Spark Plug Replacement Intervals - Why?

[Auto tech]

The older the spark plugs, the more current required to produce a spark big enough to jump the gap. So your coils get more saturated each time they fire and will run hot. You will simply end up replacing coils sooner. Failed coils suck - they can kill your cat in no time. Ask me how I know.

Also, a dull spark, even if it jumps, is less efficient. Your closed loop system will adjust with fuel trims, but in the end you won't be getting optimal mileage.

I always suggest following OEM suggestion and using OEM type plugs for these two reasons. Unless your trying to do something different - like your running a tune or whatever.

I couldn't have said it better myself. Awesome advice for the OP.

 

[Auto tech]

I had many misfires in my 6+ decades and have never replaced a catalytic converter for any reason other than rust. And have replaced exceedingly few coils. And never from leaving spark plugs in too long. Almost all the misfires have been caused by bad plug or wires.

I've done experiments just cleaning and regapping plugs and reusing them. The plugs "wear" out. What does that mean? For some (unknown to me) reason the insulation ie: ceramic breaks down where it meets the metal thread body and the spark starts leaking out there. You can see black carbon traces begin to appear.

Also invisible cracks occur in the ceramic other places where voltage/spark starts to leak and carbon traces appear.

Wires simply wear out and start leaking voltage too. I don't know what happens to that material.

To me, no big deal. You'll know it when it starts to miss and the computer will pick it up. But you absolutely have to get them out of there regularly to put some never-seize on the threads. Or you won't be getting them out. And that's a big deal.

Personally I just replace them now at the recommended intervals because I have learned that they are eventually going to fail and the cost is not that material in the scheme of things. And I have them out anyway. But again, no big deal.

Spark plugs are supposed to be installed dry without anti seize. Anti seize can also cause misfires. With that said I have used a small amount with a q tip in the past and had no issues. The spark plugs nowadays are made of materials as to not seize in the heads unlike the old days of cast iron heads and steel threads.

 

[TiGeo]

Meanwhile me at once/year/10K miles for plugs in my Sportwagen

Seems silly to run them at 2-3x recommended interval for something that costs so little to replace. Clearly the manufacturer has an interval based on something...not following the manual is a BITOG specialty! I mean what do they know.

 

[50yr Street Mech]

Ok, my 2 cents. YAY .... Copper plugs 40k tops / Platinum 75k tops / Iridium is tricky. I'd say anywhere between 100k-130k tops. ONLY because the ground electrode starts to wear, carbon buildup on it, larger gap. The coils have to work harder to make that spark jump, possibly causing premature coil failure. Iridium plugs are cheap. Coils are not cheap. Yes, you must gap an Iridium plug before you install it. Take a vice grip to the electrode and move it. Don't touch the Iridium. Very brittle and will break. Give it a few taps on a vice to shorten gap if you went too wide. Stop obsessing over $ 15 dollars in plugs and just change them. Use OEM plugs, and gap accordingly. I like to use a tiny bit of anti-seize, ( Q-tip amount ). Haven't had a misfire yet. $ 15 dollars in plugs is a lot cheaper than coils, with plugs, and possibly CATS. Stop squawking and just change them . Ok. I'm done YAY LOL

 

[oil pan 4]

Again - I see no drop in mileage. This would obviously be the first indication of electrode burn-back, and it doesn’t seem to be happening.

I have a degree in mechanical engineering. I get that things wear out - but I also know that they give indications that they’re wearing out. I don’t accept replacing a widget that’s still fully functional just because somebody put a replacement interval in a manual.

Pull one and see. Assuming they're good they'll at least need the gap reset.
High-end plugs aren't cheap. $10 to $15 each.

 

[oil pan 4]

Ok, my 2 cents. YAY .... Copper plugs 40k tops / Platinum 75k tops / Iridium is tricky. I'd say anywhere between 100k-130k tops. ONLY because the ground electrode starts to wear, carbon buildup on it, larger gap. The coils have to work harder to make that spark jump, possibly causing premature coil failure. Iridium plugs are cheap. Coils are not cheap. Yes, you must gap an Iridium plug before you install it. Take a vice grip to the electrode and move it. Don't touch the Iridium. Very brittle and will break. Give it a few taps on a vice to shorten gap if you went too wide. Stop obsessing over $ 15 dollars in plugs and just change them. Use OEM plugs, and gap accordingly. I like to use a tiny bit of anti-seize, ( Q-tip amount ). Haven't had a misfire yet. $ 15 dollars in plugs is a lot cheaper than coils, with plugs, and possibly CATS. Stop squawking and just change them . Ok. I'm done YAY LOL

And a tow.

