Since you are going to run a compression test on all cylinders, go ahead and remove all the glow plugs before you start. The car can not start with the glow plugs out because of lack of compression. However, you should hold the stop lever to the extent possible, because you don't want the cylinder you are testing to ignite.

This tends to make the task a two-person job if you have someone turn the key to engage the starter.

Of course, you can prevent any chance of starting by also removing the metal injector pump to injector lines.

Ken300D

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1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles