How to change spark plugs
Generally, speaking, changing out the plugs on a Sienna transverse mount V6 (1998-2003) with 1MZFE engine is trivial and no different for the FRONT 3 spark plugs than from most cars. Step 1. is to remove the engine cover plate (that 1 - 2 ft wide shield). Unscrew the center Toyota plastic logo, then unscrew the 3 allen key nuts in front (5mm - and use a magnetic allen key or risk dropping one of the nuts down into the bowels of a crevice like near the power steering pump and never find it again - $2 at your dealer to replace that nut and I've seen 3 siennas missing one of these nuts - so go figure!). Each of the 3 front spark plugs has a wire that clips to the boot/coil mechanism. Unclip the wire (use strong thumbs - women will likely break a nail here and pinch and then slightly rock a little side to side and slide straight back). Use a 10mm socket or box wrench and loose the boot anchor bolt on each. Take a standard 3/8th inch socket driver, 8 inch extender, and smaller of the two standard spark plug sockets and remove the spark plugs. You may need a thinner wall version of the socket to slide down the tube. Do NOT force it down. If you can lift the plug out the deep shaft, put a layer of masking tape on the socket to make gentle contact to life the plug out. Replace with your favourite iridium plugs. Remember to clean the boot and holes of dust and dirt before doing the service. Installation is reverse of removal. I put some high temp anti-sieze compound on the threads and spark plug boot grease on the tip of the plug. You do not need to gap iridium plugs.
If you tolerate mediocre sparks with factory performance, get the NGKs or Denso long life iridium plugs (around $7 - $8 each and 0.6mm or 0.7mm tip). The NGK iridium IX plugs last around 40K miles, and the Densos go around 60K miles. The dealers will claim that Toyota Tech Service Advisories say these last upto 90K miles, but my experience shows a 2 - 5% decline in performance, mileage and overall more sluggishness in acceleration after 30K - 60K. So I change them. If you want performance, go with Denso Iridium Power IK20's. Awesome. But they last only 30K miles. And then performance degrades. They have a very thin 0.4mm tip. Super hot, super narrow conductor means absolute spark with low voltage requirements. This means longer life for your ignition coils and wires and upto 5% better gas mileage. Maybe just propaganda, but my wife and I can clearly feel the difference. The engine starts fast and runs clean. Cheap gas burns without causing knock after 5K miles without any cleaner added to the fuel. I believe this works.
What's the downside? Well, the back three plugs are a nightmare. You CAN'T see them. Only feel them. You need a 3D picture in your head. But you need to compensate for a slightly different sized boot, since the rear plugs don't have their own coil. Toyota uses 3 coiled-boots that are each paired with one front and one rear boot. Get some good padding for the engine cover, get some velcro tie straps to move vacuum hoses out of the way, and hopefully you have skinny but strong forearms. Get a thin long sleeve to protect against cuts and scrapes. Just live with the bruises. If facing the front engine, you will need to reach in from the right of the rear intake manifold cowel assembly to remove the right and middle rear plugs. You access the rear left plug from the left side of the engine. The right most rear boot can be pulled out without unclipping. I take a clean microfibre cloth and clean the boot areas by feel before I start work. This will reduce the hazards of falling particles into the cylinders. The middle and the left side plugs need to have the wires unclipped. Remove both the right and middle rear boots being careful NOT to damage the wire. Boot assemblies from the factory are pricey (if the service person lubed the plug tips with boot grease - the clear stuff - life would be a piece of cake.)
Now replace the plugs using the same procedures as for the front, only you will need to do this by feel. There isn't much swing room, so hopefully you have a socket that rachets in less than 10 degree increments. There's almost no way to get a torque wrench in the back. So you need to do your best. Typically, spark plugs in aluminum block engines can suffice with around 12 - 15 ft-lbs of torque. Specs by the plug makers are around 25 ft-lbs of torque. Go figure. Who's right? Well I don't know, only, having worked on plugs for years, I go by feel and anti-sieze compound. After a small dab on the threads, and boot grease on the tip, I put the plug into the drive socket and gently slide it down the tube. If the plug drops, it could alter the gap, and hence just the right amount of masking tape or a fresh rubber socket boot liner to grip the plug lightly. I screw the plug by finger strength only until it hits bottom. Then with socket wrench, I use light force to go another 1/2 turn or more until the plug feels like the gasket is stiffening. You'll only get one hand in there, so you're using the socket wrench more like tightening the lid on a jar. You'll need to guess the force. After it stiffens, I tighten another 15 degrees to snug it down and that's all. The left hand side is nasty because now you need to switch hands. If you're a southpaw, more power to you. But by now, your back has been cantilevered over the engine for around 30 minutes or more and you will be tired. Hence, I put a pad over the engine block and sorta put my weight on it. Slide boots back on, feel them click over the plug tips, and then clip wires back on. My record is 45 minutes to change 6 plugs on a Sienna 2002. The local dealers are asking for 2hrs labour + parts. That can run $400 total. I order my Denso plugs online for around $9/ea. with shipping. Pepboys carries NGKs locally for around $7. Bruising of the forearms is just par for the course. I'm 6ft 2inch, 300 lbs and built like a line backer. If I can do it, so can you.
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