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Which decade did spark plugs become 100k items? What causes this?

1960s-80s… Used to be every 20k ?
In the 80s, I used to replace the spark plug in my motorcycle every few months, it seemed.

What has changed to allow plugs to last 100k ??
Clearly, a paradigm shift.

Is it the advent of fuel injection?
If so, how? B/c you’re still having explosions and carbon. (“The plugs are fouled”)

Less chance of oil getting on the plugs?


I’ve wondered about these things for years, and love getting all these random questions now…
It’s been fun diving back into cars, and filling in some gaps b/c things have changed since the 80s!

It started perhaps a decade ago.

In my opinion there are two reasons why this is now possible. The first is that the center electrode of the spark plugs off of which the arc jumps (on most engines) is now commonly made of irridium. Irridium is some 8X harder than platinum, and erodes much more slowly. Erosion of the electrodes used to be one of the single biggest reason for plug changes.

The second reason is that engines today run far, far cleaner than the engines of old. That means less carbon deposition, better heat distribution, less oil ash, and so forth. Fouling was the second biggest reason for plug changes.

The reasons that engines run much cleaner is twofold, first of all the fuel is far better metered. Electronicall controlled injection in each one of the cylinders, controlled by engine need sensors and a microprocessor, controls the fuel far more precisely, and the far higher pressures that injectors spray at (as opposed to the low pressure output of the orafice in a carburator) creates a much finer spray, for more surface area per volume of fuel. The droplets burn far more completely if they’re smaller.

Secondly, fuel sprayed ring behind the exhaust valve at precisely the right time means that the spray will get combusted at its most finest (pardon the wording). New direct injection engines are even better at this.

So, in summary, better plug materials and cleaner engine operation allow plugs to go far longer without suffering the erosion or the deposition that we used to see.

Having said that, plugs can stick in the hole if left there for 100,000 miles. I ilke to change them out at 60,000 instead.

Several factors contributed to long spark plug life. 1st and perhaps most significant was removal of lead from gasoline, which occurred in the late '70’s I think. Lead caused deposits on the plug tip. You could remove the plugs clean them regap them and reinstall which was helpful every 6K miles. After about 12K without maintenance the plugs might start to foul and misfire. Without lead plugs stayed clean for a long time, like forever.

Next, since plugs didn’t foul change interval for plugs increased. Electronic ignition systems improved the spark and standard plugs were going 30K miles and looked pretty good. Finally, in the late 80’s emission requirements called for cars to be meet low emission standards for 100K miles. To do this plug manufacturers worked with car makers on long lasting plugs by using harder materials on the tips and electrodes; platinum and titanium plugs came on the market. These plugs now could provide dependable spark for 100K miles since the gap changed very little due to the hard materials.

So, why don’t plugs last forever? Mostly it is deterioration at the base and the gasket where the plug ceramic meets the base of the plug. When the gasket material deteriorates, due to high heat and pressure, it can allow the current to arc to ground at the base without firing the plug. Dirt, dust, water, and oil all can accumulate at the base too contributing to the deterioration.

100K plug change intervals came into play in the late 90’s and early 2000’s as engine designs incorporated better plugs, better fuel injection, computer controls, and even lower emission standards.

What the spark plug is made from is the reason service intervals have extended.
The old copper plugs didn’t last as long as the platinum.
Other exotic metals have exteded their life too but because of the materials you then get a price increase.

Irridium is some 8X harder than platinum,

Back in the 80s, I don’t think the plugs were even platinum…
When did platinum plugs become common? 1990s?

My memory is very hazy on this but I think it started back in the late 80s/early 90s when Ford had an advertising campaign in which it was claimed their cars only needed a tune-up every 100k miles.

In the old days of leaded gas plugs would foul with deposits much easier; especially with higher performance engines. Lead has long been a thing of the past.

Some years back the Feds started mandating that spark plugs be covered under the emissions warranty for 24k miles, etc. The odds of a copper core plug, even with unleaded gas, developing a misfire are greater than a platinum or iridium plug in that first 24k miles so the car makers are going with what will likely keep them off the hook.

