Have 2011 ML350 with 246K miles and power steering issues. Have owned it since ~70K miles. Been a very good vehicle so far. So much so, I purchased a 2011 and 2012 over the last 5 years. Both have provided good service with minimal issues.

My daughter complained that the steering was difficult and brought it over this weekend. It was very difficult to turn and the pump was really groaning. I pulled the reservoir and replaced the o-ring between it and the pump and filled with fluid (yes, I'm using the correct one) . I also went through the jack it up and lock to lock routine to try to purge the air. I got it to the point where there were no air bubbles showing up in the reservoir while turning the steering wheel without the engine running. Everything seemed ok and I filled the reservoir to the lowest level on the dipstick/cap and started up the engine. The steering is much easier but the pump groans and is pushing fluid out of the reservoir cap. The gasket looks ok on the cap and it is tight when closing. I took the cap off with the engine running and there is a tremendous amount of air bubbles/froth of fluid that will overflow. I cleaned thoroughly the area around and below the pump along with the connecting points at the rack and radiator. I don't have any leaks anywhere. The only fluid leaking is from the froth and air bubbles pushing the fluid out of the reservoir with the cap on. I assume this is not normal and there is something really wrong inside the pump.

I have a rebuilt MB pump and reservoir coming. Is there anything special I need to do when installing these two items other than don't start the engine until the air purging is done? Is 50 cycles left to right to left stop enough? Could there have been damage to the rack caused but low or no fluid in the reservior/pump? Any other insight or points I need to cover?

TIA

 

Mine was fine after a pump replacement going lock to lock with the wheels in the air. I went lock to lock until I stopped seeing bubbles coming back from the return line and went back and forth probably another 10 times or so. A previous non-MB car had you do the second step of going lock to lock repeatedly while the engine was cranking with an EFI fuse pulled (to prevent the engine from starting), but that didn't seem necessary here. May not be applicable on a diesel, but a tip for gas engined GL/MLs is to replace the cam magnet that sits behind the reservoir if you ever do the pump and/or reservoir. The part is cheap and it is much easier to get to if the reservoir is already off.