A diesel engine is very simple... air gets sucked into the cylinder or pushed by the turbo, a bit of fuel is injected into it, the cylinder comes up which compresses the mixture and it spontaneously ignites.
There's no electronics to control ignition (ie spark plug) and no advance or retarding of that spark, there's no throttle controlling the air adjusting the air fuel mixture etc.
If you put your foot on the accelerator, it basically injects more fuel, you get a a bigger bang and more power.
The TD4 engine is more complex as it is a "common rail" engine, the high pressure pump provides a constant flow of fuel under high pressure and the car's ECU talks to each of the injectors telling them individually when and for how long to open and let fuel flow. This has the ability to create a cleaner running engine to meet tougher statutory requirements of engines. The L Series though is a much more old skool and simple "direct injection" engine. The injectors are essentially just nozzles that squirt what ever fuel is sent to it into the cylinder. The pump is driver by a belt from the engine, and just like the cam shaft which must have its belt set correctly to open and close the valves at the correct time, the pump uses the belt to know when to push fuel to each of the cylinders/injectors. The pump has a variable flow (thinking about it, I'm not sure if this is variable flow or duration per injection) and on even older engine, this would have been directly linked via cable from the accelerator peddle. On the L Series the accelerator peddle sends a message (eg 0% thru 100%) message through to the ECU, that then passes the message along to the pump.
It is in essence that simple. So taking the 2nd bit of
@ColinNI reply, there is wiring from the ECU to the pump to tell it how much fuel to inject. The pump is the lump of metal bolted to the front of the engine on the gearbox end with the pipes going to the 4 injectors. Disconnect the the wiring plug, inspect for any damage, clean and push back on making sure its a good fit.
Having said my description is simple, it was a "slight" simplification in a few ways. One of these is with the timing. The pump does get the basics of timing from being turned by the belt. However, (unlike the valves being pushed by the camshaft) it can adjust this timing to a small degree, which can make a big difference. To adjust this timing there is a lift sensor on injector 1 (the one at the other end of the engine near the oil filler cap). This has a wire going to the ECU so that it can compare when diesel is being injected to the engine's actual position in its rotation (presumably using the crank position sensor). It then sends the necessary advance or retard info to the pump. So taking the first bit of
@ColinNI reply - check the wire running from injector #1.
Colin is quite right that this can cause the misfiring that you describe.
The other simplification is that when the turbo is giving its maximum boost, it is pushing 50% more air (oxygen) into the cylinders. The engine is therefore behaving like a 3.0L engine. The engine, pump, injectors and electronics are therefore designed so that when you push the accelerator all the way to 100% - that is the amount of fuel to inject into that pseudo 3.0L capacity. If the turbo was not working, it would still be a 2.0L engine, so the ECU will see your 100% demand, but only tell the pump to inject 66% of max fuel - any extra would not be burnt and therefore would be a waste and blow the engine's emissions. The ECU uses the MAP (air pressure) sensor to know how much air is being compressed into the cylinders and faults with this can lead to smoking. The ECU also limits fuel demanded and injected for other reasons as well to give smooth running and protect the engine - eg it won't allow 100% at low or very high revs. Unless the ECU itself goes wrong (which isn't very common) you can guarantee that all these calculations etc are being performed correctly. So as complex as they may be there is nothing "to maintain" with them - its just the sensors to the ECU their wiring and how they are actioned (eg injectors, pump, hoses) that can "go wrong".
Good luck