Today i removed the injection pump.
There are some things i am curious about.

First one would be the relief valve. After reading some posts on different forums i deducted that this valve is located on the injection pump right here


Also here is a diagram. I am talking about element 556


When i removed element 556 it was empty. No spring or ball inside.

The misleading thing is that i also found this diagram that suggests the relief valve is located on the fuel filter housing


Second curious thing is the pump gear. I turned it to align the missing tooth with the marking on the body like so


The gear does not move smoothly. It seems to have 4 fixed positions and it just jumps/snaps into them. Is this normal? Also the o-ring (element 3 on above piture) was totally deteriorated. There was just a slight trace of rubber in the groove.

556 should be the relief valve, although my picture from the EPC looks a bit different. If yours is empty, that should explain a lot. If you remove the valve (or in your case the empty banjo bolt) and pump with the hand pump, does fuel come out of the hole?

The IP doesn't turn smoothly, because you are turning the camshaft of the IP which presses against strong springs.

The O ring can be bought from the dealer.

I found the ball and spring. On the ground under the car
Fuel flows out when i use the primer pump.
Everything is ok than. I will try blocking the return and see what happens.

We're there ever any updates on this? It's one of my summer goals to tackle my 240's rough idle in D and/or with AC on. Fast idle or unloaded in N is smooth as glass. With AC or in D, the front end shakes like a tractor.

Take the idle up fast enough that it's smooth as glass in D with AC on, and the car really pushes hard against the brakes when stopped... And the body lifts/drops in and out of neutral when trying to prevent that...

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Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (116k)
1991 350SD (206k)
1991 300D (228k)
2008 ML320 CDI (199k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (267k)

Past Diesels:
1983 300D (228K), 1985 300D (233K), 1993 300D 2.5T (338k), 1993 300SD (291k)