Yes, I did notice this problem as well on my 300SD. I'm in d'accord with Mr. JimSmith regarding the relation between a glow plug relay cut-off and diesel noise. My thinking about this relation is as follows:

When the glow plugs are still red hot and the atomized diesel fuel is forced (by injectors) into pre-combustion chambers at high pressure, the lack of air in tiny pre-combustion chambers will induce a localized but incomplete combustion/explosion process inside pre-combustion chambers. This process will create a MUCH higher pressure of incompletely burned diesel fuel/air, and forcefully rush this mixture into the larger combustion chambers (with piston contact). Thanks to narrow tubes connecting pre-combustion chambers and larger ones, the Bernoulli's Principle kicks in and this will create an extremely powerful jet of atomized diesel fuel to THOROUGHLY and UNIFORMLY mix with the larger volumes of air in larger combustion chambers, then the heat produced from piston compression will trigger the full combustion process in larger combustion chambers. A thorough and uniform mixture of air and diesel fuel is the ideal condition for efficient diesel combustion process (extracting more energy from same amount of diesel fuel). This explains why the diesel noise is relatively low when glow plugs are still on in spite of a cold engine block.

However, once the glow plug relay cut-off kicks in and the engine block is still very cold (quickly absorbing heat generated from previous short combusion process), pressurized and atomized diesel fuel will not be able to spontaneously burn inside tiny pre-combustion chambers, so the speed and pressure at which diesel fuel enters larger combustion chambers through connecting tubes are MUCH less than before. This causes a partial and irregular mixture of air and diesel fuel, leading to concentration gradients (denser near the exit of connecting tubes and thinner elsewhere). A cold engine block and bad mixtures of air/diesel fuel would adversely affect diesel combustion and may dramatically reduce its efficiency. In fact, I believe that combustion may not occur in one (or two) cylinder for a couple of seconds after a glow plug relay cut-off, especially on old diesel engines with relatively low compression pressure in some of their cylinders. The result is a diesel engine that may idle badly because the combustion process is not complete and may not take place at all in some cylinders. Of course, when the engine block is warm enough, atomized diesel fuel will spontaneously burn (incompletely) under appropriate pressure from piston compression, and will thoroughly and uniformly mix with air in larger combustion chambers to produce efficient diesel combustion, as described above. Therefore, the heavy noise will go away when the engine is warm enough.


Hi BoostnBenz: I'm pretty certain that the original glow plug relay in your 1983 300D does stay on about 30-60 seconds after a car start. You can easily check this by opening the hood and/or turn on the interior light. Start your car and wait for about 1 minute. Within this minute, you will hear a very distinctive click of the glow plug relay cut-off, and the interior light will become brighter (no more current draw from the GP relay).

Hi ck42: It's "N'est-ce pas?"

Sorry for my awkward English

Eric