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Spark Plug resistance Question

Old 02-02-2009, 07:06 AM
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Default Spark Plug resistance Question

Is measured resistance a valid number to use when deciding whether a spark plug is needing replacement?

The 6 plugs on my 200HPDI measure between 3.6 and 4.2. The plug on the 3.3Merc measures 4.9 and is about new. The used plug from the ClubCar measured 42.2. All plugs are NGK.

I recall that new plugs were close to 0.0, so assuming that use increased resistance, at what point do we change them out?

Also, is it a fair test to compare different plug sizes? The golf cart ran fine with that high-value plug in it...
Old 02-02-2009, 09:40 AM
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Default RE: Spark Plug resistance Question

bamaboy473 - 2/2/2009 10:06 AM

Is measured resistance a valid number to use when deciding whether a spark plug is needing replacement?

* No. The resistance between the center electrode and the 'contact' on top should never change regardless of how much fouling or wear is on the electrodes. If it does, it would likely go 'open' and the plug definitely should be changed.

* Plugs most often go 'bad' when too much carbon has built up on the ceramic surrounding the center electrode and creates a low ohm path for current to bleed off *instead of* jumping across the plug gap.

The 6 plugs on my 200HPDI measure between 3.6 and 4.2. The plug on the 3.3Merc measures 4.9 and is about new. The used plug from the ClubCar measured 42.2. All plugs are NGK.

* Most resistor type plugs have about 5K internal resistance. However, a little bit more or less should have no impact on the usability of the plug. The plug from the ClubCar may or may not be good. It all depends on if the resistance is correct for that plug or not.

I recall that new plugs were close to 0.0, so assuming that use increased resistance, at what point do we change them out?

* Non-resistor plugs will ALWAYS have very little resistance unless they are broken. Resistor type plugs used in cars and boats normally are around 5K ohms.

Also, is it a fair test to compare different plug sizes? The golf cart ran fine with that high-value plug in it...

* No guarantee there as different plugs can be made as the manufacturer decides. Again, other than being internally "broken" there is no all-telling resistance test that can be performed on a plug.
Good reading here also:

http://www.ngksparkplugs.com/tech_su...faqs/index.asp

Ken
Old 02-02-2009, 09:52 AM
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Default Re: Spark Plug resistance Question

Thanks, k, what I'm trying to find is a link about resistance...or did I miss something?
Old 02-02-2009, 11:16 AM
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Default Re: Spark Plug resistance Question

bamaboy473 - 2/2/2009 12:52 PM

Thanks, k, what I'm trying to find is a link about resistance...or did I miss something?
Well, what are you trying to find out? Resistor plugs have about 5K resistor in them AND their associated spark plug wires/coils also have about the same amount of resistance in the high voltage output circuit. Its needed to surpress radio interference. The resistance doesn't cause a problem with the spark because there is so little current involved there is very little spark voltage lost to that resistance.

Except for open/shorted, etc. there is no "resistance" reading that you can normally make that will definitely tell you if a plug is fouled or not. Though NGK does mention that if the resistance across the insulator falls below 10ohms then the plug will not fire. The problem is its a resistance that's very hard to read and its not a cut and dried number. In reality, it could be a lot higher than that and cause problems - especially at high speed/high loads.

In my experience, if a plug looks bad or is in any way suspected of being fouled or misfiring - it should just be replaced. Its too easy to do and eliminates one item that is a common source of misfires.

Ken
Old 02-02-2009, 11:31 AM
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Default Re: Spark Plug resistance Question

The motor is a 200HPDI with supposedly 50 hours or so. All of the plugs look the same and the boat runs fine (after adding double SeaFoam to new gas and running the dog out of it for 15 minutes to clear old fuel).

Motor will rev to 5300 RPM at 45MPH so the RPM range is right where it ought to be. I pulled the plugs for inspection and then did a resistance test and that's when I started to wonder whether a plug might look fine but might be failing internally. The GolfCart plug has 10X the resistance of the boat plugs, and it's old...but it works, so is there a correlation?

If your experience and/or training says that visual inspection is the best way...and that resistance testing isn't accurate, then that's answered my question. Thanks for your help. I'll reinstall those plugs and call it good for awhile.
Old 02-02-2009, 12:00 PM
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Default Re: Spark Plug resistance Question

The noise suppression resistors in spark plugs so equipped are not wear items, and rarely "fail" or change substantially in resistance throughout the practical life of the spark plug.

Items of concern on plugs are insulator cracks and deposits (visible or otherwise) and electrode erosion. These areas have an impact on the amount of energy delivered across the gap and the thus the quality of the spark.

Mike
Old 02-02-2009, 12:29 PM
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Default Re: Spark Plug resistance Question

The plugs were all about the same and looked as they should after 50 or so hours, so visual inspection confirmed that they're doing fine and there's no reason the test their resistance? Would anything be learned if 5 plugs measured one value and one plug was 5X or 10X that value?
Old 02-02-2009, 10:42 PM
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Default Re: Spark Plug resistance Question

bama

That last question, if you mean is there anything to be learned about how the engine is "treating" the plugs, the answer is no. It just doesn't work that way. If the resistance was that high on the one plug, which bear in mind would be 25k to 50k ohms (5x or 10x the normal value) I would probably replace it just because it was weird, but I don't think you'd ever see resistance that high (but not open) in a functioning plug, but then plug resistance isn't something you normally measure.

Notice on all four of your plugs used in outboards, your readings were in the range of 3.6 to 4.9 (I assume your meter was on the "k", kilo ohm, scale) where roughly 5 is expected, so they are all fine. (Fine as far as resistance goes. Like the guys said, other things are far more likely to cause a problem.)

I have no idea what a golf cart plug should run, but I'm kinda surprised it's >42 k, assuming your meter was still on the same "k" scale. Note if you "slipped" over one scale, or have an auto ranging meter which you misread, that golf cart could really be 4.22 k which is again about what a "normal" resistance plug runs.

This same discussion, by the way, applies pretty much to resistance measurements of plug wires as a way of trying to decide if they are worn out. If is usually just not useful.
Old 02-03-2009, 04:59 AM
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Default Re: Spark Plug resistance Question

KeyPineSavage - 2/3/2009 12:42 AM

This same discussion, by the way, applies pretty much to resistance measurements of plug wires as a way of trying to decide if they are worn out. If is usually just not useful.
Wires, too?? Darn, looks like I need to leave that multimeter in the toolbox. Thanks for the information!

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