White smoke indacates unburnt fuel. With an ohm meter disconect the wires on the glow plugs Check each one for resistance. About 1 ohm. They should all read the same. If not you may have found the problem.
Additionaly my VW TDI the glow plugs all were within spec but it still through a code. Along with rough idle and white smoke. Replacing the glow plugs solved the problem.
As I understand it. The glow plugs remain on after starting for a time.
Emissions no doubt.
-A
1999 E300 TD