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1999 Mercedes-Benz E320 4Matic
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44 Posts
Discussion Starter #1 (Edited)
When accelerating from low RPMs (basically if the car is moving and my foot is off the gas then I accelerate) I hear and feel a very slight thud or bump. It almost feels and sounds like it's coming from the back of the car. A similar although even less noticeable thud or bump happens when backing completely off the accelerator from around 20-30 RPM. This does not happen when accelerating if I already have my foot on the accelerator. I would assume it's a motor mount or trans mount but it's such a slight sound/feel that I'm just not sure.. I have driven cars that just make a little noise when the transmission engages. Other symptoms the car is suffering from that may or may not be related are a very slight vibration at idle (especially when in gear) that seems to get better as the car warms up, poor MPG (14-16 avg. I did just change the plugs today so this may improve). I am going on a short road trip (about 1500 miles round trip next week and want to make sure we are safe and not going to spend time and $$ in a garage while gone. Any help is greatly appreciated.

I should also mention this only happens when in gear and does not happen in park or neutral.
 

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When accelerating from low RPMs (basically if the car is moving and my foot is off the gas then I accelerate) I hear and feel a very slight thud or bump. It almost feels and sounds like it's coming from the back of the car. A similar although even less noticeable thud or bump happens when backing completely off the accelerator from around 20-30 RPM. This does not happen when accelerating if I already have my foot on the accelerator. I would assume it's a motor mount or trans mount but it's such a slight sound/feel that I'm just not sure.. I have driven cars that just make a little noise when the transmission engages. Other symptoms the car is suffering from that may or may not be related are a very slight vibration at idle (especially when in gear) that seems to get better as the car warms up, poor MPG (14-16 avg. I did just change the plugs today so this may improve). I am going on a short road trip (about 1500 miles round trip next week and want to make sure we are safe and not going to spend time and $$ in a garage while gone. Any help is greatly appreciated.

I should also mention this only happens when in gear and does not happen in park or neutral.
Best to record and upload the thump sound you are referring to, otherwise, it's whack-a-mole, on what you describe here.
 

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1999 Mercedes-Benz E320 4Matic
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44 Posts
Discussion Starter #3
Best to record and upload the thump sound you are referring to, otherwise, it's whack-a-mole, on what you describe here.
I dont know that its loud enough to record... Its more like something you feel rather than hear.
 

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Dallas, Texas
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I dont know that its loud enough to record... Its more like something you feel rather than hear.
Open hood. Firmly hold brake pedal. In D blip the throttle engine should move some but not lift off the left mount. Repeat in reverse.

 

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1999 Mercedes-Benz E320 4Matic
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Discussion Starter #5
Open hood. Firmly hold brake pedal. In D blip the throttle engine should move some but not lift off the left mount. Repeat in reverse.

If it moves in drive but not reverse which mount is bad?
 

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1998 e430
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391 Posts
agree on replacing both. while you're at it, do the transmission mount too.

another symptom of bad mounts is engine vibration at idle.

after i did mine, i was re-acquainted with how smoothly these cars run...
 

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1999 Mercedes-Benz E320 4Matic
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44 Posts
Discussion Starter #8
It is the mounts... All getting replaced tomorrow.
agree on replacing both. while you're at it, do the transmission mount too.

another symptom of bad mounts is engine vibration at idle.

after i did mine, i was re-acquainted with how smoothly these cars run...
Got all three from amazon... they will be here in the morning. I can only imagine how quiet and smooth everything is going to be by tomorrow afternoon.
 

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Dallas, Texas
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Amazon? Link to parts? Most of the stuff Amazon sells is cheap pure chinesium.

Real Lemforder or Corteco.
 

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1999 E320, 2000 E320, 2003 E320 Wagon, 2005 C230K SS, 2010 Accord LX w Eibach & Koni FSD's
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2,883 Posts
Check your flex discs also.
 

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1999 Mercedes-Benz E320 4Matic
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44 Posts
Discussion Starter #11
Amazon? Link to parts? Most of the stuff Amazon sells is cheap pure chinesium.

Real Lemforder or Corteco.
They are aftermarket parts... but have excellent reviews.
 

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Dallas, Texas
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They are aftermarket parts... but have excellent reviews.
Day I have pulled my thing out I have been getting very positive results. Problem is once you put bolt and nut through it I cray. 馃槀
 

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2008 E350 110k, 2000 E320 143k, 1983 380SEC 240k
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949 Posts
It's the matrix sliding back and forth in your catalytic converter. It would also explain your other symptoms. Get a rubber mallet and bang on the rear converters.
 

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1999 Mercedes-Benz E320 4Matic
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44 Posts
Discussion Starter #14
It's the matrix sliding back and forth in your catalytic converter. It would also explain your other symptoms. Get a rubber mallet and bang on the rear converters.
It ended up being a bad DS motor mount. I am getting all three replaced today. Catalytic converters are fine and after plug change MPG is back to acceptable 18-20 MPG avg.
 

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It ended up being a bad DS motor mount. I am getting all three replaced today. Catalytic converters are fine and after plug change MPG is back to acceptable 18-20 MPG avg.
TY for reporting back your resolution. (y)
Many don't.
 

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1999 Mercedes-Benz E320 4Matic
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Discussion Starter #16
So just a quick update.. they sent the wrong trans mount but it really seemed ok so just replaced the motor mounts for now. This resolved all shaking (which was pretty minor anyway) as idle is much smoother and car is generally much much quieter now.. I still get a little bump coming out of first gear but I suspect that is just the transmission being older. I drive like an old man so it should not be a problem for quite some time. Next project will be to replace the rack and pinion as I discovered a small leak.
 
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