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. Its peculiarity is that it automatically centers the steering even if the car is standing still, and is less servo assisted at higher speeds and tighter curves.
Diravi was only fitted to 3.0 liter models and it never came in RHD.
Steering rack failures are generally caused by problems in the front suspension.
If you can hear clicking noises from the left side of the rack (LHD), the bottom pinion bearing will fail. You can test for this by agitating the steering rod when the steering wheel is turned fully to the right. The bearing is quite hard to change (if possible at all) without removing the rack, and also very hard to get as a spare part. It is a rather small needle bearing. The tpp bearing of the pinion could also be at fault but this is rather rare, unless the steering assist valve leaks. Then the leaking LHM could wash out the grease from the bearing. It is possible to renew this bearing, as well as the sealing rings and the valve itself without pulling out the whole rack, but it is difficult because it's not easy to get at (LHM reservoir and front middle sphere have to be removed, also be prepared to do yoga in order to get a wrench where it's needed).
If the rattle comes from the other side, then it's the synther bearing. It can actually be replaced without removing the whole rack, but you do have to pull out the steering valve assembly at least half way to free the steering rod in the rack and pull it out.
Removing the steering rod is a rather complicated task. Note that the following description applies to LHD cars equipped with DIRASS (conventional power steering) only. As far as I know, removing the DIRAVI steering assembly is not possible without removing the engine, too.
First, depressurise the hydraulic system. Turn the steering wheel fully to the left. Remove the LHM reservoir. If you have a hydractive model, remove the front middle sphere. Then uncouple the link rods and the hydraulic conduits of the steering valve.
Undo the large hex nut holding the tensioner, remove the spring, and if you can, the piston-like part that presses into the steering rod. Remove the steering shaft by unscrewing the rubber joint, then pushing the shaft towards the steering wheel. Loosen the but and bolt holding the steering assist cylinder on the rack but do not remove the cylinder yet. After you have losened the whole rack, you may remove the bolts completely.
Pull out the heat shield. Dismantle the hydraulic assistance cylinder. Be careful, there will be a fair amount of LHM still inside, do not extend the piston rod until you have rags or a can on the end of the cylicder where the conduits go, then drain all the fluid by pulling and pushing the rod all the way in and out.
Unscrew and remove the steering valve carefully, you may need either to turn the pinion or to move the rod to accomplish this. Now the whole steering rack can be pulled out to the right. Remember not to remove the leak return hose from the front height corrector.
The rack can be now dissasembled by unscrewing the ball joint from the side closer to the steering valve. After that the shaft can be pulled out through the synther bearing on the other side. It has to be dissasembled to this stage if you want to gain access to the bottom bearing. Assembly is in the reverse order taking care to do some work to block the opening in the rack where the steering valve assembly will go, as well as the hydraulic conduits, so that dirt cannot get into the rack and the valve. Plenty of grease will also be needed. Be sure to replace all torn or worn boots. Plan on the better part of a day to do all the work...
All steering rack parts except the valve seals and rings appear on the parts microfiches, thus, can be ordered.
The steering valve can be renewed but it is a rather difficult job. First of all, be careful with second hand units: the only real way to check them would be to open them, but once you do, you have to change the seals anyway, because the old ones will never seal again. These rings are made of a black, teflon-like plastic. There is a Citroën kit for renewing a steering valve, containing three new rings, two sealing rings (simmering), a replacement rubber seal for the HP supply hydraulic conduit and a new plastic cap for the top of the steering valve (just under where the steering attaches). It may however be a good idea to go to a scrap yard and get a valve, rebuild it and have it ready to be put in instead of the old one. It will save some time.
The whole operation takes almost half a day because it's so difficult to get at the parts. You want to avoid this job unless it's absolutely necessary. There is a relatively simple way to test that it's actually the steering valve that causes most of the leakage: use an old brake valve bleed screw (usually found on brake valves for CX and BX) to close the main high pressure supply from the pressure accumulator and regulator assembly—this is the smallest diameter hydraulic pipe on the unit. Take this out (including the rubber seal) and screw in the bleed screw istead; it should be an exact fit. This will make the LHM only go through the steering part of the system, the main accumulator will just reach operating pressure and click as usual, and should remain charged since you have put all of its LHM-consumers off-line. Note that the STOP lamp will not go out because the pressure switch is also off-line.
