Because the glow plug is intermittent in the contact from the electrode to the resistance wire that actually does the heating. When cold, you measure the 0.3 ohms or whatever. When voltage is applied, it gets hot a little and expands and loses contact and does not get hot anymore. This cycle can continue and fool the unsuspecting into thinking they have a good glow plug because their handy dandy ohmeter says so.

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked