I have a '68 220D which is slowly getting road worthy thanks to this resource. The owner's manual and service manual give recommendations for how long to preglow the glowplugs when cold-starting the engine based on ambient temperature. Do you think they intended the timing to "start" when you first electrify the circuit, or when the little glowplug resistor on the dash begins to glow (about 10 seconds later with my car)? The wording is a bit vague. At 20C, they recommend about 10 seconds of preglowing. If you start timing at the pull of the knob, 10 seconds is not long enough for my engine to start. But if you start timing when the resistor begins to glow (a total of 20 seconds), then it starts up just fine.

Obviously, I just need to do what works, but I am trying to use this as an indirect measure of engine health. Thanks for your advice.