Marcb,

Since the GP are turned off as soon as the engine starts, it is unlikely that the GP are the cause of your problems. If the engine starts easily, your GP are OK.

When the speed of the engine varies and the smoke appears, does the engine seem to misfire? If it misfires, it could have low compression, maybe only in 1 cylinder. Have you done a compression test lately.

You also might take your injectors to a Diesel injection shop and have them tested.

P E H