Citroen XM Forum

All hydraulic components for the car are original except for the following;

Hydraulic pump
Pump to FDV main supply pipe
FDV to Regulator flex pipe

The issue was not initially evident so im ruling out incompatible parts for the time being, blockage or failure is my first port of call, although I find both to be unlikely problems.

D

Could the FDV filters be blocked?

I guess that they would be blown with that amount of pressure and debris would end up in the system....like always does.

Yes the filters don't last long once they pick up some gunk.

Pressure gauge arrived!

Tested from the two main supply pipes mounted on front subframe as they were easy to get to......................or the only ones i could get to.

Tested the brake/suspension feed first, this is fed from the pressure regulator. The pump runs fine on the drill but makes a maximum of 75bar and no more, hissing sound is heard coming from hydraulics on the front of the engine, no massive belt slipping resistance though. Should be seeing main accumulator pressure here, 145-180bar i would say but just cannot get more than 75bar.

Reconnected this brake suspension feed pipe.

Took the larger power-steering supply pipe off and connected to the gauge, pressure went up fast to 90bar which from memory is what the PAS pressure should be, fed directly from the flow distribution valve, this pressure cannot be bled down using the relief bolt which I remembered from the previous fault finding escapades so you just have to sit back when finished and let the pressure bleed down over time, the system isnt designed to have the PAS supply pipe capped.
what is odd was that as soon as the steering output reached 90bar the hydraulic pump locked solid, to turn by hand was impossible even with huge force so I stopped. What the hells going on? the brakes and suspension supply pipe was connected up. as the pressure bled down after 10 minutes i opened the 12mm relief bolt and the steering output pressure shot up to 75bar so the accumulator must be charging a bit but why is the pump locking up.

Im stumped here, the suspension pumps up fine, the steering works fine, the brakes feel fine but the test is showing a reg output of no more than 70bar and if the suspension or brakes are not drawing fluid the pump just locks up solid like its making ????bar.

Sooooo, i will need to think about this one for a bit, low output to suspension/brakes with hissing noise during pump operation when that supply was tested and what i think is correct pressure to steering gear but hydraulic pump locks solid when steering supply pressure reaches 90bar when testing the PAS supply output from FDV.

The more i think about this the more i want to pull that pressure regulator off and strip it down.

D

Connect tot eh regulator, not the FDV.

It sounds like the FDV shuttle may be jamming after the PS reqirement is satisfied, so the fluid flow becomes blocked. The FDV is where fluid returns to the tank, not the regulator. If the FDV jams, or the open return is blocked, there is no return path.

The steering pressure is supplied via the FDV yes hence why you can't depressurize the system if you block the PAS output as you say, i have tested the regulator, maxes out at 75bar with pump spinning relatively freely.

D

The regulator can't be supplying a maximum of 75 bar if the steering is reaching 90 bar. The 75 Bar you are seeing is at the high pressure outlet of the FDV which in itelf that says something is wrong.

Go back to the regulator and measure again. You should see 160+ bar.

I believe the problem is at the FDV.

Thats what im thinking, it can't be putting out 75bar, but it is.
Pump supplies FDV first and then regulator though doesn't it?

D

Yes, FDV first, then the regulator.
If you look at the Russek diagrams, you'll see the arrangement.
Both FDV and regulator might need stripping down - there must be a blockage somewhere - but do one at a time, or you'll never know.
I'd go for FDV first.
Check it over, put it back, see if anything changes.
If not, move on to the regulator.
______

For the regulator to be outputting the normal 145/170bar, it must be receiving at least this at its input.
Can you get the pressure gauge coupled up to the output from the FDV to the regulator?

If pump goes solid when steering hits 90bar, then presumably 90 is the max working pressure of the steering circuit, and the fluid then still needs somewhere to go. It should pass to the regulator, and excess pressure bleed off back to the reservoir. If there's a regulator fault, it might not be able to do this, and so everything goes solid. Only significant use of the steering will then provide a means of relieving the pent up pressure, and allow the pump to briefly turn again. My guess.

It does sound like good progress is being made in addressing the 'belt-eating' habit Dean, although the FDV and regulator are a b***** to access on the PRV V6 insitu so I do envy you the task! I recently had a FDV rebuilt by Pleidies for around £100, but apparently the filters are now getting hard to source! Regards, Matthew T.