Benz ML 270 cdi not without starting unless primed morning or anytime ?

I have this big problem with my ML 270 cdi it won't start once it's off until the battery go flat of I prime it with petrol (which I know is bad) I have tried many mechanics and had an engine changed but with the new engine after 1 month it started all over again I cleaned egr ,manifold. I have changed all injectors. And no air bubbles in the fuel line. Please save me