WHEN I MERCEDES 300SD DIESEL HAS A LOUD ROUGH IDLE , BUT WHEN U ACCELERATE CAR, THE ROUGH IDLE GOES AWAY, WHAT CAN THIS PROBLEM BE AND HOW DO I FIX IT?

Worn/dirty injector nozzles-do a 'dieselpurge'
Worn motor mounts

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$60 OM617 Blank Exhaust Flanges
$110 OM606 Blank Exhaust Flanges
No merc at the moment

Unfortunately it can be caused by all or any one of the following:
Air leak in the fuel inlet system
Valves that need to be adjusted
Worn out Fuel Injectors
Worn out Motor and Transmission mounts
Worn out Damper Shocks (by the Motor Mounts)
Rack Damper Pin that needs adjusting or up dated with the newer Gold colored one
Fuel Injection Pump needs to be timed
Uneven Cylinder compression
Thats all I can think of now.

On mine replacing the Rack Damper Pin with the updated Gold one did not help much.
Adjusting the Valves helped some.
Rebuilding the Injectors with new Spray Nozzzles helped about 80%
I replaced the Motor and Transmission mounts; that helped some more.
At the end of all of that re-adjusting the Gold Rack Damper finally had some good effect.
I also had a small Air Leak that replacing the Fuel Inlet Hoses down by the plastic Primary filter also took away a small amount of idle shake.

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84 300D, 82 Volvo 244Gl Diesel

Is the rough idle all the time, or just after it's warmed up?

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83 SD

84 CD

yes it is all the time , but only when i am stationary... a light, stop sign,but when i acceralate it goes away...

You have a long list of things to check, (Diesel911) but I'll suggest a fuel issue first. Start by replacing all filters. See if there are any leaks anywhere in the fuel distribution.

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83 SD

84 CD

While things do not always go by the book if there was a restriction in the Fuel Inlet System or the Secondary Filter by the book it is supposed to show up when you need more fuel; when you accelerate.
He said it is not rough after he accelerates and, he did not complain of a loss of power. I think this is an indication that his filters are not plugged.

Now a small air leak in the Fuel Inlet System could have less effect during acceleration. If you changed the Filters and during the filter change you cured an air leak; such and tightening the clamps on the Primary filter or if the Primary Filter had a tiny crack in it; a filter change could help the symptoms he described.

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84 300D, 82 Volvo 244Gl Diesel