300sd rumble or droning - PeachParts Mercedes-Benz Forum

Markus · #1 09-29-2011, 07:07 PM

I have a 1983 300sd that idles normally in P or N, but when you select D the rumbling and droning in the cabin is alarming. Replaced the motor mounts and transmission mount 2 years ago. With the car at idle, in D and brakes on, the rumble seems to be resonating from the area around the battery. In fact I can get the rumble to disappear if I push the top of the valve cover towards the drivers side of the car. Bad or prematurely failed passenger side motor mount or am I missing something else thats obvious? I do not see the exhaust touching the frame or body anywhere and shaking the exhaust does not change the rumble. Put the car in R and the rumble also disappears, same with P or N. Driving off the rumble continues but diminishes with speed, it all comes back at a stop light with your foot on the brake in D.

vstech · #2 09-29-2011, 10:10 PM

what brand motor mounts did you choose? some brands URO? are KNOWN to fail nearly overnight.
also, did you change the shocks when you replaced the mounts? they work in concert, and a bad shock will drastically shorten the mount's life.
another thing on my SD was the downpipe to transmission mount. it's a Ubolt with a steel bracket and two rubber mounts to the eyes in the bottom of the trans.
I had to disconnect the exhaust near the trans mount and adjust the angle of the main pipe, after a transmission shop reassembled them. the main resonators were slightly askew, and contacted the tunnel.

Markus · #3 09-30-2011, 08:49 PM

No I did not replace the shocks, my bad on that, it just ran so smooth afterwards did not think it neccessary. The mounts were ( I think Meyle, maybe Corteco, I remember the top face of the mount was glittery, like pressed flake metal). Do you also need the engine shock mount kits or just the shocks? I plan to remove the exhaust U-bolt mount and see if that makes a difference, the exhaust is original I think. I also plan to run the car again in D (wheels chocked) and lift the motor slightly with a jack and block on the pax side oil pan and see if I can isolate the contact point. Thoughts?

vstech · #4 09-30-2011, 10:24 PM

if the shock mounts are not broken, don't replace them, they are VERY PRICEY!
did you replace the motor mount shields when you changed the mounts? diesel or oil on the rubber will kill them quickly also.
yup. moving things, and listening to the noise is a good way to trace it down. took me months of wiggling before I found the exhaust slightly askew touching a bolt inside the tunnel...

Skippy · #5 09-30-2011, 10:52 PM

Broken air cleaner mounts?

Markus · #6 10-01-2011, 07:39 PM

Found it!! Removed the down pipe bracket entirely, ran car no help. Lifted motor on passenger side just enough to stop the rumble and discovered that the "new" engine mount had collapsed so far that the engine shock mount was rubbing on the metal frame of the engine mount. Since its the weekend and could not now buy a new part placed a spacer inbetween the aluminum engine block mount and the rubber engine mount, now have 1mm clearance and no rumble when car is in gear. I think the mount is so "soft" it must be from a gas car, or really poor quality.