Just changed spark plugs on 99 SR5, 5VZ 325,000mi and noticed that the left bank of plugs (wire side) had worn much more than the right bank (coil side).

Plugs were NGK Platinum and had been in the vehicle for a little over 60k of hard miles. (Plenty of towing including 2 cross country trips pulling around 6k. [Proper coolers, air bags and WD hitch] Plenty of mountains at 4k+, WOT for several minutes at a time.) Right bank were only .005 over new plugs (Iridium) going in but Left bank was almont double the gap. The skinny part of the electrode was almost (if not entirely) worn away. Wires, and boots were also NGK and same change interval as plugs. The only codes I had were misfires which occured on damp mornings until the engine warmed up a little. Same codes/issue that prompted the change 60k ago.

I'm curious what would cause such different wear side to side. Is this normal for the wire side of the shared coil or is it a symptom of something else? I'm not exceptionally worried, and am not looking to spark the great plug preference debate, but am curious if anyone else has seen something similar.

Yes, I have replaced hundreds of 3.4L plugs and this seems to be pretty normal. I have also found the factory plugs to be different types on both banks. Denso on LH side, NGK on RH side.

The difference in plugs between heads is fabled to be based on the factory assembling the heads at the time. Obviously there is little advantage to following this practice today.

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Its because of the wasted spark system used

Kinda accurate... cheap plugs will wear faster/inconsistently in a waste spark system.

Waste Spark Ignitions - NGK Spark Plugs

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Good call....

Yup wasted spark systems will were put twice as fast as standard spark ignition. So if the plugs are normal use are 100k miles. In the 3rd gen v6 its only 50k.

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Recommended factory service interval for dual electrode spark plugs is 30k miles so I would expect to see a lot of wear since you went twice the recommended service interval.

Thank you MaineRunna, good to know this sounds normal.
And for the record I don’t think those plugs owed me anything for the mileage they were run, just thought the difference between banks was odd.

Doesn't that assume standard copper plugs though? Platinums should last a little longer.

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Platinums will last longer, but not twice as long.

From what I observed @badluck is correct they will not go twice as long then the standard plugs because of the double spark. So if they are supposed to last 100k. They be extremely warn at 50k. Because they did 200k worth of work. If your following what we are trying to explain...

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I have the 6EK-11 double platinums in my Saturn. I’ll tell you what the erosion is like. The regular EK’s wore down .005-.008 in 50k miles.


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Wasted spark, as others have mentioned.

Because the plug is making the jump one way in one bank and the other way in the other, you will see the cathode wear more in one bank and the anode wear more in the other. This is typical, and you'll typically see this as the tip wearing a lot more in one bank than the other (doesn't impact ignition in a meaningful fashion).

I ran NGK Iridiums for 100,000 miles - put them in at 190 and replaced them at 290. Didn't look factory new, but damn close, and no issues at all.

Platinums will wear 2x as fast, the replacement interval is ideally going to be 25k-50k.

Very good point!!!!


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