Spark plug resistance

apbright

apbright

Senior Member
Hello,

Is there any known/established upper limit on spark plug resistance, particularly for the inner conductor?

I pulled one plug from a cylinder from which I was not getting any significant power (shorting the spark wire to ground did not result in any significant decrease in engine idle rpm). I checked its resistance with a multi-meter, which showed about 1.8k ohms, which seems high to me. Other plugs pulled from cylinders operating as expected showed 0.2 - 8 ohms.

Anyone else ever run into this kind of problem?

-Andrew B.
 

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I don't know of any go/no go limit but, like you, I mostly find resistance of the positive electrode in the less than 10 ohm range. I would throw out a plug at 1.8K ohms.
 
Interesting. I have never done that. It should be a straight shot with little or no resistance. I think there are resistance plugs for newer cars but I don't think they were ever made for Model A's. The resistive carbon wires for newer cars reduce the amount of electro-magnetic energy coming off the wires and actually deliver more energy to the plugs. But that is another story.
 
I've never checked the resistance. If one isn't working, I toss in another.

They are quite fragile, a little tap in the wrong place can crack the porcelain which is not easily seen.
 
This a good example for use of the old time Champion spark plug tester. With an air line connected and pressure regulated to equal engine compression, the tester allows you to see how the plug performs under compression. Because air has resistance and higher pressure means more resistance, a plug that fires fine laying against the outside of the engine, may be very weak or not fire at all under compression.

This tester has a model T coil inside. Install the plug, hook up the high tension lead and press the button. The spark is visible through the window via a mirror.
 

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Newer Plugs have resistance built in. Google it, There are resistance charts. I would think it would make no difference if your coil is good--it should be able to easiliy overcome 1.8k resistance.
 
aside from the resistance on the center electrode, which really is not a concern for the high voltage characteristics of conductivity.
The issue could be more of an insulation leakage to ground thru the insulator.
Those are take-apart plugs, not that I recommend taking them apart as routine maintenance, but, you could take apart the one not working and check for burning thru insulator. More than likely there will be a burn scar inside the assembly where the spark is jumping to ground.

The real answer is to just replace the plug as there is not much you can do to fix the insulation.

Just thoughts, John
PS. aircraft plugs are resistor types, typically considered "good" if resistance measures less than 4kohms.
 
For the insignificant cost of a replacement plug, why not just replace the suspect plug and get on with enjoying the vehicle.
 
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