Also a thought I’ve had, but good luck to us finding those diagrams...
Over on STD you'll read about people removing fuel elements to replace them with larger units. I’ve seen what the elements look like. If that’s my issue, I’m in way over my head.

I ordered a lift pump reseal kit in the meantime. That is at least within the realm of my ability.

__________________
1981 240D 4sp manual. Ivory White.

Okay, it's getting interesting. At least for me.

When I pinch off the supply line, the volume still drops. I've done this a few times now, always the same result.

When I pinched off both the supply and return line, I got no visible drop.

However... the car wouldn't start. For context, I usually have ignition on the first hit of the starter. I let it crank a few times over and then got out to have a look at the lines. I hit the primer a few times and watched a small column of bubbles rise from the lift pump toward the spin on. I cranked two additional times and then it fired. I hopped out and watched the trapped air escape through the return line back toward the tank.

In my mind, this leaves only one explanation.

Fuel is seeping past the lift pump bore and into the IP until the pressure differential between the IP "crankcase" and the fuel circuit is zero.

One thing I've noticed is that sometimes the drain back is a very slow process that reaches its peak only overnight. Othertimes, I'll happen moments after shutdown. Not the full drop mind you, but at least an inch or two. It's possibly that the position of the piston, either extended or retracted creates a slightly better seal.

The only problem with my theory, or something I cannot account for:

When I pinched both the return and the supply, I saw trapped air rise from the lift pump as I primed. I can explain where the fuel went, but not where (or how) the air entered. Because this is happening in a static condition, if the piston is back (in a lift/pull hit) the main chamber of the lift pump would be under negative pressure. When the piston is forward (in a push hit), positive pressure.

The obvious question is, "is your oil level rising at an appreciable level?"
Not that I can tell... which suggests (I'm reaching here) that while the lift pump is running, there is either a better balance of forces within the pump itself, or the circuit.

I'm going to re-rebuild my lift pump. It's possible I knicked the o-ring when seating it...

Thoughts?

__________________
1981 240D 4sp manual. Ivory White.

If the inlet and return lines clamped and no leak back the seal in the lift pump is sealing.
Air when priming and continues most likely seal in primer,new does not mean good.
The lift pump valves are not ck. valves,they leak,will return to tank pressure by 20 min.

Suggest:remove fuel cap,remove vent,no pressure no vacuum,what is result over night?

Forgot this,if lines blocked off and fuel pressure holds no leak out of injection pump.

I'm sorry, this doesn't make any sense to me.
If the lines are clamped, it's the clamps that are doing the sealing.
How does that give any indication whatsoever that the pump is sealing?

I've tried taking the fuel cap off, removing the vent, and even leaving the oil cap off. No difference.

Would a leaking seal in the primer cause drain back? I don't get how that works...

__________________
1981 240D 4sp manual. Ivory White.

Well, re-rebuilt the lift pump and same same.

At this point, the only thing I can imagine is a bad element seal in the injector pump.
If that's the case, I quit.

__________________
1981 240D 4sp manual. Ivory White.