 

[edyvw]

Meanwhile me at once/year/10K miles for plugs in my Sportwagen

Seems silly to run them at 2-3x recommended interval for something that costs so little to replace. Clearly the manufacturer has an interval based on something...not following the manual is a BITOG specialty! I mean what do they know.

Yeah, but you track your vehicle. Most people don’t realize how much strain tracking is putting on vehicle and why 95% of vehicles cannot be found on the track.

 

[JamesBond]

I’m getting embroiled in an argument on another forum, and I think I want to ask a broader audience.

I have a 2019 Mazda CX-5 with a 2.5 L turbo engine that has 140,000 on the factory spark plugs, and is still going strong. I have no drop in fuel economy, nor do I have any apparent misfire events causing rough idle or other drivability issues.

Can anyone convince me that there’s a good reason to replace a spark plug with platinum/iridium electrodes on a mileage interval just because the owner’s manual says I should?

Just leave them. Everyone knows Mazda and “big spark plug” are in cahoots and trying to nickel and dime you 4 plugs at a time. Don’t fall for their tricks.

Whatever you do, don’t take them out and inspect them.

 

[HowAboutThis]

It’s actually very easy, but I adopted this philosophy when I had a different vehicle that required pulling the engine to replace the plugs on the rear cylinder bank.

I’m just looking for a better answer than “because the manual says so”.

If you're this stubborn over replacing plugs once every 100-125k miles, I want to see the soles of your shoes. Or do you mostly go barefoot out of spite for the shoe industry? Personally, as a self-proclaimed minimalist, from May 1st to October 1st I'm either barefoot or flip flops. But I do buy Reef flip flops because I want people to know I have a small amount of pride. But I refuse to wear my new ones (they're still in the box) until the old ones are unwearable.

TLDR: I understand your hesitance with your plugs based on my flip flop lifestyle.

PS: I changed my plugs at 110k in my Toyota even though the manual says 120k. I'll probably go it again at 200-225k, too, if I still have the car. It was under 30 minutes and $60.

 

[Chris142]

I would change them now and note if the car runs any better. If it runs the same then maybe you are correct in that special metal plugs do not need to be changed ever.

I bet you notice it runs better though.

 

[NibbanaBanana]

Spark plugs are supposed to be installed dry without anti seize. Anti seize can also cause misfires. With that said I have used a small amount with a q tip in the past and had no issues. The spark plugs nowadays are made of materials as to not seize in the heads unlike the old days of cast iron heads and steel threads.

That's what they say. Actually, the aluminum heads are worse in my experience that the cast iron. Never had a misfire due to anti seize in my life.

 

[Lawnman23]

Subaru recommends changing the factory NGK iridium plugs at 60k for my Outback. I waited till ~105k to change them. They looked great and could have easily gone longer.

I replaced them with the NGK ruthenium plugs, don’t plan on touching those for another ~120k.

 

[Propflux01]

Go ahead, swap them out. It won’t break the bank, and you Will notice a difference. Your coils will thank you, and whether you notice it or not, you probably have spark scatter. Not to mention the cycles eventually weaken the ceramic nose.

 

[4wheeldog]

I’m getting embroiled in an argument on another forum, and I think I want to ask a broader audience.

I have a 2019 Mazda CX-5 with a 2.5 L turbo engine that has 140,000 on the factory spark plugs, and is still going strong. I have no drop in fuel economy, nor do I have any apparent misfire events causing rough idle or other drivability issues.

Can anyone convince me that there’s a good reason to replace a spark plug with platinum/iridium electrodes on a mileage interval just because the owner’s manual says I should?

Eventually the plugs become “One with the head” if not removed/replaced. I would never leave originals in for more than 50k before removing and maybe putting them back in with some anti seize on the threads.
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