Hi

I think you made a good diagnosis. The leak can easily be small enough
to not be noticable while driving, but drain the pump and filter of
fuel while not running.

I have had problems like this myself and know of no clever way to
find out where it leaks. Part of the pump is fuel filled and basically
any fitting or bushing can leak :-(

It makes sense to start checking the easy parts:

First check that there are no hairline cracks where the transparent
hose attaches to the pump. I've had a crack in the underside of the hose
just next to the fitting, and almost pulled a perfectly good 1.9TD
pump due to this.

Check the removable fuel input and return fittings on the pump and the
copper washers. If suspect replace copper washers.

Check all fuel return lines (yes it can drain backwards even when the air
leak is in the return system)

Failing all this, the next step is to remove the pump and check the bushings
and o-rings for the various shafts. You probbaly want the VW fuel
injection manual for this.

Alternatively: put a low pressure fuel pump at the tank and run it a
little before starting.

Hope you don't mind me asking, but where are you from?

Good luck,
Martin (From Sweden, but now in Canada)

   Date: Sat, 18 Nov 2000 17:40:54 +0100
   From: Ognjen Maric <oggy@etf-bl.rstel.net>
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   Hello,

   I've read your pages about VW diesel engines and found them very
   informative and helpful in solving some Bosch-pump related problems.
   However, I still have one problem with my Golf I can't solve and I
   couldn't find the answer anywhere, so I figured I might ask you for some
   help.

   I have some difficulties starting the engine. I suspect that the
   injection pump is the cause of the problem. I've observed the hose which
   supplies the pump with the fuel from the filter, i.e. tank (the very end
   of the hose is transparent). When the engine is working, everything
   seems to be working properly and that there is no air in the hose.
   However, immediately after I turn the engine off, I can see a small
   bubble of air coming from the injection pump, through the hose and back
   to the tank. I assume that the fuel is flowing back from the pump too,
   and I assume that's why I'm having problems with starting. Based on the
   schematics that can be found on your injection pump page, I assume that
   a dirty regulating valve seat could cause this. In your opinion, is this
   the case?

   Thanks,
   Ognjen