So I undertook a power steering flush on my 2017 Rapide S, and turns out it was probably overdue even though the car has low mileage.

The power steering system on my car is quasi universal, sporting a generic ZF fluid reservoir and cap. The part is shared with a lot of other cars, which makes getting service kits quite easy. Fluid wise, mine runs on Pentosin CHF 11S fluid. The system itself was originally designed with ATF fluid in mind, so the reservoir cap either has "ATF Only" or "CHF 11S Only" fluids . The CHF 11S and cheaper equivalent CHF 202 has a LOT of aftermarket equivalent, some are CHF 11S certified ( Feb BIlstein CHF 11S ), others have the same specs ( Liqui Moly 1127 ). I went with the original Pentosin branded CHF 11S as new fluid as I wanted to compare the degradation / oxidation of my original fluid so having new CHF 11S was going to make it easy to compare. You can also buy Mercedes / BMW branded Pentosin fluid, sometimes cheaper than the non co-branded fluid itself!

I went for a BMW service kit, which included a replacement reservoir and 2x 1L tins of PS fluid. I bought it during Black Friday for cheaper than Aston's retail price of 1x PS fluid tin. Note that there's 2 versions of the reservoir, mine has the outlet/inlet barbs at an angle.




The PS reservoir has a filter membrane, in it,, which turned out to be a good thing. When I sampled the PS fluid in my system, couple things stuck out:



1. The fluid level was low
2. The fluid was very oxidised with rubber black having leeched into it
3. Looks like the dealer topped it off with the wrong fluid with a different density, it sat at the bottom of the bottle when I emptied the reservoir!!!

New Vs Old:



I went ahead and emptied the OE reservoir, removed the Oetiker clamp and plugged the return line to an empty jug so I could flush the system. By doing so, I was able to add new fluid to the reservoir, and pump it through the system by turning the steering wheel and even running the pump for a very short time. I did so until the liquid on the return line was clean and new...

You will need a 1" Oetiker clamp... Here's old vs new (post-install):


Plugged the return line to a jug and cycled the fluid through with steering wheel action until new CHF 11S was on the return:



After replacing the reservoir, I opted to keep the original cap as it has the CHF 11S requirement on it. The OE replacement didn't.



Once the new reservoir was installed, I filled up to the Min line (the Cap has Min / Max lines ) and then worked to flush out any air or gaps in the system. Ran the car and often checked the level.



After I was satisfied the system was purged correctly, I filled 1/2 way to Max and called it a job...


One last thing... The reservoir cap is vented directly to atmosphere, and if you ever overfill, it'll spill out. Conversely, it can draw in air directly from the engine bay... with dust and all. So I used a trick I learned from the BMW community and made a small filter... Don't judge me!





In the end, left with about 1/2 L of PS fluid, and a successful flush that was probably overdue given the color of the old fluid I pulled out.

 

Great job, thanks for the write-up!

 

Well done

 

At which service interval does Aston recommend changing the power steering fluid? Every other fluid has a schedule.

 

No interval on AM. Oddly enough.

BMWs with the same system is usually 2yr or 5yrs, 30k miles

 

Interesting. I'll add it this years annual which is normally in November.

 

A couple items of note:
1st, Thanks for the tip on the cap little vent filter addition, i'll have to go check mine out.
2nd. When pumping the fluid out, via turning the steering wheel, top off the fluid about every 2 lock-to-lock cycles, that way you don't lose the prime and or introduce more air into the system.
3rd. There is a recommended warm up and purge air procedure that Aston1936.com has written up, when doing this i would add "Do not do this in one location" move the car about 1 ft every 2 locks, i did this because i did not want to flat spot my front tires. If you do that on asphalt it could really chew them up. This purge procedure worked well, i could feel a lot of jitter in the steering wheel at the beginning, but none when completed.

 

All great tips. Especially the last one.

For tip 2. I actually checked the level quite often, like twice per lock to lock turns and added fluid as soon as it would drop. Very important to not introduce more air in the system..

For tip 3, Absolutely don't do go lock to lock on the steering wheel with the car static. it's probably very hard to do with the engine off!! and with the engine on, it probably strains the pump.

In my case, I forgot to mention my work setup didn't require me to rock the car back and forth...

 

Thank you for the write up. Is it not necessary to add an additive to the fluid? I saw on Aston 1936s video that he adds "Librizol" to the fluid. I need to to do this procedure on my v8v but not sure how to obtain the sdditive.

Thanks Again!

 

I wasn't sure about the additive either, so i added it.
Pelican Parts Lubrizol 6178 about $12.50 it is listed under Mercedes-Benz

 

Thank you! Did you obtain the kit through Pelican as well?

 

Thanks for the tutorial…it’s now on my list..lol

 

No kit, and i did not replace the tank on mine.
I went with Amazon for the Pentosin CHF 11S, I ended up buying 2 cans.
Home Depot for the 1/2" ID clear hose, 00 rubber stopper, and double ended barb fitting.
Temperature probe.

 

The Lubrizol additive is required for the pre 2014? steering racks. It's called out specifically in the owner's manual. In fact, I dont think your cars use CHF-11S originally. The Service Bulletin for your model year calls to flush out the OEM fluid, and swap to CHF 11S w/ the Lubrizol additive.

The steering rack on my 2017 car doesn't require it so I didn't add it.

 

Nothing more visually satisfying than replacing PS fluid! Thanks for the write up!