Hi all
I never had the return hoses run diesel when i changed them before. As i changed the hoses diesel just ran like it was under pressure. Changed two other cars this year and never saw this before. The car had been run about 2 hours before. Is there a vent in the tank? When i take off the cap there is a sucking sound.
Looked at the hanes manual no clue. what do you guys think?
car runs fine. This is my 1983 300sd. thanks Bill

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94 dodge cummings,86 ford 6.9,75 mbz 240d[ sold], 82 mbz 300sd,83 mbz 300sd

If these are the small black hoses attached to the fuel filter and looping from injector to injector ending with a short plugged hose then “return lines” is a misnomer. These lines carry filter fuel under pressure to the upper portion of the injectors. This fuel apparently acts as a lubricant to the upper workings of the injectors. Fuel is under pressure coming from the fuel filter while the engine is running. As there is a least a check valve preventing the reverse flow of fuel back through the fuel pump perhaps pressure may be retained after engine shut down. there most likely is a check valve in the return fuel circuit to the tank that would possibly have a high opening pressure to ensure that fuel is directed into the IP an only allowing return to the take at a relatively high pressure. There is a return line on the engine side of the IP that returns excess fuel back to a banjo fitting on the fuel filter on my 603 engine.

Re: the sucking sound at the fuel tank cap could be the result of an obstructed fuel return line, maybe clogged maybe a faulty in-line check valve or a faulty tank breather. Fuel is sucked from the tank under vacuum but is not returning under pressure hence a vacuum is created within the fuel tank! Theoretically a condition of fuel starvation could exist either at low ROMs reduced suction and return pressure or at high rams with greater suction and increased fuel tank vacuum.