Euro 500SEL hydropneumatic suspension (SLS) low

[AlexTheSeal]

Hello all,

Need help here with a Euro 500SEL with the full hydropneumatic suspension option (almost identical to that in a 450SEL 6.9).

When I first got the car, the suspension would pump up to full height on all four corners after running for 15-30 minutes, but the ride was extremely stiff. I figured that meant the struts and probably the leveling valves as well were OK, but the nitrogen spheres were shot. I've done the following work to the car:

• Replaced all five spheres
• Replaced large filter inside the fluid reservoir
• Cleaned out the gunk from the bottom of the reservoir
• Replaced the high-pressure hose from the pump on the front of the engine to the regulator on top of the reservoir
• Removed the metering bolt from the rear of the regulator, changed the O-ring on the bolt, cleaned out the orifice in the bolt, and cleaned the small bronze filter inside by soaking it in brake cleaner overnight
• Replaced the connecting rod from the rear leveling valve to the sway bar
• Flushed the system repeatedly with Shell Tellus T-15 hydraulic fluid (which as far as I can tell, has properties identical to the official Febi fluid at 1/10 the price---I will change to the Febi fluid as soon as I know for sure that it's going to stay there).

Now the car rides as it should---nice and smooth over bumps. However, sitting on a flat surface, the body is level front-to-back and side-to-side, but overall it is three or four inches too low (the tops of the tires are covered by the fender lips). Pulling out the height control knob does not lift the car, and neither does moving the control valve arms by hand. The suspension warning light on the dashboard is always on.

I have bled the system by running it with the high-pressure line disconnected from the regulator until the fluid comes out with no bubbles. There are no leaks in any of the connections to the hard lines. There are no externally visible leaks on either of the leveling valves. One of the flexible lines to the struts is weeping fluid, but not severely (it's just a little bit darker than the others). The knob on the dash seems to move the rotating control disk on the top of the pressure regulator as it should.

I'm confused by some of what I have read on this and other forums, as well as in the Star Motors troubleshooting guide which seems to be considered one of the definitive references on the hydro suspension. The latter refers to the fluid tank as a "pressure reservoir" and gives "lack of sufficient pressure in reservoir" as one of the first things to check if the car is low all the way around and the suspension warning light is on. I find it hard to believe that the tank is really a pressure reservoir in the same sense as, say, the coolant overflow tank. For one thing, if I open the filler/dipstick cover on top while the car's running, there's no sense of pressure being relieved (like there is with the cooling system); for another, the dipstick cover itself is not designed to seal tightly (it doesn't screw in, just a press fit).

There also seems to be some disagreement as to whether the system constantly recirculates fluid---in which case the health of the pump would be extremely important---or whether the pump is only used when the load at the front or rear of the car changes significantly, to add fluid to that end if necessary, and the rest of the time each set of struts and spheres becomes a closed system---in which case the pressure of which the pump is capable would be less important (it would just mean the car would respond slower to changes in cargo load). If the former is the case, then a bad pump could, it seems to me, cause the behavior I'm seeing (since it is operating at a lower pressure than spec'd, the pressure in the struts all the way around the car is lower than it should be).

Any tips or advice? I haven't yet tested the pump for flow rate, but that's next on my list. If the pump flow rate is low, I'm going to swap it with a used one to see if that helps. Otherwise I'm kind of at a loss. I don't want to start swapping struts, regulators, and control valves without knowing for sure that the ones on the car are bad, since new ones cost the earth and used ones are practically impossible to find. I'd be willing to pay for Star Motors to rebuild them, but only if I know it'll fix the problem.

(Apologies to anyone who already read this message---I've posted it to a couple of other forums as well trying to tap as much wisdom as possible.)
Thanks!