more or less the same as that of the regular LHM.
A sphere is only as good as the neoprene membrane that contains the pressurized gas. Occasionally, the interface between the membrane and the retaining plate inside the sphere is to blame for leakages, but usually any problem is associated with the membrane itself. Like anything that's under constant flexing, it'll break down over time.
Any work on the hydraulic parts below the car should be carried out with the car safely supported on axle stands or standing over an inspection pit. Never work or allow anybody to work on the car (not even for a small adjustment or even just inspection) when it's only the high setting of the hydraulic system that keeps the car up.
Needless to say, you should always use the correct grade oil in your gearbox. Total manufactures it for Citroën/Peugeot with a grade of 75W. This is sometimes referred to as 'motorcycle gearbox oil'. Substitutes, ATF for instance, or any gearbox oil with incorrect grade, will leak from the top of the box, through the plastic tube where the gearbox lever enters the housing. Even with the correct oil it will leak slightly (it seems that the gearbox was not actually designed to operate in the slanted position it is fitted in).
First of all, shop around for the clutch--the original Citroën ones are very expensive (exceeding 500 EUR), so I actually went to the shop and opened up a clutch kit. It turned out it had parts from Valeo and Sachs (I have been told some also have Luk parts). I looked up the suppliers, and found an XM kit from Sachs with a three year warranty. When I checked it out it turned out to be exactly the same kit down to the Valeo parts. The only difference was that it cost less than half the price.
The easiest way to change the clutch is actually to have the whole engine out.