6.9 Hydro oil and foam

[daantjie]

Hi guys

Would like to gauge the collective wisdom of the 6.9 guys on the appearance of the hydraulic oil as it gets bled from the system. There seems to be a lot of anecdotal advice as to air or lack thereof in the 6.9 hydro system.

When I open the front bleeder valve under the battery tray, the fluid comes out under high pressure of course, and appears quite foamy in the catch bottle. This fluid settles to a clear liquid in about 10 minutes.

What I am gleaning from other forums is that this is normal behaviour of fluid under high pressure, and not an indication of air in the lines, which would of course be undesirable?

Any thoughts on this?

Cheers
Daniel

 

[Styria]

What you glean from other forums is correct. Regards Styria

 

[daantjie]

Thanks Syria, one less thing to worry about!

 

[Oversize]

Foaming can only come from 2 sources. A faulty pump, or the cells.... There should be no air/gas in the fluid and poor handling will be the result

 

[Styria]

Mark, not quite right. There is constant pressure in the system so obviously when you release it, it will be foamy initially. Picture the situation of even clean oil and you blow compressed air into it - it will start to foam. Try it. Regards. Styria

 

[Oversize]

Pressure doesn't equal foam. The air/gas has to come from somewhere. I wonder if it could happen due to to a sudden pressure, or temperature change?

 

[Styria]

I don't know if this is right, and if we can compare this with hydraulic oil. However, take soapy water. If you had it locked in under pressure, and you'd release that pressure, I reckon I'd be right in stating that it would be all bubbly - right ? If then you'd leave it in a container for fifteen minutes, the bubbles would probably disappear. Make sense ? Maybe, yes - no ? Regards. Styria

 

[daantjie]

What do you guys think are the chances of the foam being caused by pump cavitation, perhaps due to a restriction on the suction side?

 

[john erbe]

I'm going to really stir the pot. Once upon a time (this is a true story)when I still had the hydraulic suspension on my 6.9, I thought this Febi oil is rather pricey, and, I wondered if there was a substitute. After doing some research I discovered that rape seed oil has a higher compression rating ( I was also looking for a stiffer ride). Rape seed oil aka canola oil is cheap and obtained at your local market. So I dumped it in and voila! it worked fine but there were bubbles/aeration clearly visible on the dipstick. My next thought was to find an antifoaming agent. Never followed up on that having decide to toss the whole system and converted to shocks, thusly no Nobel Prize. I think I'll have another glass of wine.

 

[daantjie]

I know some guys run a thicker Hydraulic Oil such as a 22 or 32 weight to give a firmer ride. I believe the Febi ZH-M oil is 15 weight. I think as long as you stick to a good quality mineral oil, and still thin enough not to stress the pump you should be fine. I think "ARAL 1010" was the brand originally specced by Benz but good luck finding that today. On the Benz spec sheet "343.0" they only state the Benz oil and Castrol Vitamol ZH-M. I think running the Febi stuff is the best bang for the buck if you want to stay close to MB spec.