C4 Tech/Performance L98 Corvette and LT1 Corvette Technical Info, Internal Engine, External Engine

Spark Plug Change Intervals

 
Old 04-14-2019, 08:29 PM
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RetroGuy
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Default Spark Plug Change Intervals

Since I can't easily access all 8 of my spark plugs (some are quite hard to access), how often should one replace the plugs if the engine seems to be running well and there's no visual inspection done on them?
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Old 04-14-2019, 08:48 PM
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84 4+3
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I do mine every 3 years or so. You can leave them in there way longer if platinum or iridium. I don't see much benefit stock so I run coppers. IIRC it was something like 30k miles or 3 years on the early cars?
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Old 04-14-2019, 09:26 PM
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I tend to go with about 2-3 years.
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Old 04-14-2019, 09:56 PM
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My biggest fear is to have a plug stuck in the head. I really wouldn't go past 3 years on any of my cars. Had 2 sets of heads removed because of stuck plugs before.
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Old 04-14-2019, 10:41 PM
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Lots of opinions on this. I personally replace the plugs around 60,000 miles.
They say you can go 100K but I think thatís too much. If you push 60K a bit I donít think thatís a problem.
However, now that I am no driving the car to work anymore, I might consider a lower mileage.
Don't think there is one right answer.

Aside from the runability of the car, I think the plug deteriorates at 100K and pieces may break apart and gaps may open.


Last edited by pcolt94; 04-14-2019 at 10:48 PM.
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Old 04-15-2019, 10:17 AM
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Thanks for all your info on this. I'll go to my respected and trusted mechanic and have mine changed out soon. I wish I new when the previous owner had it done, if at all.
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Old 04-15-2019, 01:03 PM
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I would pull and inspect 2-3 of the plugs that are easy to reach and they will represent the condition of the rest of the plugs. If you aren't sure if they have ever been changed, I definitely would break them all free to be sure they are not seized... and if you go this far and have a socket on them anyway, just replace them with new platinum or iridium plugs. At this age maybe new wires too.
If the engine is stock, in good condition and you drive it normally, you can easily go 60-80k on a set of iridium or platinum plugs.
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Old 04-15-2019, 01:33 PM
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Originally Posted by DGXR View Post
I would pull and inspect 2-3 of the plugs that are easy to reach and they will represent the condition of the rest of the plugs.

If the engine is stock, in good condition and you drive it normally, you can easily go 60-80k on a set of iridium or platinum plugs.
It would be, IMO, a GUESS of the condition of the plugs. I've had 1 bad or fouled plug and the rest good.

You could. That is a 5 to 10 year deal before you have to do it again, I don't have faith in that for 2 reasons. I have had larger than stock gaps out of plugs that were supposed to last 100K and IIRC, got better mileage with new stock plugs. Also, it might not be as fun to pull the head if the plug sits in it too long. BTDT and didn't enjoy pulling the head for machine work when the plug couldn't come out without drilling or whatever it is they do. It's cool if you get away with it but the one time you don't, well....
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Old 04-15-2019, 01:38 PM
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Isn't the plug gap supposed to grow slightly over tens of thousands of miles?
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Old 04-15-2019, 01:44 PM
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Originally Posted by DGXR View Post
Isn't the plug gap supposed to grow slightly over tens of thousands of miles?
Sure but in my case, it was, IIRC way more than slightly. 50% more than spec gap on this truck. It ran "just fine", depending on your definition, for a 70K vehicle. Changed the plugs and picked up a few mpg on the highway runs (I usually make over 50 mile runs each way). Same motorcraft plugs, same everything.
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Old 04-15-2019, 01:56 PM
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Yah some members of another forum (tacomaworld) say they have run the same set of plugs 250,000 miles or longer. When they finally pull the plugs and see the ground electrode whittled down to a wisp, they always say they wish they had changed the plugs sooner! I understand that excessive plug gaps can strain the ignition coil(s) and lead to premature failure.
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Old 04-15-2019, 06:34 PM
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Yes, they are a huge PITA to change. It took my wife and I longer to replace the back passenger plug as it did all the others combined. I agree with most, 2-3 years is cheap insurance for what the plugs cost.
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Old 04-15-2019, 06:44 PM
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Originally Posted by DGXR View Post
Yah some members of another forum (tacomaworld) say they have run the same set of plugs 250,000 miles or longer. When they finally pull the plugs and see the ground electrode whittled down to a wisp, they always say they wish they had changed the plugs sooner! I understand that excessive plug gaps can strain the ignition coil(s) and lead to premature failure.
If the gap is too large, it might be running inefficiently. Problem is that the SOTP dyno is inaccurate and I'd even actively disregard it. These things creep up SLOWLY so that it IS hard to notice. It is hard to compare today's result with what you had 5 years ago. The difference between today and 5 years ago is huge but the difference between yesterday and today is negligible
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Old 04-15-2019, 06:50 PM
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I'm not as particular now that I no longer drag race my Vette. Back then it was every year.

If you have Al heads don't forget to use some Li grease on the spark plug threads to prevent seizing.
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Old 04-15-2019, 11:07 PM
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84 4+3
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Originally Posted by DGXR View Post
I would pull and inspect 2-3 of the plugs that are easy to reach and they will represent the condition of the rest of the plugs. If you aren't sure if they have ever been changed, I definitely would break them all free to be sure they are not seized... and if you go this far and have a socket on them anyway, just replace them with new platinum or iridium plugs. At this age maybe new wires too.
If the engine is stock, in good condition and you drive it normally, you can easily go 60-80k on a set of iridium or platinum plugs.
I've seen it many times that the easy ones to get to are in fact just fine... but the pain in the *** ones haven't been changed in a while...
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Old 04-15-2019, 11:12 PM
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Originally Posted by 84 4+3 View Post
I've seen it many times that the easy ones to get to are in fact just fine... but the pain in the *** ones haven't been changed in a while...
People are lazy and want to do something so they can feel good and not have to put forth the effort?
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Old 04-16-2019, 08:09 AM
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84 4+3
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Originally Posted by aklim View Post
People are lazy and want to do something so they can feel good and not have to put forth the effort?
Pretty much, nothing surprises me. It's usually a gmt800 with a 5.3l, the plug angle makes #8 a bitch. Ours had the factory 03 plug in there when I did it last year. All 7 others were replaced.
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Old 04-16-2019, 08:38 AM
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Originally Posted by 84 4+3 View Post
Pretty much, nothing surprises me. It's usually a gmt800 with a 5.3l, the plug angle makes #8 a bitch. Ours had the factory 03 plug in there when I did it last year. All 7 others were replaced.
I wonder why I am a particular dick when I am buying a used car.
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Old 04-16-2019, 12:30 PM
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Originally Posted by aklim View Post
I wonder why I am a particular dick when I am buying a used car.
The best part is that it was the dealer who did that.
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Old 04-16-2019, 12:43 PM
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Originally Posted by aklim View Post
I wonder why I am a particular dick when I am buying a used car.
Caveat emptor... Another person may be very happy to sell you their world of problems. So, it is SMART to be very detail-oriented when buying any used item.
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