How many miles should spark plugs last?

[shew01]

I’ve cleaned the plugs in our car every 3-4 thousand miles or so. However, we’re still running the same plugs that came with car. So, that’s about 16,000 miles in a little less than 5 years. (I have no idea how many miles were on the plugs before we bought the car.) The car recently developed a miss under load, and cleaning the spark plugs this time did not help much.

How many miles should a set of spark plugs last?


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[Bigsnapper43]

Ihave been driving a's 65 years I have never bought a new 'plug. Decide which one is missing through it away. get another used one.

[Synchro909]

How certain are you that a plug is causing the miss? There are a number of other possibilities worth investigating.

[J Franklin]

When the electrode is burnt away so it can't be gapped, it is time for a new plug.

[shew01]

Originally Posted by Synchro909

How certain are you that a plug is causing the miss? There are a number of other possibilities worth investigating.

Good question… This is what I’ve tried.

- Swapped out coil with known good coil; still no power under load
- Swapped out condenser with known good condenser; still no power under load
- There is no carbon tracking on the distributor body
- The rotor button looks okay
- The distributor cap looks okay
- The high tension coil wire looks okay
- Sparklite indicates strong spark
- Timing has not been changed; car ran fine before the miss developed
- 04/30/2024: Tried a jumper across the junction box to bypass the ammeter; still no power under load; ammeter is probably okay
- 05/01/2024: Reset point gap—it was too closed; improved, but still seems to miss under load
- 05/02/2024: Cleaned plugs and checked gap; #4 was very sooty; improved, but still has a slight miss under load

Should I be looking at something other than the plugs?


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[shew01]

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Originally Posted by J Franklin

When the electrode is burnt away so it can't be gapped, it is time for a new plug.

Hmm… That sounds like spark plugs can last forever. The electrodes look fine. The gap was correct.


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[shew01]

Originally Posted by Bigsnapper43

Ihave been driving a's 65 years I have never bought a new 'plug. Decide which one is missing through it away. get another used one.

That sounds like a good idea, but I’m not sure how to determine which plug is missing.

[Synchro909]

Originally Posted by shew01

That sounds like a good idea, but I’m not sure how to determine which plug is missing.

The missing one is the one where there is only a hole in the head!!!
Seriously, the usual method of shorting out one plug at a time won't work because you are obviously driving (and under load) when it acts up. All I can suggest from here is try replacing each plug, one at a time and when the issue disappears, you found it. Do one plug at a time.

[700rpm]

I worked in a service station in high school in 1961, and one of the services we offered was cleaning plugs. We had a special machine and some kind of electrical device that told us whether the plug was good or not. I wish I had bought that machine when the station closed. I’ve never seen or heard of anything like it since.

(PS: or service vehicle was a 1930 Model A pickup.)

[shew01]

Originally Posted by 700rpm

I worked in a service station in high school in 1961, and one of the services we offered was cleaning plugs. We had a special machine and some kind of electrical device that told us whether the plug was good or not. I wish I had bought that machine when the station closed. I’ve never seen or heard of anything like it since.

(PS: or service vehicle was a 1930 Model A pickup.)

Was the spark plug tester somewhat similar to this?

Spark Plug Tester Dual Holes, Automotive Engine Ignition Coil Tester 0-6000rpm Working Frequency, Car Spark Tester Tool with Protective Cover for 12V Gasoline Vehicles https://a.co/d/7oXPzzE


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[nkaminar]

Buy a new set of plugs and install them. If you still have the miss then it was not the plugs.

[WHN]

Model A Ford owners are known to be cheap. This is not always a compliment to the way some of our cars are being maintained.

If your to the point that never changing your spark plugs is a thing of pride? Your car most likely has other items that have deferred maintenance issues.

It may still be running, but how much better would it run with a set of new plugs? Do you ever change the points or just keep filing them down? Still driving on 20 year old tires? Haven’t changed your radiator hoses ever? Still running steel original engine fan? Rebuilding worn out springs? Why replace a really old radiator, just add a little vinegar and repaint it, it only boils over when it’s hot out? Car only shimmies when I hit a bump or railroad tracks?

When you go to sell your car. If it has been well maintained? You will see that return in the price you will be able to get for the car.

Preventive maintenance! Even antique cars can benefit from it.

[shew01]

Originally Posted by WHN

Model A Ford owners are known to be cheap. This is not always a compliment to the way some of our cars are being maintained.

If your to the point that never changing your spark plugs is a thing of pride? Your car most likely has other items that have deferred maintenance issues.

It may still be running, but how much better would it run with a set of new plugs? Do you ever change the points or just keep filing them down? Still driving on 20 year old tires? Haven’t changed your radiator hoses ever? Still running steel original engine fan? Rebuilding worn out springs? Why replace a really old radiator, just add a little vinegar and repaint it, it only boils over when it’s hot out? Car only shimmies when I hit a bump or railroad tracks?

