Flywheel/flexplate on autotransmission?

[mobilejo]

I have a 2011 e70 X5 xDrive40d with 40,000 miles. It has a vibration at idle which is strongest in the steering wheel but can be felt in the dash and the rest of the cabin. It also vibrates when driven and held at 1200RPM in any gear at any mph - so long as its at 1200rpm you will feel it. Slightly worse in D or R, but present and still quite bad in P and N. The vibration is there all the time but is worse when the vehicle is warm and is at its very worst after alot of stop start driving like in city traffic.

So far, 3 mechanics (inc. BMW) have looked at the vehicle without dismantling anything and can't see anything that may be the cause - engine mounts 'look' okay, belts and pulleys look okay, exhaust mount and heatshield looks okay, smooth runnigh test on the diagnostic machine passed, no misfire, no CEL, no vacuum leaks, no visible signs of any problem, MAF tested and working fine. Obviously no one will start pulling things apart without being paid for labour.

When coming to a stop, there is a slight metallic buzz sound that can be heard just as the vehicle stops. The same buzz can sometimes be heard when releasing the brake and starting to move off. It lasts only a second or so.

One mechanic said he would start by inspecting the automatic flywheel where it connects to the torque convertor but I have not found a single case of the flywheel on these cars needing replacement. Does this even sound plausible?

[Doug Huffman]

Isn't 1200 rpm the change over point for the variable stiffness motor mounts?

[mobilejo]

Originally Posted by Doug Huffman

Isn't 1200 rpm the change over point for the variable stiffness motor mounts?

My first suspicion was motor mounts - see my previous thread: http://www.xoutpost.com/bmw-sav-foru...ml#post1057441

But BMW tested the vac system and that was working within spec, and every mechanic who has physically inspected the mounts has said they look and feel fine. When punching the gas with foot on brake, there is no visible engine movement other than perhaps a cm or so. So I have almost ruled out engine mounts.

Also, the buzz sound (which started after I suspected engine mounts) seems to be related - and I don't think engine mounts could cause that noise without there being alot of engine movement.

[ninja_zx11]

Originally Posted by mobilejo

My first suspicion was motor mounts - see my previous thread: http://www.xoutpost.com/bmw-sav-foru...ml#post1057441

But BMW tested the vac system and that was working within spec, and every mechanic who has physically inspected the mounts has said they look and feel fine. When punching the gas with foot on brake, there is no visible engine movement other than perhaps a cm or so. So I have almost ruled out engine mounts.

Also, the buzz sound (which started after I suspected engine mounts) seems to be related - and I don't think engine mounts could cause that noise without there being alot of engine movement.

Don't know if this test "When punching the gas with foot on brake, there is no visible engine movement other than perhaps a cm or so" is adequate for diagnosing our modern active mounts.It sure applies to older rubber type engine mounts.I think your best bet would be to change mounts at this point if you are not seeing anything obvious.I felt a difference after new mounts but it all depends on how worn your existing mounts are.The difference was not much but it was there.

[ard]

Originally Posted by ninja_zx11

Don't know if this test "When punching the gas with foot on brake, there is no visible engine movement other than perhaps a cm or so" is adequate for diagnosing our modern active mounts.

Originally Posted by ard

If you place a jack under the motor trans and slightly lift it off the mounts, still does it?

Any thoughts on my technique? The idea is to change the loading on the mounts to see if that affects the resonance/vibration...

[ninja_zx11]

Originally Posted by ard

Any thoughts on my technique? The idea is to change the loading on the mounts to see if that affects the resonance/vibration...

Yes makes sense as it can be easily doable while the vehicle is idling.

Another thing OP can check all the exhaust mounts and there are couple of them.

[ard]

Get the wheels off the ground and run it, see if you can replicate the vibration when stationary. You might try and SLIGHTLY lift/hold the motor/trans and see if that changes things.

Just a WAG

[mobilejo]

Originally Posted by ard

Get the wheels off the ground and run it, see if you can replicate the vibration when stationary. You might try and SLIGHTLY lift/hold the motor/trans and see if that changes things.

Just a WAG

It happens at idle already. Which does make it easier somewhat to try and diagnose

[ard]

It can be a motor mount thing without a lot of motion. We don't know where resonance issues will occur.

At idle in park? or in drive?

If you place a jack under the motor trans and slightly lift it off the mounts, still does it?

[mobilejo]

You guys speak sense! The car is back with the supplying dealer to fix and I have told them my theory re: engine mounts. I suspect that if they can't find an actual cause, they will start with the engine mounts and hope that's it. At least I will be able to eliminate that from the list of suspects if it doesn't help.

I wish I still had the car here now to test the mounts by jacking up the engine slightly.

I will keep this thread and my other one updated with what happens.