If you disconnect the fuel line and tank filter is pretty clear you should have a reasonable stream of fuel. Usually a new filter is not required as it is removable and cleanable if partially obstructed. I do not know the line layout on a 190d. But best area to disconnect for test is near the tank if possible. It just occured to me that your tank may be under the car instead of in the trunk. Just means you have to be a little more creative to initiate fuel flow sometimes. I just still assume mercedes continued with an in tank filter on your model. I also have not timed a 601 so do not even know the proceedure.