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1984 mercedes benz 300d
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10 Posts
Discussion Starter #1
So my car is rocking really hard at idle but only after its warm. If I get to 700-900rpm the rocking stops. Any ideas??
 

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1985 300CD Turbo coupe, 2006 E320 CDI sedan
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573 Posts
Motor mounts, probably all four - but specifically the little shock absorber pair.

//greg//
 

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1985 300CD Turbo coupe, 2006 E320 CDI sedan
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573 Posts
The turbo models only had 3.
Two per side, and transmission mount.
+1 on rack dampener pin adjustment.
Well, I've never physically raised an '84 sedan on the rack for inspection, so for purposes of this thread I am referencing my '85 coupe. And I can tell you that there are four on mine; two mini-shocks and two conventional hard rubber mounts. I've replaced all four, which fixed a original owner vibration problem that cracked the Z-pipe and broke the air filter housing mounts. That said - at 265k miles - I'm still on the OE transmission mount.

I find the rack damper suggestion interesting though. Again, 265k miles - never touched it once.

//greg//
 

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W-1-2-3 Go!
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16,161 Posts
Well, I've never physically raised an '84 sedan on the rack for inspection, so for purposes of this thread I am referencing my '85 coupe. And I can tell you that there are four on mine; two mini-shocks and two conventional hard rubber mounts. I've replaced all four, which fixed a original owner vibration problem that cracked the Z-pipe and broke the air filter housing mounts. That said - at 265k miles - I'm still on the OE transmission mount.

I find the rack damper suggestion interesting though. Again, 265k miles - never touched it once.

//greg//
Ah, I see where the confusion was. I don't consider the shocks as mounts.

The turbo 300 diesel had 3 mounts (2 for engine, 1 for transmission) for the main engine + transmission, plus two shock absorbers (one per side). Although irrelevant, the other mount is of course for the differential.

Maybe yours has been updated already. The spring inside wears out over time and some adjusted is necessary to compensate for that.
 

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'73 450SL, '83 300CD, '01 E320 4matic
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2,446 Posts
If the rack damper bolt adjustment doesn't prove successful, before you go much further, remove the injectors and perform a compression test with the engine warm.

Speaking from (annoyed) experience, a weak or dead cylinder causes the engine to violently rock- My #5 cylinder has only 200psi warm, the rest remain at 405. I'm sure it's stuck rings on the #5 piston due to the PO's attempt at rebuilding the injectors on his kitchen table.... (the #5 injector was constantly draining fuel into that cylinder).

A cylinder imbalance can cause this to a lesser degree.

It's a good test to determine the health of your engine as a whole, anyway. Remember to purchase new injector seals (heat shields) they are inexpensive, and should be replaced each time an injector is removed for any reason.
 

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1984 300D
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5,070 Posts
What I had to do to get rid of the my when Engine hot vibration/shaking:

Raise the Idle speed within the normal range (this helped some; it was around 700rpm). I rased it to about 750 rpms.

I bought a new Rack Damper Bolt but it did not help much (like changing the Air Filter Bolts it will not cure vibration caused by other things; will come back to that later)

Rebuilt my Injectors with $16 each brand X Spray Nozzles (This took away most of the shaking)

Doing a Valve Adjustment (took away some more shaking)

Changed Motor and Transmission mounts (took away some more vibration)
If you can do the Valve adjustment youself do that first as it will not cost you anything except the cost of an new Valve Cover Gasket.

Found that I had a small air leak that showed up when the Engine was hot due to hard fuel inlet lines that did not seal even with the clamps tight.

Finally was able to use the Rack Damper Bolt to tune out a little more shake. (that bolt was made to reduce the shake on a new engine; not to compensate for a bunch of worn and out of spec parts

So, the Valve Adjustment and rebuilding the Injectors took a way the largest amount of shaking.
 
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