1989 300TE, 240,000 miles.

I had a leaking rubber hydraulic hose (to the rear suspension). Following advice from this forum I replaced the rubber portion of the hose using the metal fittings from the old hose. I bought a rubber hose (from a local hydraulics shop) that had similar specs to the original Gates hose for around $20. The job was not too difficult. Rear suspension self-leveling is now functioning normally.

HOWEVER, there is now a loud buzzing noise invading the passenger compartment whenever the engine is running. The frequency of buzz is proportional to engine speed. An engine stethoscope verifies that the buzz is coming from the hydraulic line, but placing the stethoscope on the pump itself reveals no such noise (just the normal whirring sound that bearings make). I begin to hear the noise in the stethoscope as I move along the rubber portion of the hose just after the pump.

Presuming that the pulsations in the line were normal, I proceeded to check all the rubber attachment points for the hydraulic line as it heads for the rear suspension. All the mounts were good and the line was not touching the frame directly at any point. I proceeded to remove the attachments one-at-a-time, to see if any one of them was the culprit, but the noise in the passenger compartment just diminished slightly with each mount I released (still loud). I now conclude that the pulsations (buzzing) in the line are NOT normal and that the rubber mounts were not designed to isolate that amount of vibration.

I see two possible sources of excessive pulsation: (1) perhaps the pump was slightly damaged during the hose replacement operation (debris?) and is producing an abnormal pulsation in the line, or (2) the pulsation is normal and the replacement rubber hose I selected is somehow amplifying the normal pulsations to abnormal levels.

Would someone with a 300TE please check their hydraulic hose (adjacent to the coolant reservoir) to see if it has a pronounced buzzing vibration? If you don’t have a stethoscope, you can just use a screwdriver between the hose and your ear.

Any other thoughts or suggestions?

Thanks, Eric

mine was the speedo cable, I had to detach it and force some speedo grease down into it, and it is OK. Good be a different problem, as the speedo this time and the the hydraulic leak you fixed?

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Al Lueb
1999 C280 54K miles
1979 450 SLC 144K miles

I am facing exactly the same problem now, having replaced this hose and refitted the old fittings on a new piece of similar hose. I didn't use a Gates hose, but with one of same specs.

My first assumptions was that there might be some air trapped in the system, and the repair manual also mentions that if you disconnect this hose from the pump and empty the pump, you must bleed it afterwards. I will do that tomorrow, but I am a bit sceptical about that possibility. It sounds more like a fundamental resonance property of the tube.

I was reinforced in this belief after I came across an article yesterday that describes the phenomenon. It is apparently a well-known issue with hard (steed-braided) hydraulic hoses. I guess that is the reason why the original which was quiet, had fibre-braiding. I still have the old hose, cut into pieces to get out the plastic flow speed dampers - and it is clearly of a different type, much softer and flexible and without the hard steel-braiding. This of course means a shorter life, but on the other hand, it provides the needed sound absorption to make the hose silent.

I'm afraid it is an issue without resolution, other than getting the original hose. Unfortunately, I have screwed up the nut on the banjo, so it will be a drag to try to dismantle the thing again.

This also explains why it was nearly impossible to drive in the fittings all the way (a few mm missing). The new hose is hard as rock.

/Viggo

I faced a similar problem long time ago in another country where I had to repair a high pressure hose for Power steering, the new one was steel braided and was very loud, The original hose was fibre braided.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)

Is the original fibre braided hose availible anywhere?

When it comes to hydraulic parts it's best to buy OEM. Recent research at a large pump manufacturer showed that they could take an average of 5 dB(A) out of a standard power unit merely by changing the configuration of the hydraulic hose.