Buying spark plugs, time vs mileage?

I might have worked thru this and answered my own question, that replacing the conventional plugs every 2 years vs. platinums every 8 years might be more expensive in terms of labor, and about equal in parts price.

My Tundras are not daily drivers. They are purpose driven for truck duties. As such, I can barely put 1000 miles on either of them in a year. Heck, I have only put 400 miles on my lifted 4x4 in a year! I just replaced the plugs with the Denso conventional, under $2 per plug.

Premium iridium and platinum plugs will go the most miles. But are they are still subject to retirement due to their time/lifespan? Conventionals last less miles.
It's not easy to find how many miles/years plugs will last.

The Tundra manual states to replace plugs non-platinum plugs at 2 years or 24k miles. And for all others at 120k miles or 8 years. That seems to be the absolute limit on plugs, 8 years, regardless.

Likewise, I have a Dodge Challenger and in the owners manual it states the (platinum) plugs must be replaced every 80k miles or 8 years, or thereabouts.

I think the time/life expectancy is based upon the fact the plug materials become brittle and may fail, with the tips or the ceramics breaking, or they may get stuck in the block. Hence the approximate 2-8 year replacement recommendation.

Cost of plugs.
Conventional: About $2 each, or $16 for a set of 8.
Platinum and Iridium: $5-7 each, or $40-$60 for a set.

Here's my thinking for the Tundras. In the next decade, I probably cannot put 10,000 miles on either of my trucks. But the plugs will likely need to be replaced in a couple years, to a decade. If the plugs have a time/life expectancy, is it wasteful to put in the more expensive plugs?

Next question, I just replaced Iridium plugs with unknown miles but they looked good. They could have many years and tens of thousands of miles, I just do not know. I replaced them with Denso conventional plugs. Truck runs great with either set. Was this smart or should I have cleaned and put the old ones back in?

Yes, this is a trivial "value" we're talking about here. Fifty bucks. But I am trying to make the best decisions and learning about plugs.

 

Doing plugs is a good way to find out how your engine is feeling. I think platinum and iridium are less conductive so require more voltage, but my ‘02 came with platinum. That extra voltage might burn through coppers faster, but i dont know how much faster. The iridium plugs i pulled out were fresh, so i cleaned em, gapped em, and put em on the shelf.

i need to check the torque, when i changed mine they were all just above finger tight, and the spec was something silly like 13ft.lb. and i dont want em to rattle loose. But didnt use antisieze.

 

Very good points.

The iridium plugs I just pulled were also snug, but barely-so. Changing or inspecting plugs on these is really easy. So I'm leaning toward the less expensive plugs. Seems to me if you're inspecting them anyway, may as well swap out. And I'm thinking fresh conventional plugs may be better than 7 year old iridium?

I did decide to keep my old iridium plugs like you've done. Put them in the used parts box for an emergency, keep a couple in the truck too. I once blew a plug out of a Chevy on the highway!

 

Interesting topic . . . my 06 DC has the orig. plugs from the factory, and the truck just turned 60K miles.
So maybe at 15 years I should put in a new set even through the mileage is low relative to the trucks age.

 

I have a 69 VW beetle convertible that is still running the same spark plugs I installed in 1981 when I rebuilt the engine. It only has about 15K miles on the engine over those 40 years. I plan to change the spark plugs at 30K miles in 2061

 

At 15 years, and 60k miles, you're many times over the suggested limit on the plugs according to the manual (at least, my 2000 manual, yours may be different). I'd suggest carefully doing so. Lots of brittle plastic clips, and plug ceramic you do not want to break. I'd urge you to spray some liquid wrench or breakfree on every plug before attempting to remove. A broken plug is a disaster.

This week I did a plug job on my recently acquired 2005 Mercury sedan. I suspect the original plugs and 100k miles. One of the insulators was broken off, at some point, and consumed by the engine and caused a small misfire. No idea when. New plugs cured the misfire, but that breaking insulator could have caused serious damage in the engine. Thankfully it now appears to run perfectly and no damage, crossing my fingers and saying prayers.

 

I would do plugs immediately at that age. This week I did a plug job on my 2005 Mercury sedan that I recently acquired. I suspect the original plugs and 100k miles. One of the insulators was broken off, at some point, and consumed by the engine and caused a small misfire. No idea when. New plugs cured the misfire, but that breaking insulator could have caused serious damage in the engine. Thankfully it now appears to run perfectly and no damage, crossing my fingers and saying prayers.

 

none of my old ones really stood out as being the worst, i think the white on the strap (on all of em) is from a lean burn, and theres a little brown, but nothing to worry about.

 

how a spark plug go bad anyway, if a spark plug could go bad over time, wouldn't they have a expired date on the box???

 

Not enough attention in its formative years.

 

Sitting on the shelf, indefinite life. In the engine exposed to combustion, heat, cold, moisture, rust/corrosion, etc. I don't know. But my vehicle manuals DO put a date on this, and it seems to be consistent with at least a couple manufacturers - 2 years on conventional and 8 on iridium or platinum plugs.

A few thoughts, from what I have experienced or read around the interwebs (take it for what it's worth).
* Stuck or seized in the threads, due to corrosion
* Ceramics may crack/break (this happened to me, not sure if due to miles or age or both).
* The bond on the tips could weaken and fail.

Seems like one should at least pull and inspect at the recommended age intervals. And they are so inexpensive if you've pulled them you may as well replace them in my view unless they look pristine. Maybe not. I'm unsure about this.

 

Drilled out a friend’s plug at just a little over 100k miles and 8 years. I’d wait 1/2 that time before replacing and avoid that possibility. I’m sure there are people who took out plugs with over 20 years on them but I wouldn’t make that gamble myself.