injector return hoses
 

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

Return Lines?
 

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.