Contrary to popular belief, the hydropneumatic suspension system used in Citroën vehicles is remarkably reliable — provided it is properly maintained. The main cause of problems is not the complex design, but neglected maintenance.
How the hydropneumatic suspension works
The hydropneumatic system replaces conventional springs and dampers with a system of spheres filled with nitrogen gas and LHM hydraulic fluid. Pressurised fluid generated by the main pump keeps the body at a constant height regardless of load. The height corrector allows ride height adjustment — a unique feature of Citroën's system.
The BX, equipped with a simplified MacPherson variant, may feel firmer than older CX or DS models — this is a design characteristic, not a fault.
How to check the suspension yourself
The following procedure allows a preliminary assessment of the system without specialist equipment:
- Start the engine and allow the vehicle to reach full working height — this should take 30–60 seconds.
- Press gently on the front bumper — the front should drop approximately 10–12 centimetres.
- Release — the suspension should return to the normal position smoothly and without vibration.
- Repeat the test at the rear of the vehicle.
- Switch off the engine and move the height lever to the service position — the vehicle should slowly but clearly rise with the engine off (indicating a healthy pressure accumulator).
LHM fluid condition
LHM fluid (a green mineral hydraulic fluid) is a critical component of the system. Its condition directly affects the suspension, braking system and power steering on all vehicles with Citroën hydraulics.
- Colour: a bright, vivid green indicates good fluid condition. Brown or dark fluid must be replaced.
- Level: check regularly, with a cold engine and the vehicle in the low (parking) position.
- Filter: the LHM filter in the reservoir (right side of the engine bay) should be cleaned at every fluid change.
Warning: Never mix LHM fluid with other hydraulic fluids. Using the wrong fluid (e.g. ATF or DOT) rapidly destroys the seals throughout the hydraulic system — repairs are then expensive and time-consuming.
Common problems and diagnosis
Firm ride
If the vehicle rides firmer than it should, check in turn:
- Suspension sphere condition — simple self-test (bumper press)
- LHM fluid colour and level
- LHM filter condition
- Height corrector settings
- Wheel alignment and tyre pressures
Tyres have a significant effect on Citroën ride comfort — soft radial tyres (e.g. Michelin) noticeably improve the vehicle's behaviour.
Rear suspension dropping quickly
Does the rear suspension drop immediately after switching off the engine? The most common cause is the brake distributor valve not holding LHM flow under high pressure. This does not require immediate repair, though the fix can be costly.
Pump running frequently
If the hydraulic pump runs almost continuously, the system is not holding pressure. The prime suspect is the pressure accumulator — see the test procedure above in step 5. Other possible causes:
- Leaks in the system (pipes, fittings, struts)
- Incorrect pressure regulator valve setting
- Worn front suspension struts
Vehicle does not rise after starting
Failure to rise after starting indicates a serious main pump failure, significant system leaks or a blocked regulator valve. A workshop visit is required.
