If there was some way to completely block of the Fuel Returning to the Fuel Tank I suppose it would make it harder for the Pintel in the Injector to move up and Inject Fuel as it would cause a counter pressure.

There is also that littlie Pin Hole that is in the Secondary Fuel Filter Houseing. While I do not know how it is possible some say it can pull Air through that hole assuming Air is in the Retuning Fuel.

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84 300D, 82 Volvo 244Gl Diesel