If both the pump and the flow regulator are OK, and you still get a whistling sound, the steering valve is worn. Incidentally, this is also a good way to test the main pressure regulator for leakage—with this bleed screw in place, the pressure in the main accumulator should remain there for a very long time (possibly even days).
After the test, you must depressurise the regulator before removing the bleed screw, remember, that's where the LHM would normally come out to be used by the suspension and brakes!
Removing the valve is real pain in the neck. You will have to remove the driver's side wheel, the LHM reservoir and the middle sphere, just to be able to get at it.
You need to uncouple the steering wheel (unscrew the rubber damper from the valve assembly, you will probably have to leave half of the rubber damper plate assembly on the valve itself because of limited space) and push it upwards. The steering shaft itself is actually made of two coaxial shafts that allow for shaft compression in a crash. This effect will give you just enough space to be able to remove the valve later.
Before you loosen the two hex wrench bolts that hold the valve on the steering rack, you will need another huge hex wrench (22 mm I think) and unscrew the tensioner in the steering rack. Careful, there is a spring in there. To get to it you have to remove the middle sphere. Once this is done you can unscrew the hydraulics off the valve, then loosen the two hex screws that hold it onto the rack, and then pull the valve assembly upwards and out. You will need to turn it slightly to get it out.
When you have the valve out of there, the first thing you'll need to do is pop out the two sealing rings. Removing the circlip makes it possible to push out the rotor of the valve together with the main bearing. Once you have done that, the rings are dead, you must replace them. You will also notice that the main HP supply goes into the stator part of the valve through an adapter nut. This is because the valve is Bendix and the hydraulics up to that point are Citroën: the pipes have completely different endings. Unscrew the adapter nut carefully, you will find a simple one-way valve mechanism under it that needs to be cleaned. There is also a one-way valve of similar nature in the return pipe. Before replacing the rings themselves, clean the old grease out of the main bearing carefully and verify that the bearing runs smoothly.
When you have done all this, you will need to change the main valve rings. Do not remove all the old ones, there are only three rings while there is room for four. If you look inside the stator part, you will clearly see where the old rings were—there will be three very smooth bands on the inner wall. You will also notice that the bottom ring that is missing serves no apparent purpose—in fact, this is the outer seal that prevents leakage of the LHM towards the main bearing. The point is, you will see that instead of replacing the bottom ring, you can add the new ring to the free groove. If you look at the rotor and stator you'll find that they both serve the same function. This way the new ring will not go over the smoothed out part of the stator, and will also have the old ring as backup. The other two rings need to be replaced, so cut them out.
The way you change the rings is very tricky. I actually used a piece of an old fizzy tablet can (calcium tablets as I recall) because it fitted the diameter exactly, and was made of very thin aluminum alloy so it didn't add much to the diameter. It's a 18 cm long tube of about 2.5 cm in diameter. Using a conical tool I first stretched the rings (very) slowly, then plugged the thinner end into the aluminum tube and transferred the rings from the conus to the tube, all three of them. Then you put the rotor part into the tube, and slowly, one by one, slide the rings off until they pop into place. Be very careful to get it in one go! When you have done this, leave the rings for about 2-3 hours, they will shrink back, although not completely.
The whole assembly is put back together in the opposite way. You have to be very careful as the rings on the rotor go back into the stator part one by one, not to knick them. A 45 degree conical angle is provided on the edge they will be going over, which makes things a bit easier. Before you start, use a bit of fresh LHM to lubricate the rings and the inside of the stator. It may still need a few taps with a plastic or rubber hammer to get them in. You will notice that the rotor will now turn with a lot more resistance. The resistance should be uniform. Go on with the sealing rings, and a liberal amount of grease for the main bearing. Then the whole thing goes back into the steering rack (apply a liberal amount of grease to cogwheel. Then tighten the valve mechanism back into the rack, connect the hydraulics, tighten the tensioner and reconnect the steering wheel. Finally, if you have loosened it, secure the rack.
The hydraulic conduits from the steering valve to the steering assistance hydraulic cylinder seal using large conical seats. Unlike Citroën hydraulics in general, these need to be very tight to seal as intended, and you have to be very careful not to drive any dirt into the seats. The way I did it was to tape over the seats on the steering valve, then took off the tape when the valve was already in place.