Thanks for the tips! Yes it does consume a little bit oil, and the valve stems has not been replaced. Oil consumption is about a quart every 3000+ miles. I forgot about the valve stems. I do believe they need replacing. The other plugs look clean and fine, by the way. I'll follow up on these tips and let you know how it goes! I'll try and replace that plug I put in yesterday with another new one and see if it helps a bit since I have another laying around.

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99 W210 E220 cdi sedan automatic (248.000 km) 210.006