I found that I had 3 dead glow plugs which affected starting from cold when the engine was below about 10°C. However when the engine was between 10° and 15°C the glow light doesn't light, but I'm sure the glow plugs are still used to smooth the idle. My TD4 did suffer rough running when starting above 10°C, until I changed 3 of the plugs. I would have changed 4 plugs, but 1 simply refused to move.