Spark Plug Wear Patterns

[The Critic]

Most people determine spark plug wear based on the amount of visible gap (compared to new) and the wear of the center electrode.

On most of the long-life Iridium plugs that I have changed out, even on the ones that have been in-service for over 120k, the gap and center electrode are in a like-new condition.

However, some of my technician friends have mentioned that the resistance of the plug can increase overtime, and this may have a detrimental effect on the lifespan of the coils. Obviously this is not a visual condition, and without having a baseline, it's hard to verify this condition.

With that said, my car calls for a spark plug replacement at 120k (or 150k in a CARB state), and I just turned 90k....

Can anyone comment on how to "judge" the amount of wear on a spark plug?

Thanks

 

[PandaBear]

Based on what I see in a Lexus IS250 TSB, it asks you to check resistance between side electrode and center electrode. If it is less than 10M Ohm, it is gone (too much carbon buildup that can "short" an arc).

I personally would just look at fuel economy and use corrosion to see if it is time to replace.

Since you have an OBD adapter and factory diagnostic software, I think a better way to check is look at the amount of misfire.

 

[The Critic]

Interesting, thanks.

I am definitely not seeing some of the fuel economy numbers that I used to, but then again, I have also changed wheel fitments and driving environments, so that makes it hard to get an apples-to-apples comparison.

The cost of a set of Denso SC20HR11 plugs is only $37 from RockAuto, so it is hardly a significant cost.

Maybe I should just change 'em.

 

[PandaBear]

You can also buy some cheap plugs just to get the crush gaskets, then you can really inspect and reinstall the plugs without worrying about reusing the gaskets.

 

[BISCUT]

Originally Posted By: The Critic
Interesting, thanks.

I am definitely not seeing some of the fuel economy numbers that I used to, but then again, I have also changed wheel fitments and driving environments, so that makes it hard to get an apples-to-apples comparison.

The cost of a set of Denso SC20HR11 plugs is only $37 from RockAuto, so it is hardly a significant cost.

Maybe I should just change 'em.

$37 makes it hard NOT to change them. I like to change them a bit early. I feel (because I can't substantiate it) it ensures optim performance and lessens possible fatigue of coil overs.

 

[Mackelroy]

Ive changed out 2 sets of irids on two different cars, with 120,000 miles. plugs looked good for that milege but the were shot. But not as bad as a motorcycle with 50,000 miles, they were so bad I was shocked there wasn't a performance issue, huge gap and everything eroded.

The core electrode has the iridium coating witch tends the hold up, the ground electrode tends to be platinum coated and that wears faster. You don't gap irids, when they need to be gapped you replace them.

 

[Vikas]

I am not sure which resistance are you talking about here. The designers actually put resistor in the circuit to reduce RFI.

The 10Meg is measuring the insulation between the center post and the ground.

Bottom line:- Logically, using your DMM on spark plug is not going give you any useful data.

 

[Chris142]

what about a lazy 02 senser too? my jeeps were slow at 110k,never threw a code.

 

[Quest]

spark plug ceramic insulation is way over 10MegOhms, and cannot be measured by conventional means (you need laboratory grade meters with specific set ups, etc.) insulation resistance may vary over time by seldom goes bad, for the fundamental electrical property requirements is that the insulation must be able to handle in excess of 10s of thousands of volts throughout it's service life and not fail (thus the use of aluminium oxide, aka "ceramic" electrode insulator).

modern spark plug contains a resistor within (for RFI suppression) and that resistor's resistance variation (or drift) doesn't really matter, so long as it mantains over a few kiloOhms' worth of resistance.

Copper core resistor means the conductor inside the ceramic insulator is copper alloy. It doesn't imply that the electrodes are made of copper (electrodes in those standard grade plugs are typically made of nickel alloy).

most important aspects when choosing a spark plug is (a) heat range/self-cleaning ability. Too "cold" of the range the plug, the tip might foul up easily (carbon deposits will form), which will ground/short out the electrode eventually; Too hot of a plug will cause pre-ignition similar to when low octane gas hits some glowing deposits inside the combustion chamber during compression phase. (b) sealing properties between the ceramic body and the metal portion.

Also: spark plugs require additional cooling as well, by heat conduction from the metal body threads portion out to the cylinder head (water jackets not too far from them).

Lastly: modern electronic ignitions for automobiles, from old single coil, spark plug wires configuration type to individual coil over spark plug types, are capable of very, very high voltage indeed, so gap variation no longer a prominent issue.

The purpose of coil-over-plug type of configuration is that computer can control/vary each coil (you can look at them like each coil = each plug) sparking timing individually to achieve the best idling "evenless" (despite of variables such as ambient temp, gasoline type/grade, other factors, etc.) and also best regulation over the course of the engine RPM speed throughout due to the lack of the archaic, mechanical+ vacuum distributor type ignition spark distribution.

Q.

 

[PandaBear]

Vikas and Quest,

I think what Toyota/Lexus said was to use resistance to check for excess carbon buildup ON the insulation. The insulation shouldn't go bad and therefore should stay above 10M Ohms. Yes, as I mentioned earlier I wasn't talking about the internal resistance of the plug's center electrode that suppress radio interference, but the insulation resistance.

Most home grade multi meter can measure <10M Ohm.

 

[tom slick]

The resistance of the conductor, regardless of material, is trumped by the resistance of the air gap. Wear on the electrodes would cause the voltage requirement to go up but I don't know if that causes excessive wear.

A megaohmmeter is needed to do a real insulator test but not worth the trouble when plugs are cheap. An ociliscope would give you a real view of ignition system condition.