I have a ford focus zetec tdci 1.8 diesel. Everytime I start it from cold it lets off white smoke from the exhaust and judders when I drive it. This lasts for about 10-15mins then it drives okay. It does'nt lose power. The engine management light came on and I was told the dianostic computer said egr valve so I have had a totally new egr valve and manifold fitted. BUT it still persists on doing the same thing even though the engine management light as not come on this time! What could this be and why is it doing it?Please help what could it be?