Yes, changing out the spark plugs on Toyota's tranverse mount V6 engines is tough. I have a 2002 Sienna which is similar to all models 2003 and prior. The front 3 plugs are easy. But the back 3 are really tough to reach.
2001 and prior Siennas only need the Denso platinum plugs PK20TR11 which I think are dual head plugs. But older Siennas can probably still benefit from pricier iridium plugs because they fire more consistently at lower voltage and can take higher temperatures than platinum. I did some research on the web, and folks are raving about Denso Iridium Power IK20s. They are a bit more than the NGKs, but on eBay, I found a set of 4 for just $33-buy-it-now in CA. So I bought 3 sets (12 plugs total).
Tools you'll need - a common 3/8 inch square rachet socket driver. The overall length must not exceed 8 inches inches long and mine is a cheap one from any store. But I would recommend that the rachet have a push button socket release and a fine tooth rachet that clicks every 15 degrees. Next, you need a med-long extension. The spark plug is deep in the tube and you'll need ideally, 5 3/4 inches and no more than 6 1/2 inches max. Of course you need a standard spark plug socket. Additionally, you'll need a 5mm Allen wrench. and lastly, a 10 mm closed-end crescent wrench. Length should be no longer than 7 inches.
Installation: pop the hood, and remove the plastic engine cover. I assume you know how to remove the front plugs. So I'll only focus on the rear plugs.
Start on the right side. Unclip some of the vacuum hoses and spread them out to the sides of the opening. You should see a hole that extends back underneath the air intake and next to the exhaust manifold. Reach back there with your arm. feel for a 1 inch square block and securing hex bolt. Using feel only, use the 10mm closed-end box wrench and loosen the securing nut. A hard counterclockwise tweak should break it loose, and about a half dozen more small loosening motions should loosen the bolt so you can switch out and use your fingers. Remove the bolt and place it away from the engine in a safe place. Lift the boot assembly out. Watch to make sure the rubber grommet doesn't slide off and fall in the oblivion never to be found.
WithOUT the rachet driver, get the socket and extension and slide down tube, then reach in with rachet and unscrew the spark plug. Remove the rachet, and lift extension and spark plug out. If your socket doesn't grip the spark plug, take it out, wrap the lip with a little masking tape). Installation is the reverse of removal.
Get a new spark plug. Slide it inside the spark plug socket and make sure it grips but not too tightly. The plugs are pre-gapped at the factory and have a plastic sleeve to protect the head. You must NOT allow the plug to drop down the tube because the impact could bend the tip and affect the gap or damage the plug.
There are two metal compression washers on the spark plug. There's no room for a torque wrench back there, so tighten the plugs down until the compression washers just begin to feel like bottoming out. Then twist another 30 deg. Do NOT slide the boot/coil assembly back over the spark plug yet. The first boot must remain out from its tube to allow the second boot room for removal.
The middle boot assembly is a bit harder. It's just like the first, but you need to disconnect the wire harness clip to each boot/ignition coil.. With a single hand, rock the boot assembly a little until it lifts up and off the plug about 1 inch. Then turn the boot assembly clockwise a little and use the technique you practiced to unclip the wiring harness from the boot assembly. There is only room for one hand in there. Be persistent and follow technique and practice. Eventually, you should be able to get the wire clip off.
Now the boot/coil should lift out easily. Then use the spark plug socket and reach back in there and slide the socket down the tube and then attach the rachet driver and remove the spark plug. Installation the reverse of removal. Again, be careful about dropping any parts down into the engine.
The 3rd boot is like the 2nd, because you need to disconnect the wiring clip, but it's different in that instead of reaching in from the right side, most of the extraction uses the opening from the left side.
My dealer claims this can take 2 hours of labour. I used simple tools and did all six myself in 1 hour 15 minutes making lots of little mistakes. If you follow these directions and have simple tools and do not exceed my dimension recommendations, you should be able to get in there and do this in under 45 minutes. Good luck.
For more information on this and related procedures please visit: http://www.siennaclub.org/forum/index.php?showtopic=12402
After posting my original answer to a question on a different forum, I've noticed how wiki.answers.com started posting this blog. So I thought I'd add comments here. Well, I had to change my plugs again... putting lots of miles on the Sienna. This time I did this in just 45 minutes! The one danger I'd advise here is the pulling of spark plug wires. I recommend that folks use plenty of spark plug lube on the heads to avoid sticking over time and easier removal the next time you gotta do this.
Another reader comments: If you remove the windshield wiper cowling and tray, things are easier (but not easy) to access. I was able to change the plugs on my 2002 Sienna without removing any hoses or anything, just a ground wire that was in the way. To do this:
1. The engine should be cold. Open hood. Remove caps at base of wipers, remove wiper nuts, and wriggle off the wipers.
2. With needlenose pliers, squeeze the plastic holddown thingies under the front rim of the wiper tray, while pulling up on the gasket that runs along the top (hood gasket). Start at one end and work across until the gasket is removed. Remove wiper cowling by pulling up and towards the front of the car. There were 2 plastic holddown thingies on the sides that I couldn't remove, they just broke off. Disconnect wiper washer hose at connector on passenger side, remove hose from clips in tray, and lift out both cowling pieces.
3. Undo bolts holding the wiper tray, but not the two bolts on the front of the tray near the air filter (these hold the wiper motor assembly to the bottom of the tray).
Don't set the bolts in the tray, they could roll down a drain hole into the engine compartment. Lift the tray as far as you can (it's heavier than you would expect).
Disconnect the wiper wiring connector and push the wiring cable harness thing out of it's hole. Now you can remove the tray.
4. Remove the fresh air intake duct (for the passenger compartment, not the engine) by removing 3 plastic nuts.
Now you can actually see and reach the rear part of your engine. Since the spark plugs are all under the intake manifold, you pretty much have to work by feel. You can reach under it from either side once you remove the ground wire attached to the intake manifold assembly on the passenger side. (It's pretty obvious which wire, because it's in your way). With the proper selection of socket extensions, and lots of patience, you can replace the spark plugs without removing anything else, except for that little plastic cover on top of the engine.
The 2001 Toyota Sienna spark plug gap is .023. Most spark plugs will be pre-gapped by the manufacture of the spark plug.
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Good and tight, but don't break the thing.
The spark plugs on the 2nd bank are behind the engine under the crow bar. They can only be felt with hands follow the spark plug wires. Change the spark plugs are very easy but needs the proper spark plug socket and the wrench extension.
Same relative position as the front plugs. They are just a lot harder to reach.
There is an article with video on how to change spark plugs at ; http://www.carbasics.co.uk Look in their 'how to' section.