If your car is warm enough it won't hurt your engine to do a short or partial glow, but it will wear out your battery.

This is because the GP's build up resistance as they get hotter and thus draw less amperage from your battery. So if you crank while they're still drawing a lot of amperage, your putting that amount PLUS the starter draw on your battery.

The amount of time you should glow depends on how "fast" your GP's get hot.

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'79 300SD
'82 Chevy Chevette diesel