On my 91 300D, I knew I have one bad glow plug among 2-5 (because it had rough start symptoms but there was no fault revealed through the indicator...the FSM mentions this). So I measured the resistance of each plug. Plugs 1-4 were all about 0.5 ohm. But plug 5 is 9 ohms. So clearly that's my bad plug. What puzzles me is that I would have thought that a bad plug would have a much higher resistance value...like 50 ohms.

Can anyone help me better understand different resistance readings on bad plugs?

Thanks.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 159k miles
06 E320 CDI "Rutherford", Black on Tan, 179k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 145k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 77k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife