One for the experts on vacuum systems!

[mobilejo]

I am trying to diagnose a problem with the vacuum operated hydraulic motor mounts on my LCI e70 Xdrive40d - the diesels use mounts that have vac lines going into them, they work as follows apparently:

The damping characteristics of the hydraulic mount are set softer or harder by means of a vacuum. In this way, the vibration transmitted from the engine to the body can be influenced specifically.” (Quote from BMW training manual)
Failure of the pneumatic vacuum system will prevent the engine mounts from entering the ‘soft’ state which is activated at idle and low engine speeds.

Engine Mount Control DDE 4.0

The controllable engine mounts are switched to "hard" or "soft" corresponding to operating conditions.
Function

The pressure converter connects the engine mounts to the vacuum supply by means of hoses. When activated by the DDE control unit, the pressure converter switches vacuum through to the engine mounts so that they switch to "soft".
The engine mounts are activated at:
Road speed < 60 km/h and
Engine speeds < a value which depends on the coolant temperature as follows:
Below -40 °C: 1200 rpm
From -40 °C to -10 °C: 1200 rpm to 1050 rpm
Above -10 °C: 1050

Troubleshooting

Fault code 3520 "Engine mount control" is stored if a defect occurs in the activation of the pressure converter.
The engine mount control is switched off if one of the following faults occurs:
0500, "Driving speed signal"
0115, "Coolant temperature sensor"

Realoem diagram of relevant parts:



Now my car vibrates at idle and low revs, suggesting the mounts were not entering the soft state. To test I disconnected both vac lines ot both mounts - no change in idle at all. On a 'working system' doing this should cause idle to get "vibratey" because this would effectively switch off the soft state. No change indicates it was never in the soft state, which is what I suspected.

So I disconnect the main vac line (no. 16 in diag.) where it joins no.12 in the diag (i.e. before it is split to the 2 mounts), there is no vacuum at this line. This line (no. 16) goes in one complete length to the solenoid (no. 13).

I don't have any of the fault codes mentioned above so I can't understand why no vacuum is being delivered.

So, what do I need to test/look at next? I will try and get a new part 13. but I would have assumed a fault code if this failed? Where does this solenoid get it's vacuum from (i.e. what is on the other end of vac line 15?)

Thanks

[Doug Huffman]

(Quote from BMW training manual)

I can't help but would like to learn. Do you have a link URL to the BMW training manual, please, to share?

[jfoj]

Since most Diesels have very poor intake vacuum, most systems have a vacuum pump driven from the engine. I would find if the engine has a vacuum pump and/or vacuum reservoir and make sure there is enough source vacuum.

I assume there should be source vacuum available on hose 15 in the diagram.

The more bandaids I see on these Diesels, they happier I am that I do not own one.

[bawareca]

^^^ Since N62 is using no throttle body to control the engine load it has a mechanical vacuum pump and it is another problematic downside of this engine.
But I digress. The vacuum is also used to control the turbo[s], so if there is a low vacuum or no at all there will be a problem with the boost control and it should be pretty noticeable.
Here is the vacuum control diagram

RealOEM.com - Online BMW Parts Catalog

[mobilejo]

Originally Posted by Doug Huffman

I can't help but would like to learn. Do you have a link URL to the BMW training manual, please, to share?

Yeah, I downloaded it here: http://www.kaross-chip.lv/m_download...ommon-Rail.pdf

[mobilejo]

Originally Posted by bawareca

Since N62 is using no throttle body to control the engine load it has a mechanical vacuum pump and it is another downside of this engine.
But I digress. The vacuum is also used to control the turbo[s], so if there is a low vacuum or no at all there will be a problem with the boost control and it should be pretty noticeable.
Here is the vacuum control diagram

RealOEM.com - Online BMW Parts Catalog

I think my problem may be at the electrical switch at the end of hose 16. I disconnected both vac lines from this switch, and there is vac going into it (from hose 15) but nothing when I cover the nozzle of the switch that hose 16 goes over.

I might make a temp bypass of the switch by just connected the 2 hoses together and see if I get vac at the other end of hose 16 (the engine mount end). Any reason why this wouldn't work?

[bawareca]

Could be the pressure converter itself. It will be better to check for activation first- "+" and "-" on the connector while on idle. The BMW document mentions that this function will be deactivated at certain conditions. Did you check fault codes in the DDE?

[ninja_zx11]

Originally Posted by mobilejo

I think my problem may be at the electrical switch at the end of hose 16. I disconnected both vac lines from this switch, and there is vac going into it (from hose 15) but nothing when I cover the nozzle of the switch that hose 16 goes over.

I might make a temp bypass of the switch by just connected the 2 hoses together and see if I get vac at the other end of hose 16 (the engine mount end). Any reason why this wouldn't work?

I went through all this couple of months ago HERE trying to find the cause of faint vibrations and eventually found that my E70's mount system was working properly even thou i feel fine vibrations at my steering wheel at idle.
So this is what i found after doing some research. and here is the confusing part:
On E65,E6X,E70 mounts are switched to hard when vacuum is applied.
On E46,E53,E83,E9X mounts are switched to soft when vacuum is applied.

It means mounts are in soft mode when there is no vacuum at idle.

So do one test.Unplug your vacuum hose from the mount and rev the engine past 1000~1500 rpm and check if vacuum is applied to the mounts.If it is ,means your pressure converter solenoid valve is working correctly and your system is working properly.

Hope it will help you...

[mobilejo]

Originally Posted by ninja_zx11

I went through all this couple of months ago HERE trying to find the cause of faint vibrations and eventually found that my E70's mount system was working properly even thou i feel fine vibrations at my steering wheel at idle.
So this is what i found after doing some research. and here is the confusing part:
On E65,E6X,E70 mounts are switched to hard when vacuum is applied.
On E46,E53,E83,E9X mounts are switched to soft when vacuum is applied.

It means mounts are in soft mode when there is no vacuum at idle.

So do one test.Unplug your vacuum hose from the mount and rev the engine past 1000~1500 rpm and check if vacuum is applied to the mounts.If it is ,means your pressure converter solenoid valve is working correctly and your system is working properly.

Hope it will help you...

DUDE! That is amazingly helpful! I have been racking my brain wondering why there was no vac at idle! This answers that question. So my mounts are soft with no vac, and hard with vac? This would explain why my idle seems slightly worse when I bypass the solenoid and connect the mounts to a source of always on vacuum.

So next question, how did you resolve your issue? I read your thread but I didn't see a resolution?

[mobilejo]

Also, can you remember the following: when I put my thumb over the nozzle on the mount itself (i e the nozzle that the vac pipe would go onto), I feel fluttering air coming out of the mount, this suggests that while idling the spring in the mount is bouncing up and down a little. Hastens in both mounts.

Did you ever check if your mounts did this?

If this bouncing of the spring is basically allowing fluid to move into and out of the chamber that controls soft/hard setting, then surely this is a problem on idle? Would this fluttering not occur if the vacuum system after the mounts is air tight?