BMW E61 Hydraulic Liftgate Pump Rebuild

The liftgate hydraulic pump in my 2010 BMW 535xi Touring lost hydraulic fluid level about 2 years following a quick fluid level top off. This time, there were two issues: i) bad seals on the liftgate cylinder; and ii) failure of the seal in the hydraulic pump accumulator piston, leading to leakage below the pump. 

I'll do a writeup on replacement of the lift cylinder separately. This post describes the replacement of seals and o-rings in the pump assembly.

BMW Hydraulic Liftgate Accumulator bore, piston and spring following dissasembly

When your liftgate starts struggling to reach the full open position, troubleshooting is fairly straightforward. The pump is located in the rear compartment, directly below the liftgate cylinder on the left hand side of the car, attached the left rear fender. To access, remove the left hand rear storage compartment door, then remove any audio components hindering access. Be very careful disconnecting the fibre optic connections on the audio components.

BMW Hydraulic Pump - low fluid level, fluid weeping from the accumulator (left side of pump)

Once you can see the pump, you'll note the "+" sign on the side of the plastic fluid reservoir. The oil level should be in the center of the "+" sign. In my case, the fluid reservoir was nearly empty, with traces of hydraulic fluid dripping from the accumulator housing (to the left of the fluid reservoir). 

Unfix the pump, lift to show bottom of accumulator and fluid weeping from vent hole

There are some gymnastics required to unfix the pump. Some can remove the pump without removing the left rear cargo area trim panel. If you're removing the pump for a rebuild, it may be easier to take the time to remove the trim panel. 

Once you have the pump removed from the car, and the hydraulic lines to the cylinder disconnected, you're ready to work on the replacement of seals. In the photo below, working clockwise from 12 o-clock, you have the motor top right. You have the mechanical pump mid right - between the motor and reservoir. The reservoir is bottom right. To the left of the reservoir is a hydraulic accumulator (a piston and spring designed to maintain hydraulic pressure, which holds the liftgate in the open position without the motor / pump running.) Just above the accumulator is a valve body, where the hydraulic lines connect and where the pressure sensor (top left) connects. 
Right side: Motor, pump and reservoir. Left side: Accumulator, valve body, and pressure sensor.

Removing the pressure sensor is straightforward. Remove the two hex head cap screws, and pull the pressure sensor straight up. The pressure sensor is sealed with a single o-ring - 9mm x 2mm. (All o-rings mentioned in this post are described by inside diameter (ID) then thickness. For outside diameter (OD) - double thickness and add to the ID.)

Pressure sensor with cap screws, and o-ring seal at base

I removed the motor, it is sealed to the pump with a single o-ring (41.6 x 2.4mm). I made no attempts to remove any pump components, I removed the pump, tried not to change the indexing of the drive spring, replaced the o-ring and replaced the motor. 
Motor removed. The o-ring stayed in the pump recess, it removed with a pick. 

In order to remove the accumulator piston, you need to split the two halves of the pump assembly. There are two long hex head countersunk screws which hold the two halves together. When you split the two halves, you'll note 5 oil passages which are sealed with small o-rings (4 x 1.5mm). Replacing these o-rings is very simple, just pop out of their recesses and clean any debris with a clean, lint free rag. 
Left and right halves separated - note 5 sealing o-rings.

Once you have the two halves separated, you can disassemble the accumulator. There are four hex head countersunk screws which hold the bottom plate to the accumulator body. Remove them slowly, and remove them equally (a few turns on each screw in rotation) because the bottom plate of the accumulator is under spring pressure. By separating the two halves of the pump, you ensure that the piston is at the top of the bore by removing any hydrostatic pressure remaining under spring tension. 

Accumulator piston removed from bore, note bits of piston seal disintegrating, dirty fluid

The piston will only come out of the bore with the two halves separated. Try to remove the piston square to the bore - to avoid the metal edges of the piston from scoring the aluminum bore. Same when replacing the piston, avoid rocking the piston in the bore, and insert squarely. The piston is a urethane U-cup seal, 35mm ID, 45mm OD, 7mm tall. 

