Originally Posted by Diseasel300

Worn/collapsed hydraulic lifters are common on the 60x engines, but they won't cause hard starting issues. They're just noisy and irritating. Eventually after tens or hundreds of thousands of miles they can cause wear to the valve and seat, but that's really about it.

Impedance of the glow plugs are only part of the equation. Is the light coming on on the dash when you turn the key to preglow? The glow plug controllers can and do wear out. The points can fail inside, the capacitors can dry out and cause an excessively short glow cycle, the strip fuse inside the lid can develop a micro fracture and "look" ok but be open and cause no glow.

Set up a multimeter to your DC scale and measure from a glow plug stud to the block. Have a helper turn the key to preglow. You should read ~11V if the glow system is working. The glow plug relay should glow the plugs when the key is turned to the preglow position and stop the glow cycle when the key is turned to the start position. These early cars (pre-1990) do not have afterglow.

I assume it's still somewhat warm in Morocco? The 603's with decent compression tend to start reasonably well even without a preglow down to ~50˚F. Even my 300SDL with cylinder walls like glass would compression start down to ~55˚F. 5-6 compression hits and it would light off. Rough running for 3-5 seconds, but it would settle down pretty quickly.

Hard starting and rough running could be air ingress issues. If you still have the fuel preheater thermostat present, bypass it so that the fuel line goes straight into the lift pump. The preheater thermostat is a very common source of air ingress and is unnecessary. If the fuel lines look old, they need replacement. The correct size is 7.5mm. Using 5/16" or 8mm line will cause air ingress issues on these cars, the flare on the end of the metal lines is not large enough to get a reliable airtight seal on the suction side of the lift pump. How greasy/wet is the top of your injection pump? The 60x engines use a rubber O-ring seal on the injection pump delivery valve holders and are prone to leaking. A bad enough leak can cause an aired up fuel system when the car sits. The O-rings and internal copper crush washers are a DIY replacement job, look on this site for a very well written how-to article. Reassembly requires a torquing sequence that is very important.

The tapping/knocking noise can be injector noise or it can be lifter noise. Typically the lifter noise is a hollow "clanking" sort of sound. Injector nailing/knocking is typically a very sharp "metallic" noise, not unlike a hammer striking an anvil. Injector noise can be caused by worn injectors, worn delivery valves in the injection pump, air in the fuel system (especially at startup), injection timing, low compression, etc.