From E30 Zone Wiki
A single diesel engine, the M21, was available for the E30 in two models; the standard 324d and the turbo-charged 324td. It featured an indirect injection fuelling system driven by a Bosch pump. The same engines were fitted to the E28.
While the main components of this system are similar to petrol engines, a diesel system employs a pump which also functions as a multiple injector. This combined unit is capable of injecting diesel into each cylinder in turn at pressures far beyond that of a standard petrol engine. It is located under the intake manifold. Even though the in-tank pump is no longer present, the same 55L fuel tank is used, albeit with a different fuel level sender.
The diesel system was available in two flavours. The earlier system fitted to the 324d was cable-driven and directly controlled by the accelerator pedal. This was replaced by a later fly-by-wire system for the 324td that read data from a throttle pedal sensor. Both of these systems employed a belt-driven pump with a DDE ECU to control injector cycling, with the later system employing a second DDE to read the throttle data. The pumps on these two systems are clearly different, and not interchangeable.
Injection pump timing on the M21 is controlled by a solenoid operated pulse valve inside the injection pump which varies the pressure applied to the pump distribution (advance) piston. The VP-20 control module monitors signals from a coolant temperature sensor, engine speed and TDC sensors, and the lift sensor located in the #4 fuel injector (needle lift sensor.) The VP-20 computes ideal injection pump timing based upon sensor signals and controls operation of the pulse valve accordingly.
The pulse valve varies advance piston pressure by providing an additional pressure bleed when it is open. The control module cycles the solenoid on and off a fixed number of times per second, varying the amount of "on" time during each cycle in order to control the pressure bleed orifice. When advance piston pressure is reduced, injection timing is retarded; when pressure is increased, timing is advanced. Action of the pulse valve only controls piston pressure, and does not affect the amount of fuel injected.
If you do not happen to have flywheel/injection pump locking pins: The flywheel can be pinned with a 5/16" drill bit (non-cutting side into the flywheel) to lock the crank at #1 TDC. The "O / T" mark on the harmonic balancer confirms #1 TDC because it lines up with the notch on the upper timing belt cover. The injection pump cog can be pinned with a 3/8" drill bit through the lower pinning hole in the cog (~7 o'clock) and into the hole in the IP mounting bracket.
Because diesels do not use spark plugs to ignite the fuel, but instead use immense pressure to create heat for diesel detonation, the fuel needs to be warmed before the engine can start from cold. This employs the use of glow plugs, controlled by a pre-heating control unit. To activate the glow plugs, the ignition key is turned to IGN but the engine is not cranked. The key is held in this position until the glow plug light in the dash cluster goes out.
If your diesel is reluctant to start in the morning, cycling the glow plugs twice (warming, then shutting off the circuit, then repeating) will ensure that the plugs warm to their maximum heat. If this starts your car, it is a sign that your glow plugs are ageing and need replacement. There are six of them, located in the head under the inlet manifold.
Because of the complexity of the diesel fuel pump, most engine issues are connected here. The simplest cause of the problem is an ageing or corroded fuel solenoid, or the connections to it.