C250 turbodiesel first registered 1999/2000. It feels like its running on less than the 5 cylinders when starting when engine is cold, the glow plug dash light remains on for 2 or 3 minutes. The engine runs as normal after 2 or 3 minutes. Its been this way for several months. I had a look under the cylinder head cover: all seemed fine but I did notice big build up of soot in the air intake hose. I drive less than 500 miles per month and mostly short journeys where the engine temp often doesn't get up to the normal 80 degrees celsius (176 fahrenheit).
I've put in a cleaning additive in with today's tankful of diesel in the hope that it will clean any soot or carbon build up from the valves.
My question is how serious is this and what needs to be done. I shall be very grateful for your thoughts.

I am NOT familiar with your engine, but I'll say on older diesels that may be a case of failing glow plugs. My CD stumbles (just for a few seconds) at first start up, I know the plugs read 1.7 ohms. I believe the GP's stay on a bit even after the engine has started to help smooth the idle.
Soot would be normal in any diesel, I would think.
Good luck, and post any resolution.

EDIT: I'm a Scot descendant.

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