Jeremy, I think you're way overthinking this. I personally don't care what the resistance of a hot glow plug is. As long as it's much higher than when it's cold, you know the plug is working. It's like a light bulb. It either works or it doesn't. FYI when I say hot I mean right after the plug was glowed and not just hot from engine heat which is not hot enough.

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2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL