Been working on this exact problem today as i'm still tackling the rough idle problem on my UK spec 311 CDI. Having plugged the end of the hose connecting a recent fuel tap i put in the tank i left the van idling for ten minutes hoping the engine note would change, suffice to say it made no difference. So (as i seem to do each Sunday) i got the tools out and started poking around, this time around the fuel filter as having had that lot off as part of a service and then again when removing the tank and lines i thought it a good place to start.
First mistake was to undo the bleed screw slightly to see what would happen, nothing but hissing then the engine died, no bother, popped the pre-heating valve off & syringed a load of fuel back into the filter to top it off, then reconnected then popped off the inlet (tank connection) and sucked the diesel back up to the filter and quickly refitted the pipe. I turned the engine over for about 30 secs with nothing happening so i stopped - mild panic setting in.
Second mistake was whilst checking the filters connectors for leaks i could hear hissing coming from the fuel line connector, the one that's locked by the white 'c' shaped fitting. In fact by now with my ear next to it its making the same noise a balloon makes when you let the air out by hand - sort of high pitch squeaking/bubbling noise. So on deciding to investigate this further i manage to snap one of the tabs off the connector that the white fitting locks - great!
I decided to refit everything and leave the filter alone, & feeling utterly dejected I went to turn the engine over again expecting nothing and to my surprise it spluttered back into life - all good.................. i think.
However on closer inspection of the filter its leaking not only on the fuel line connector but also around the WIF sensor judging by the staining on the bottom of the cannister. I'm hoping the purchase of a new fuel line that connects the LP pump to the filter and a new filter with no WIF sensor may just solve my problems. Having watched said fuel line there's an almost constant stream of very fine bubbles in the line so i'm hoping this is the culprit as many of you pointed out to me.
Also i'm thinking of fitting either a hand primer bulb on the inlet just before the filter, (i've gone off the taste of diesel having done this a few times) of maybe a check valve in the same place, does this sound like a good idea? If the engine doesn't quieten down after that i'm gonna go the re-design route like bubbasprinters done.
At the 20sec mark below i put my phones mic directly next to the fuel line connector fitting and you can hear the squeaky / bubbling noise
https://photos.app.goo.gl/PLnPQseUbGY7Br347
And this one you can noticably see bubbles in the fuel line.
https://photos.app.goo.gl/YVjinEiopjpu8fCUA
Cheers