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I notice that mechanic make a point that they change spark plugs annually i.e. every yearly service.

However it seems to me spark plugs can have a long life e.g. google says they are four fifths worn by 80k miles which is still long and cars rack that up over years.

When mechanics say the change park plugs every yearly service, do they actually mean that and its only a preventative measure, or do they mean they inspect them to see if they need changing and change the required ones(therefore they may not have been changed for years if they were ok)? How often do they actually change them?

  • I saw an article awhile back suggesting that one replace suspension bushings every year. Which is complete BS, as they can last decades. Don't listen to mechanics. Your owner's manual has a detailed recommended maintenance schedule, including when to change things like spark plugs. Listen to the manufacturer. They are, by definition, the World's Greatest Living Authority on the car. – 3Dave Sep 10 '18 at 22:24
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The service interval is dependent upon the type of car which the plugs are installed in ... most plugs in today's cars should be changed about every 100k miles. O2 sensors are usually about the same amount. Unless someone does a lot of driving, they aren't going to come anywhere close to needing them changed once a year. Most vehicles will probably go quite a bit longer (time wise) prior to needing them changed.

Newer types of spark plugs have super-hard alloys which are used at the tips of the electrode and ground strap to transmit the electricity produced by the ignition system. These materials are things like platinum & iridium. These two, along with copper (Note: copper is soft and isn't used as contact points, but rather to transmit the electricity through the electrode) are used the most, while there are other exotic metals used but very infrequently (in comparison). "Newer" in this sense is spark plugs manufactured in the last 20 to 30 years. Spark plugs before then would need to be changed out a lot sooner, more like the 30-40k mile range. As plugs get older and the electrodes begin to wear away, ignitions systems today can adjust for this and continue to provide the optimum spark, which allows a longer life out of the plug as well. Older ignitions systems, such as points ignition and even early electronic ignitions couldn't adjust like you'd see today.

There are some generic tell tale signs spark plugs might need to be changed prior to their recommended change interval. No mechanical thing is going to be perfect, so they can fail prior to their expected lifespan. This website gives six general things to look for:

  1. Engine has a rough idle: Your engine idles when it is a stationary and in this position the engine normally produces around 1000rpm. The sound the engine gives off is constant and smooth but if your spark plugs aren’t performing as they should, your engine will produce a rough and jittery sound while producing larger vibrations through the car. Not having this checked can lead to costly damage being done.

  2. Having trouble starting your car?: Many people put their car not starting down to being out of fuel or having a flat battery. One possibility you may overlook is having bad or worn spark plugs. If your spark plugs don’t produce the spark needed to get the vehicle moving, then you’re going nowhere. It is also possible that faulty spark plugs are causing your battery to drain. If so you need to have your battery and spark plugs changed as soon as possible.

  3. Your engine misfires: When your engine misfires it causes the vehicle to halt for a fraction of a second and then continues it’s usual movement. This means the vehicle isn’t functioning as smoothly as it should because one or more cylinders aren’t firing properly, which can also lead to higher amounts of emissions.

  4. Engine surging: When a vehicle sucks in more air than usual in the combustion process it can cause the vehicle to jerk and then slow down or continually start and stop, which means the vehicles engine is working inefficiently. This is also known as engine hesitation and dangerous situation can arise if this occurs in traffic.

  5. High fuel consumption: If your spark plugs have deteriorated you’ll notice that your vehicles fuel economy can decrease by up to 30% due to incomplete combustion. If you notice you’re having to fill up more often than usual it can be caused by deteriorating spark plugs. To get back to your vehicle’s optimum level of fuel consumption all you’ll need to do is have your spark plugs changed.

  6. Lack of acceleration: If your vehicle is accelerating poorly it is fairly easy to tell. It feels as if the vehicle doesn’t want to respond when you put your foot down, or it does but not instantly as you’ve become accustomed to. It can also feel as if your vehicle is trying really hard to pull itself along. This ‘sluggishness’ can be easily fixed by having the vehicle serviced with a spark plug change.

The thing to remember is, your mileage may vary.

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They change them to make money. Modern platinum type plugs easily go over 100,000 miles in a typical engine. At first , I didn't believe and changed at 50 K miles, the old plugs looked like new. I have left them in for 120 K ( Nissan Titan, 5.6 L), I could hardly tell the old ones from new , no change in MPG , no apparent change in performance. I follow manufacturers recommendations of 100 K.(Although about 30 years ago I did have a set of defective Bosch platinum plugs.) , Platinum Plugs are an outstanding improvement for an old person who had to change regular plugs at 20 K because they were fouled by the leaded gasoline ; and could not be cleaned. The Google story is BS.

  • For 63 years I have always changed my own plugs, so I have seen a few. – blacksmith37 Aug 10 '18 at 19:06
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I just ran into this same situation just a few weeks ago. I have a 2017 VS Passat R Line 1.8 Twin Turbo. I purchased it with around 32k miles on her and when she got 50k I took her in for an oil change and service that included changing the platinum plugs which cost $140 for the four of them according to the mechanic doing the service that day. I normally work on my own vehicles and I have built several Harley's and a couple of other motorcycles and several VW bugs and buses. So I opted not to change the plugs till I have at least 100k miles and because 90% of the mileage I put on the car is not in traffic I will probably go to 150k.

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