I have a 2005 with 110k miles. Has original spark plugs. Car runs fine but have read that you should change plugs at 100k. Is that true? Even though my car runs fine, might new plugs improve performance?

 

It's more of a maintenance item rather than doing it for performance, but if the plugs do sit in the engine too long, it may eventually lead to misfires, poor fuel consumption, and general lousy running.

If they plugs are REALLY worn or fouled up, you may feel a noticable increase in the engines output and smoother running though!

 

If your car is running without any issue at the moment, changing the spark plugs will not give any noticeable different.
However, during the replacement process, you might find oil after you remove the coil pack due to worn seal.
Just make sure you use "Real" OEM parts

 

OE replacement interval on factory plugs is 120k so you're good. Go back with OE Denso plugs and you're good for another 120k after.

 

I changed mine at 47K miles 12 years. The plugs were dry and no difference in performance were detected. Once in 12 years I am OK with replacing them for maintenance to avoid wondering if they need to be replaced.

 

Similar here, did it at 10 years / 100k miles, now at 150k. If I still have car in 2025 (20 years old) I will have to consider, as mileage will likely be around 160k-170k, driven fairly gently... Car is smooth. I also did my trans fluid around 95k (dealer did exchange, not empty/refill. That's all they would do). I experienced some transmission hesitation for a while (bad if attempting to make a left turn into traffic!!) but that fortunately resolved itself completely after a month or so. Not sure if the issue was tied to fluid exchange (don't remember the exact timing), becoming primary driver of car instead of wife (trans relearning?), or other items. Not so sure I want to mess with the trans fluid again now that it is working so well. Probably wait until 200k miles rather than 10 years since prior change. I don't believe in lifetime fluid but kind of want to leave well enough alone. But spark plugs seem less risky to change, although I guess anything can be screwed up

 

In your case, I wouldn't be worried about the mileage, rather, the 18 years those plugs have sat in the cylinder heads. Personally, I'd replace them now.

As the others mentioned, if you don't notice anything wrong now, you shouldn't expect much (if any) of a change/improvement. The placebo effect or is it affect (lol), may be realized and you may experience smoother idling and better acceleration and see unicorns and rainbows. Fuel economy COULD change slightly also depending on what type/style plugs you replace the OE with.

As others have also stated, I'd only consider iridium NGK or DENSO. (The specific style of those brands isn't as important as only using iridium plugs.)

Are you considering doing the work yourself or taking it somewhere? Whomever does the work, I strongly caution you to BE EXTREMELY CAREFUL OF THE WIRE HARNESS CONNECTOR GOING TO THE IGNITION COIL! I accidentally pulled a wire out, and troubleshot for hours why it was misfiring badly!!! Not to mention the plastic clips holding them in place will be brittle, etc. so make sure you're not rushing it. I'd allow at a minimum, 2 hours of your time.

Again, I'd replace the plugs because of their age and the dissimilar metals of the cylinder head and the spark plug gasket may simply make the job more difficult if you wait another 10k miles.

The plugs sell anywhere from $7-10 each.

Keep us in the loop!

 

I second that. In fact, it applies to all the plugs under the engine.