I might check out the possibility of something else causing the slowness as well. Things that should really be checked out on any new or recent purchase of an old Mercedes diesel.

All too many of these cars have not seen engine attention for almost decades now. They do need a version of a tune up periodically as does any engine. Basically this usually takes time rather than money to do. Except for dealing with the injectors perhaps.

Is the arm on the injection pump moving as far as another injection pump of a similar type.? Is it possible that someone backed off the stop. Thinking they might get a little more fuel at one time?

I assume the function of the stop is to prevent too much stress being applied to the injection pumps internals. Possibly something is bent now inside the pump? Especially as it will not manually shut off. Yet apparently it will still shut down when vacuum activated. So I might leave that for now.

If one assumes the stop was never adjusted. Then a good used pump may be in order. As a percentage of sweep of the arm not very much is missing. So there may be some other issues as well. Usually I never measured them but it seems to me that the lack of lever movement with linkage problems was greater on average than what you have shown.

Many simple tests for a low powered vehicle. Ever take a moderate drive and check the wheel temperatures? I find a lot of dragging calipers sometimes rather than just one getting really hot. Fuel is flowing from the relief valve on the injection pump? If so close the line off and see if there is a real difference. If it is not flowing you are low on fuel pressure is another possibility. Maybe with age you have a really stretched out timing chain. Easy to check. When this is present the cam timing is off and the injection pump is retarded.

I would not expect as much power loss as you are experiencing.. Allow it has been more than a few years since I dealt with linkage problems and the lack of the injector pump arms throw. On 240ds. Your oil consumption is fairly typical for many still decently running 240d engines. The quick startups are also a good sign.

These engines just do not have the power reserves to allow for problems and issues to exist without an obvious penalty. What I am suggesting is not a total waste of time. You in essence are just working backwards to what seems to be an obvious possibility.. What you find could be interesting. You may or may not find something. You might find several things. I might start by seeing what the valve clearances are. This can indicate how much neglect has existed. For example if they are all far too tight. The engine can lose compression as it comes up to temperature. The simple tests involving the relief valves output can clear or condem several things as well.