Accumulator disassembled - piston, seal, spring, base and screws

Old accumulator piston seal on the left, new urethane U-cup seal on the right

New seal on the piston, old seal on the right
The final seals which are replaceable are the reservoir to pump seal which is a 39.4 x 3.1mm o-ring, and the reservoir drain/fill port, which is a 6.1 x 1.6mm o-ring. Reassemble in the reverse order of disassembly. Ensure that the accumulator is reassembled before assembling the two pump halves. 

After having rebuilt the pump, and disconnected / reconnected the cylinder hoses, you will need to bleed air out of the system. Reinstall the pump in it's normal location behind the left rear wheel well, but keep the reservoir off the pump. I used some 1/4" vinyl tubing to draw oil from my replacement oil can, and to reject oil and froth from the return line to an empty aluminum can. 

Bleeding the pump using vinyl hose. Note froth returning to the aluminum can. 

I used AeroShell 41 Hydraulic Fluid for this repair

Once the froth turns to a consistent air free oil flow, you can stop bleeding, remove the vinyl hoses, and partially replace the reservoir cup. I then refilled the reservoir using a syringe and vinyl hose to get the fluid level back to the "+" sign on the side of the reservoir. I used a regular stainless steel band clamp to hold the reservoir on the pump. 

Refilling the reservoir with the pump mounted in the car

Reservoir replaced, topped up to the correct level. 
I purchased extra seals when I completed this project, I'll put some seal kits up for sale on eBay for anyone interested in doing this repair.

Let me know if you have any questions in the comment section below. 



  1. how long did it take you to get all the air out of the system?

    1. It wasn't long - 3 or 4 cycles and it was cycling fine.

    2. Well I am having a problem that it doesn't take it up form the moment where it is fully closed but only when you help it with the first bit and it doesn't fully open aswell. Still cant find what it exactly is, did the process of bleeding the air out which was my first thought.

    3. Couple of ideas - have you replaced the non-powered strut on the right side? Is the range of motion set to full in idrive?

    4. Yes both done already as well, maybe it is just the pump which lost his full power? The tailgate was used without the hydraulic unit (strut) applied for some time because it flicked of the little balljoints all the time... so maybe the hinge got out of shape of something like that?
      Thanks for replying so fast btw!

  2. I did replace that strut already as well and checked if the range is set to full motion
    . Still cant find what the actual problem is, maybe it is just the pump but i want to be sure because it is very expensive to get a new one.
    If you have an idea what it could be let me know

    1. If you've redone the accumulator seal - then that will allow the spring pressure to transfer to hydraulic pressure to help move the tailgate. New strut, new fluid, fluid degassed, and all the mechanical elements verified (ball sockets and balls - no excessive movement) - then you're left with the function of the limit switches in the hydraulic strut (connector needs to be properly connected up above the rear speakers under the headliner) - and the function of the pump. Maybe start looking for a used pump on eBay - that's about all I can add.

  3. My problem is that the tailgate does not hold it's uppper position. Check
    The pump is tested and it seems that the pump does not hold its pressure. Do you think that renew all seals as you did will solve the problem?

    1. Daniel - some questions - 1. How old is the right side gas strut? Ever replaced? 2. Are the glass hinge springs in good condition - when you open just the glass window - can it hold itself open part way? 3. do you have a puddle of hydraulic fluid under the hydraulic pump - which may have flowed down towards the spare tire well? Have you already topped up fluid in the reservoir?

      In the video your hatch closes quite fast following opening - there is an accumulator and accumulator spring in the pump body - see photos above, which act to hold hydraulic pressure when the liftgate is open. This is used to maintain hydraulic pressure when the pump shuts off to hold the hatch in the open position. A weak spring could be an issue. I suspect there may be a ball check valve within the aluminum body with a broken spring or stuck open. I'll add a photo above. The seal kit wouldn't resolve this issue, but it would prevent the pump body from leaking following a repair. I hope this helps, looking forward to hearing from you on the three questions.