When you go to sell your car. If it has been well maintained? You will see that return in the price you will be able to get for the car.

Preventive maintenance! Even antique cars can benefit from it.

Thanks for the response. However, this is not a matter of me being cheap. I already bought a set of plugs, but I haven’t had time to install them yet. (I’m thinking the new plugs will likely “fix” the miss, but I’ve been out of town and have not had a chance to work on the car for several days.)

Personally, I’d like to know what the real issue with the miss is; so, in my initial post, I asked how long a set of plugs should last. I don’t think anyone has posted an answer to that yet. I don’t see any point in stretching parts beyond their expected lifespan, but tossing out something that should still have life in it is probably not a good plan either.

I was just trying to get a reasonable expectation of how long (in miles, not number of years of minimal driving) for the lifespan of spark plugs.

I do make every effort to keep the car well maintained, including frequent oil changes and chassis lubes.

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[Badpuppy]

I've been running OEM in my 2008 Ranger 4.0L for 96K miles so far. Runs so well I never have given the plugs a thought. Throw money at it. Plugs are cheap enough.

[Gary WA]

83,467 miles on a set of plugs!

[shew01]

Originally Posted by Badpuppy

I've been running OEM in my 2008 Ranger 4.0L for 96K miles so far. Runs so well I never have given the plugs a thought. Throw money at it. Plugs are cheap enough.

Yeah, I agree. Newer cars can run extended periods of time on plugs without maintenance. I have 108K on my 2012 Honda Odyssey plugs and 118K on my 2008 Mazda Miata plugs. I have not cleaned or gapped either set since new, and both cars run like new.

My first car was a 1971 Buick, and I do recall having to clean the plugs often in the 1970s. A new set of plugs frequently “fixed” performance issues back then.

I see a LOT of old spark plugs at swap meets. Does that mean that old Model A plug tend to last forever? (I’m guessing not.) I’m thinking a lot of those old swap meet plugs had issues when they were removed from the cars.


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[Fullraceflathead]

1 of the few benefits to our modern gas that is lead free is that your spark plugs will last almost indefinitely and you have minimal carbon build up!!!

[shew01]

Originally Posted by Fullraceflathead

1 of the few benefits to our modern gas that is lead free is that your spark plugs will last almost indefinitely and you have minimal carbon build up!!!

Newer plugs are often made of tougher metal too, correct?


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[rotorwrench]

How long the electrodes last depends on what they are made from. The resistors are another fail point in plugs so equipped. If you purchase iridium electrode plugs then the resistor will likely be the first weak link to go.

In aviation we had to clean the plugs on every 100-hour inspection. The Champion spark plug cleaner/tester was the best back in the day. The plug is held in a rubber bushing while abrasive blasting cleaned the electrodes at the push of a button. Compressed air was connected for testing. After cleaning, the plug is screwed into the test chamber and 120 psi of compressed air was applied to the chamber with a little thumb wheel attached to an internal pressure regulator. The tester has a vibrator coil to operate the ignition coil that is connected to the plug with a high tension lead. The push of a button activates the high tension sparking of the subject plug to be tested. The operator then watches through a view port that has a mirror to reflect the window into the test chamber so the spark can be visually verified for at least one minute. If it holds up under that pressure for one minute then it is good to go. These testers cost over $1,800 bucks at some sources now days so most folks can't justify them.

A bad plug will quit sparking before it reaches 1 minute. The test has to be performed on a plug that has been cleaned and properly gapped to specs. We used iridium fine wire plugs on all the helicopter engines since they would last as long as 800 hours with careful operation and proper maintenance. Aircraft use hour meters instead of odometers so everything is in hours. On the small piston type helicopters, I always figured 60 to figure for 60 miles an hour average speed between hovering and 80 miles an hour top end so it is just an approximate of mileage and not real accurate at that but that figured 800 times 60 to equal 48,000 miles. The plugs cost around $105 each so a set of plugs was well over $800 for a set of 8 plugs for a 4-cylinder aircraft engine that has two plugs per cylinder. Some folks thought that too high so they purchase the heavy electrode plugs for around $50 each. They were worn out by 250-hours so a person was ahead of the game to purchase the iridium fine wire types even though they were more than twice the price.

Automotive plugs don't take near the beating of an aircraft plug so they can last a lot longer. Platinum and iridium plugs are available for them too but they are expensive. I use the iridium ones on my wife's 4.6 L Ford V8 due to the difficulty factor in replacement. The coils last around 100,000 miles before one or two start getting weak so I replace all 8 coils every 100,000 miles along with the plugs.

Here is a good tester. https://pilotjohn.com/new/champion/c...hoCMWkQAvD_BwE

[shew01]

Wow. Interesting information.

I do recall spark plug cleaners from my teenage days. Even K-Mart sold some sort of cheapie, dumbed down device “for the masses.” I think it looked something like this.




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