Spark plugs do not always last 100k miles either. It’s quite common to have minor to major performance problems due to aged plugs. Sometimes the car owner is not even aware of the subtle problems and no code may be set even in this situation. In many cases damaged plug wires and coils are caused by the long term running of iffy spark plugs. There’s also the issue of plugs freezing in the cylinder heads when left in place that long.

100K plug life is possible because…

VASTLY improved ignition systems can fire almost ANY plug, even badly worn ones…
The plugs themselves are vastly improved, better materials, better seals…
VERY clean running engines that form almost no deposits to foul plugs…
Modern engine designs with carefully controlled combustion temperatures are easy on spark plugs…

When you add those factors up, 100K spark plug life is no problem…Coming next are “life-of-the-car” spark plugs that never need changing allowing engineers more freedom in engine design where spark plugs can be located in completely inaccessible areas…

When you add those factors up, 100K spark plug life is no problem…Coming next are “life-of-the-car” spark plugs that never need changing allowing engineers more freedom in engine design where spark plugs can be located in completely inaccessible areas…

Sounds like a plan that will backfire badly. (pun intended)

I put a set of advertised-to-be-100K plugs in the last time, but at 60K when I changed then out recently, the gap had increased from 30 to over 55 (1/1000 of an inch units I think). I was noticing a little pinging too on going uphill with the old plugs, and some hesitation on rapid accel, neither of which I have now with the new plugs. I’m thinking now even 60K is too much, at least for my econobox and my driving habits, whitc is mostly put-putting around town. I’m not complaining about the 100K plugs mind you, just making a comment on my experience. I’ll be replacing these newly installed plugs at 30K and see how they look then. Replacing the plugs is an easy chore, maybe it takes 20 minutes, and the new NGK plugs only cost about $2 ea, so there’s little reason not to do it every 30K.

On all of my vehicles and family members vehicles copper core plugs come out every 30k miles and platinums at 50k miles and that’s the outside limit. I’ve never found them to not need replacement.

To the OP: Can you explain the significance of knowing this fact?

There are many similar questions such as follows;

When did air filters come into common use and how could auto mfrs not realize early on that air filters are vital to engine life, especially on unpaved roads?
Oil filters?
Effective oil filters?
Tubeless tires?
Piston rings that ended normally expected exhaust smoke?
Why did Chevrolet six engines normally burn oil well into the mid 1950s?
Why did flathead Ford V8’s have cracked blocks so frequently?
Oil that extended oil change frequency?
When did leaded gasoline begin and when did it end?
When did designers learn to fillet or round inside corners on metallic machined parts to reduce stress risers?
When did rayon tire cords begin and when they go away?
When did cotton tire cord use end?
When did external contracting brakes change to internal expanding brakes?
Why does the original brand and heat range of spark plug usually work best in your vehicle?
How fast did people drive on a paved road in 1920? 1930? 1940s after the war?
How could Henry Ford with his Model A four cylinder let himself be blindsided by the Chevrolet six?
Why did Henry Ford resist hydraulic brakes until 1939?
Why do the oriental brands permit advances to be made by the likes of GM such as improved antifreeze life, improved auto trans fluid life, introduction of OnStar, satellite radio, computer simulated oil life monitor?

OK is absolutely right. Like many items in cars, longevity occurs often because of mandates. Connecting emissions or safety to mandates is a no brainer for the consumer. Auto manufacturers are capable of producing parts that last a long time, it just isn’t as profitable. They need " encouragement". The ripple effect is that parts and survice need to be rethought and other items along with bogus repair “insurance” need to be instituted. Next, I would like to see brake system mandates in place.

This increased longevity like spark plugs comes at a cost, as more jobs need to be transitioned. Look at Midas and exhaust systems . A "profitable " item mandated nearly out of exhistance for a lot of these satellite companies… These situations all put auto parts and service and ultimately auto industries in tenuous situations. Reliability is NOT profitable in the long run.

Why did Henry Ford resist hydraulic brakes until 1939?

Contrary to what some folks think,Government mandates are sometimes a good thing.If you dont believe me ,follow a pack of vehicles down the road that are emission exempt.That being said,I despise gov’t meddling in our lives-what drives the free market is basically the love of “Mammon”,very little altruism now.Thats just the way it is-Kevin

Wha Who? B/c I like to know things. I have wondered this from time to time, and now I have the answer, thanks to this forum…