    2. Thinking about this some more, I'm more convinced it's a check valve issue, it may be relatively simple to resolve. I have a spare pump in the garage, let me have a look and see what I come up with. Get back to you tomorrow. It could be something simple like a bit of debris holding a check valve or check ball open - like a piece of a broken o-ring, or a piece off a torn accumulator piston seal.

    3. Hi,
      1. The right side gas strut was replaced when installing the pump (it's a retrofit project ;-) and the pump comes from Ebay.
      2. Yes, no problems
      3. No hydraulic fluid, no leakage at all. The reservoir was already full.

      I also think it's a valve issue. Anyway, I have to open the pump and I will see the result. I will post some photo's...

  4. My problem is the following:
    The pump is tested and the conclusion is that it doesn't hold it's pressure. Do you think that renewing all seals as you did will solve the problem?

  5. I was thinking...what happens when you switch the hydraulic hoses of the cylinder at the connection of the pump (blue vs red hose)? I ask this because the blue and red colours are not visible anymore.

  6. What size vinyl hose did you use to bleed the system.

  7. Hello everyone, I have a similar problem as in the comments.
    The trunk does not always stay open and starts to close under its own weight, and now I have to help it open the first 30 cm otherwise the pump turns off.
    Perhaps someone solved this problem?

    1. Have you checked the fluid level in the pump reservoir? That's the first step - it should be between the marker triangles moulded on the side of the plastic reservoir bottle. With fluid at the proper level, and assuming that the range of motion of the tailgate hasn't been adjusted in idrive, you can start troubleshooting from there. If you have a puddle of fluid under the pump and loss of fluid - it may be coming from the accumulator piston compartment of the pump. An o-ring replacement will take care of that problem. I hope this helps.

  8. Awesome job documenting and providing assistance with this particular chaos!

    I have a new-to-me e61 and the hatch will not raise or lower under its own power - but I can hear the pump working, and the hatch stays up after I open it manually.

    So I'm thinking the helper strut is OK, and the pump is operational.

    I have removed all the trim (chaos!!!) with help from this guide

    The reservoir level is basically empty, with fluid barely visible in the corner and a died puddle underneath. There is also sign of sticky fluid residue on the bottom of the accumulator.

    So I'm thinking full rebuild with your 9 o-ring kit

    I'm in Calgary - are the eBay listings the best way to buy, or can you offer direct sale with etransfer?

    1. Sounds exactly like what I experienced. The best way to purchase the kit is to go through eBay - everything is desribed, there is buyer and seller protection, and my sales workflows all go through eBay - much easier for me. Let me know if I can be of any more assistance.

    2. how do you run the pump to prime the system?

    3. I put the rear seats down, opened the hatch glass, climbed into the tailgate area and actuated the tailgate. I had to hop in and out of the car a few times, but it got the job done.

  9. Hello, seals are NBR or FPM ?

    1. NBR has a lower working temperature range than FPM, so I've gone with NBR. I've had the replacement NBR seals in my car now for about three years with no leaks, so they seem to be holding up fine.

  10. The ripples you created with this blog are still touching others. Thank you! I just did this on my 06 530xi with 53k. It works! It took me about 2 hours (I am an old VW/Audi mechanic from way back so I know a thing or two). I didn't remove any extra panels. I did trim the hanging sound absorber so I could pull it out of my way and make it slightly easier to get at the top bracket bolt (a tricky one to get back in!). With the bracket free and the pump off the bracket there's plenty of space to take off the reservoir and accumulator cover (didn't even unhook the wires or hoses). All I replaced was the accumulator seal. I tested it several times with the pump just hanging to confirm function. Both my pistons were fine; I did pop them off to lube the ball joints. That was a very satisfying job! Thanks again!