poor idle

my 300sd has developed a very poor idle and a lack of powre could it me the fuel filter is this a typical symptom

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Check for vacuum leak also.
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Fuel starvation isn't noticed at idle because so little fuel is used then. It would be felt during acceleration and hill climbing when the engine needs fuel to make power.
Poor idle when? At cold start or always?
The engine can lack power for several reasons:
Sudden onset of poor performance suggests bad fuel or gasoline mixed with the diesel fuel. If that's the case you can drive it and so burn the fuel or, if it's really bad, drain the fuel tank. Bad fuel would account for sudden poor idle and poor power.
This old diesel engine has mechanically adjusted valves. If the problem came on gradually and includes difficult cold starting the valves may need to be adjusted to restore the engine's full compression.
Regrettably one also needs to consider that the engine may be badly worn given these symptoms and mileage in excess of say 250K miles (400K km). A compression test is the only definitive way to know if the engine needs an overhaul or valve adjustment or neither.
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