Some diesels have a throttle control to speed the engine RPM's when warming up...I doubt if this one has one. All diesels need new/clean fuel and air filters and dirty ones will hurt performance and starting. Injectors should be checked also. If it has glow plugs they should be checked along with the circuit that operates them. May be as simple as adjusting the throttle idle speed...may not be set at specs.

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1983 300SD 343K everyday car
1983 300SD 285K from junk yard-tooks parts from deer car- runs great. Brothers car.
1984 300SD parts car-Hit deer
1979 300D 175K non-turbo "Doctor"
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