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Just creating a post to give moral support to anyone considering changing their 3rd gen HL sparkplugs. With a decent set of 1/4" and 3/8" drive metric sockets, a low range torque wrench and moderate skills and experience working on cars you should be able to change your plugs in around three hours, taking your time. I recommend first watching the 20 minute YouTube video made by The Car Care Nut for the general procedure and a few hints to make it go smoothly. I bought all my parts (six spark plugs, intake plenum gasket, throttle body gasket) at a Toyota dealer for $148, but you can pay less if you shop around on the internet, just stick to OEM parts. The dealer quoted me $560 to do a spark plug replacement, so you can save a little more than $400 by doing it yourself. A few pics follow:

Apparently I have been hosting a mouse Airbnb, as this is what I found when I removed the plastic engine cover. Fortunately the mice had not yet chewed any of the wiring under the cover.
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The one tricky hidden bolt you have to loosen is at the rear of the intake plenum below and to the right of the throttle body opening. Here are two pics to help you identify it.
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Here is a shot of the plugs removed (and a new one for comparison) showing minimal wear after 61,000 miles. All gaps were between .029-.030" so there was no measurable wear of the electrodes, since the new plug gap spec is .028-.031". The plugs were also fairly clean, with no carbon buildup or oil fouling.
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I was a bit hesitant to do this job initially, but it was not a difficult job. A step stool will help you to reach the rear plugs more easily. I recommend you take photos of stuff before you disassemble it to make sure you replace everything correctly and just take your time.
 

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Thanks for the encouragement. I'll be doing mine later in the summer, so it's good to see some positive reinforcement, and the pics will likely come in handy.
I know parts are a bit more at the dealer (although surprisingly not always), but I have decided to save myself compatibility/equivalence anxiety and just start getting my parts there. A few bucks is worth piece of mind, and the extra cost is minimal compared to the DIY savings.
 

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As someone who's finally got the courage to do this as a certified YouTube Mechanic. It was very straight forward and look forward to doing it again at 120k miles. It was a great morning spent!
 

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Those old plugs look good. I bought a set recently as I'm currently at 54K. Maybe I'll extend the change interval to 75K.
 

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2014 Highlander XLE
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Thanks for the words of encouragement! I'm about to embark on the same 3-hour tour.

I'm kinda loyal to NGK so I'm going with the NGK 94122 LFR6AHX-S Ruthenium HX Plugs, they're pre-gapped at 0.044. Apparently they're supposed to outlast iridium. OEM for the plenium and FI gaskets.

I'm curious if there are any other DIY jobs that I minus well do while all the crap removed? Maybe the serpentine belt?
 

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2014 Highlander XLE
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Ok just a quick follow up. Successful 4 hour adventure following Ahmed’s tutorial. 2014 highlander XLE. A few observations:
  • Give yourselves enough room to lay out all the parts and bolts removed, there’s a lot!
  • The infamous plenum L bracket is actually easy to get to on my year/model. I don’t doubt it’s harder on other models. Now I’m not sure I even needed to take off the tray below the window which took about 20% of the time.
  • Check that you have the right spark plug socket before you start, it helped to use a couple smaller extensions as my long extension was too long to fit.
  • Look up additional torque specs for the wiper motor.
  • Take pics before you start, this was helpful for the vacuum lines.
  • There’s not much else that I needed to do that would have helped with everything removed. But it’s a good time to wash out the engine bay.
Although I wasn’t expecting to feel any difference while driving, that hesitation when accelerating is a bit better now.
Good luck, it’s not difficult just time consuming!

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