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Why is my diesel car emitting blue smoke upon starting up?

I have a 2007 Peugeot 207 1.6 HDI, with 83,000 clocked miles. Basically, the longer the car sits unused, the more blue smoke it puts out when it starts and also the harder it is to start (cranks slower the longer it is left). Sometimes if the car is left for a full week, it will almost not start. This is even with a relatively new 640CCA 64AH battery. I'm not certain if the blue smoke issue is linked with the slower cranking but who knows. The recent cold snap is also taking its toll and increasing the difficulty of starting. The vehicle is normally used twice a week and covers approximately 40 miles for both days it is used. Another similar vehicle is used in the same manner and has no such problems. Performance when pushed to full throttle is as expected and it also returns similar MPG to the other vehicle. After a run, there is a faint odd smell from the exhaust, hinting that it is continuing to burn some oil as it is driven, however, no smoke is visible when driving, even at full throttle. The oil level on the dipstick does not drop at an abnormal rate so it can't be burning a lot. Engine idle speed is stable even immediately after a difficult start (although the start itself may be quite rough). It does not overrun when switched off.

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The most obvious reason is one or more failed glowplugs failing to ignite the fuel/air in one of the combustion chambers leading to incomplete combustion and hence the smoke on start up. One the engine is running to temperature, ignition is by compression so it will then run fine.
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