I have a car that is giving me some running problems so I cleaned the old spark plugs and then decided to measure the resistance of all the high tension ignition parts. The resistor plug wires measured 500 to 1000 Ohms depending on length. The dizzy cap measured fine with no shorts between terminals. Then there were the spark plugs.
These are standard plugs (not resistor plugs) so I expected to measure Zero (0) Ohms between the plug’s terminal and the center tip at the firing end. I only found Zero resistance on about 3 out of 10 plugs. (Once I started making measurements I brought out ALL my old spark plugs to measure). Apart from the handful of plugs that measured low resistance, MOST of my used plugs measured several hundred thousand Ohms or more.
All these plugs were presumed working when removed. I have never had an engine start misfiring due to a failed plug so apparently the resistance I measured does not affect the firing of these plugs… much.
So have you ever measured the resistance of new and old spark plugs? Have you ever seen a published value that says “if the resistance is above X don’t reuse the plug.”? Please share your thoughts and experiences.
Thanks,
Doug L.
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