Spark Plug Replacement Intervals - Why?

[leeharvey418]

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?

 

[SC Maintenance]

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.

 

[leeharvey418]

Your closed loop system will adjust with fuel trims, but in the end you won't be getting optimal mileage.

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.

 

[I<3oil]

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.

Then don't. It's as simple as that. They are just the "recommended" intervals. Sometimes things last less, sometimes things last longer.

 

[SC Maintenance]

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.

OK. So you came here to argue? Got it.

Would you be able to measure say a 1 or 2% drop in fuel mileage? How are you tracking it? You seem to ignore the coil part altogether.

You asked for the reasons and I gave you the generally accepted and proven ones. You can feel free to ignore them - its your car, you don't need permission from us.

And I also have an engineering degree, which point is meaningless to this discussion.

 

[Vercingetorix]

Helps prevent corrosion from welding the plug to the head.

 

[leeharvey418]

I didn’t come here to argue, I came here to get a reason besides “the manual says so”.

Okay, so the coils will run hotter - is that necessarily going to kill them sooner? Will it manifest itself in some other way than a drop in fuel economy? Is there some other indication of worn plugs that is readily apparent?

 

[leeharvey418]

Helps prevent corrosion from welding the plug to the head.

Welding or corroding?

 

[slacktide_bitog]

Is it difficult to change them in your car? Is that why you're asking?

 

[spasm3]

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?

When it starts to misfire, it will throw raw gas into the catalytic converter. That will kill it in short order. They are expensive, plus labor.

The old days of just leave it in until it misfires then change it, will cost you a cat. With DI, you will also throw a lot of fuel into the oil, if it misfires.

 

[edyvw]

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?

When you do replace them, you will replace coils too, as mentioned above.

 

[leeharvey418]

Is it difficult to change them in your car? Is that why you're asking?

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”.

 

[Oillover]

I suggest running them longer than mentioned in the owners manual (clearly that's what you want to do anyway, so who are we to convince you otherwise?).
If the coils/cats fail, then you have your answer.

 

[leeharvey418]

When it starts to misfire, it will throw raw gas into the catalytic converter. That will kill it in short order. They are expensive, plus labor.

The old days of just leave it in until it misfires then change it, will cost you a cat. With DI, you will also throw a lot of fuel into the oil, if it misfires.

But does that happen that suddenly? Wouldn’t there be excess oxygen registered by the sensor upstream of the catalytic converter before misfire events got frequent enough that you’d be pushing a consequential amount of fuel through the cat? And again - wouldn’t you see a substantial drop in fuel economy well before you got to that point?

 

[SC Maintenance]

I didn’t come here to argue, I came here to get a reason besides “the manual says so”.

Okay, so the coils will run hotter - is that necessarily going to kill them sooner? Will it manifest itself in some other way than a drop in fuel economy? Is there some other indication of worn plugs that is readily apparent?

My experience is the coils work, then the don't. They heat degrades the insulation between the windings, and eventually they short. Once they short they don't work at all or very poorly - so its pretty much immediate not a long slow degradation.

Once one goes and your running on 3 your closed loop system can't compensate for that. One cylinder is now pouring raw fuel into the cat. So your cat will be dead in just a few miles probably, if you don't stop and get towed.

Your closed loop system is going to adjust for your old plugs. So no, you will feel no drivability issues. Its not going to idle poorly with old plugs. Your mileage might drop 1 or 2%, not half. This isn't 1975. The system will adjust. But it won't be running at peak efficiency. Should you care? My question - is why wouldn't you want peak efficiency if you can have it with something that needs changing anyway. But again, your choice.

Is the 100K number random? Its probably based on MTBF with some safety factor. If your 100K is all highway, then maybe you can go longer. But how much longer? We would all be guessing. For me I wouldn't bother with the gamble. If for some reason you want to - feel free - I don't care.

But those are the reasons - which is what you asked for.

 

[Blang]

I love the idea of preventative maintenance. You can replace them now or wait until it becomes a problem. Every once in a while I see people have issues on '100,000 mile' plugs before that 'magical' number... and it wasn't necessarily a situation where something seemed a little off for a bit... it was a 'Oh crap I have an obvious misfire right now so I have to change my plans this weekend.' But I've also seen people get well over 100k on factory plugs. I don't mind planning preventative maintenance for this weekend... but I'd rather not change my plans this weekend because I didn't do preventative maintenance.

 

[leeharvey418]

My experience is the coils work, then the don't. They heat degrades the insulation between the windings, and eventually they short. Once they short they don't work at all or very poorly - so its pretty much immediate…

Thank you. That’s a legitimate reason.

Going to get new plugs in the very near future.

 

[HemiBenny]

No argument will convince you. Run it into the ground. Cheers!

 

[BHopkins]

Most service intervals are intended to keep the equipment, in this case a car, running at optimum performance. In ideal preventative maintenance, wear parts are replaced before they start to fail, instead of after. In calculating PM service intervals, variation in failure rates has to be considered, and a conservative PM service interval is used, to assure that the majority of parts that may fail before the mean failure rate, are also replaced before they fail.

Some, like yourself it seems, may be comfortable with waiting until performance starts to degrade before changing parts. Sometimes, like engine air filters, there is minimal risk of doing service this way. But other times, waiting until performance degrades before doing service, caused premature failure of other parts. For example, waiting too long on an oil change. I suspect spark plugs on modern cars is one of those that can result in premature failure of ignition coils.

 

[HemiBenny]

The Auto manufacturer and plug manufacturer give the life expectancy of the spark plug! It's a known maintenance routine that must be performed! It's not that expensive and doesn't have to be done that often!
Ignore it, and it just cascades into other issues that cost money to remedy, or at the very least, you'll have to change plugs when it's not convenient for you.

Anyway, Cheers!