My 300d will not start if it sits for 48 hours. My indy says that there is a check valve in the IP that prevents fuel leaking back that may not be working and thus permits a drain to the tank,and/or I have a small air leak in the fuel return line that permits air to displace fuel in the pump.

The car runs well as long as I use it every day, or at least start it and let it run for a few minutes....but let it sit two days, forget it. I have to try to start it about 10x then: it starts, runs at idle for 6-7 seconds, stops. An hour later, it starts, runs 10 seconds. I go through this routine which is a PITA as you can figure...

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71 220D 169K wrecked
83 240D 118K sweet 4 speed
91 350SDL (one of the 60% good engines) 156K
84 300d (loaner to my sister) 189K
79 300SD (partswagen)
86 420SEL partswagen
70 220d (partswagen)
68 280s GASSER!!! under construction now
85 300sd 310K miles winter beater car retired
93 300d 2.5 turbo 168K wife's car
83 280SL euro 5 speed 155K
69 250S newest project 54K

Suspect your indie is on the right track looking for a fuel leak.

Try keeping the tank +1/2 full and parking with the nose pointed downhill or set the rear wheels up on a 2 x 4 and see if that helps.

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Terry Allison
N. Calif. & Boca Chica, Panama

09' E320 Bluetec 77k (USA)
09' Hyundai Santa Fe Diesel 48k (S.A.)

There is a Over Flow Valve (pressure control valve; controls the fuel pressure and fuel return from the IP) on ther rear top of the IP near the idle adjusting screw that has a hose going to the fuel return on the fuel filter. It is not there to stop fuel bleed back.
However, the fuel supply pump on the side of the IP has at least 2 one way valves.
As stated before it sounds like an air leak any where from the primer pump back (if you have one).
After that it is possible that the supply pump valves are not seating due deposits as the mesh on the little plastic filter is not that fine and over time stuff can build up.

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84 300D, 82 Volvo 244Gl Diesel