Mercedes C250 Cold Start

Our 1996 C250 diesel (no turbo) performs exactly as specified by MB re performance, economy etc and has now covered 133,000 k's (80,000 miles).
However,cold starting is rough and becoming more so. Glow plugs are not
indicating as the problem as dash light remains on and goes off exactly as specified.
Our workshop manual suggests maintenance of injectors at 100,000 k's (60,000 miles) should be provided. Mercedes Benz garage reponse is "Never touch the injectors, usually."?
Our diesel is poor (high sulphur) but improving vastly soon. Only slight smoke when pulling very hard and starts very smoothly when just a little warm. Rough cold start is as if only a few cylinders firing and whitish smoke emitted. A little bit of throttle quickly clears up problem.
I have no doubts a diesel shop could fix without problem but we rather like looking after this magnificent piece of engineering ourselves. Can anyone help and by the way the cold start ambient temps are presently around 10 degrees fahrenheit. Many Thanks
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Install new Bosch or Beru glow plugs. That will clear up the problem.
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Many thanks, new plugs bought - installing Saturday. Will advise outcome which I believe will confirm your advice. This possibility eluded me because I became convinced that any glow plug problem would be revealed via the dash light but this seems to not be the case.
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New glow plugs installed. Cold start now perfect. Injectors now beyond suspicion so will leave well alone! Had to remove intake manifold to install although manual did not suggest this was necessary. Thanks for your trouble.
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