Probably the diesel particulate filter. The DPF on this engine is very prone to choking if the vehicle is repeatedly used for short runs from cold starts. The DPF is supposed to gather unburned hydrocarbon particulates on start up, then burn them off later in the vehicle's journey. To make this happen the engine periodically cycles extra diesel into the combustion chambers. But if the DPF never gets a chance to burn off the particulates it clogs up to a point where it can't regenerate and the extra fuel sinks to the engine sump. So check the sump level. if it gets too high, because the engine is compression ignition, it can start to run uncontrollably on its excess sump oil. If you can manage to take the van for a 50 mile run at more than 2000rpm there is a chance that the DPF can regenerate.