I recently replaced the spark plugs in my '04 3MZ-FE 3.3L V-6 Highlander. Here is how I did it along with some photos.
Notes: I disconnected the battery before beginning. I used Denso iridium plugs SK20R11. They were installed with anti-seize compound on threads and torqued to 222 in. lbs. (18.5Ft. Lbs.). Dielectric grease was used on all boots before reinstalling coil packs.
3/8 drive ratchet, 3/8 drive in. lb. torque wrench, 6 in. and 12 in extensions, 8mm allen wrench with box end wrench attached for leverage, 3/8 drive sockets 10mm, 12mm, 14mm and 5/8 spark plug socket, stubby gear wrenches 12mm and 14mm, 5mm allen wrench with "T" handle and combination wrenches 12mm and 14mm.
Removed engine cover using 5mm allen wrench (yellow arrows). Then pulled up near the back of cover to disconnect clips below cover and removed cover.
This will expose the front 3 coil packs. Remove 3 bolts (yellow arrows) with a 10mm socket.
Lift out coil packs and replace these 3 plugs.
Now for the back 3. I removed 2 nuts (yellow arrows) with a 10mm socket and repositioned the long bracket with all those hoses and tubes to uncover the intake plenum bolts.
The intake plenum is held in place on the front side by 4 allen head bolts (yellow arrows) and 2 studs with nuts (yellow circles). 14mm socket for nuts and 8mm allen wrench with 8mm box end wrench attached for leverage to remoe bolts. (I did not remove the 2 studs because I don't have any female torx sockets. (This would have made removing the plenum a little easier).
At this point I decided it would be best to remove the plenum and throttle body as a unit because I couldn't get anything on the lower aft bolt to separate the t/b from the plenum. I removed the flexible air intake duct. There are 2 brackets holding the plenum on the backside and 1 bracket holding the t/b. Not much room to work between the plenum and firewall area. Removing the brace between the struts gave me a little more room to work. After loosening the 3 bolts I used the stubby gear wrenches to back the bolts out. Note: the end bolts are 12mm and the center ons is 14mm on my eng. I marked all the hoses before disconnecting them. There are 2 coolant hoses on the t/b. I took them loose before I realized it and spilled some coolant before I got them plugged. Next time I will drailn the radiator. The photo below shows the plenum removed and standing side ways on the right side, coil packs out and plug being torqued. (This is when you change the 3 rear plugs)
NOTE: On the center bolt Use a 14mm open end wrench (it's a little shorter than a combination wrench ) and with one hand find the bolt head and the other hand guide the wrench onto the bolt head. I think this is the easiest way to do it.
The next photo shows the 3 brackets (red circles) that are behind the plenum and throttle body with the plenum removed. On my engine the center bolt was 14 mm and the other 2 were 12mm. These bolts were hard to get started. I finally left everything loose until they were all started. Also, the one on the t/b was easier to get to by removing the part of the t/b assembly with the butterfly in it and dilsconnecting the wire harness. 10 mm socket for these bolts.
Finally, reinstall everything is reverse order of removal. Refill the radiator, start up and check everything out.
That's it. It took me between 4 and 5 hours. Next one should be about half that time.