Definitely debatable...
I used to have to do this to an older GMC pickup (company vehicle) to get it started. Used a burlap bag, sprinkle a little gas on it and laid it over the intake to fire it off on cold mornings. It was really due to a weak battery but couldn't tell our mechanics that! They knew everything...

How did you find you had a bad glow plug? I'd keep testing as they are the defintive tool for hard starting unless she (the car) is just starting to get too old to fire off.
In that case there is lots you can do but none of them very quick.

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'99 S420 - Mommies
'72 280SE 4.5 - looking to breathe life into it
'84 300SD Grey - Sold
'85 300SD Silver - Sold
'78 Ski Nautique