The "deck plate mod" as it's known is one of the simplest and cost effective modification that can be made to the 3.4L V6 motor. This modification allows the motor to breath easier and adds between 5-8 RWHP. I've personally dyno tested this mod and know for a fact it works.
For this modification you'll need:
4" deck plate (available at any boat shop)
Ultra Black sensor safe silicone
(6) stainless steel 3/4" screws
Metric sockets and a socket wrench
Phillips head screwdriver
Start by removing the factory air box. Put a plastic bag inside it to catch the debris from the cut you're going to make. Then use a 4" hole saw to cut a hole in the front of the box. Be sure to center it about an inch to the right side to give enough room to remove and replace the deck plate. Use some 220 grit sand paper to clean up the hole and MAKE SURE you get all the loose pieces of debris. Remove the plastic bag from the air box being careful not to dump out the debris it contains. Then THOROUGHLY vacuum the air box out. I can not stress the vacuuming step enough as any small plastic pieces you leave in the air box have a good likelihood of being sucked into the motor. I personally vacuumed mine 3 times to be sure, tapping it on a table on each side to make sure I got any hiding pieces.
Once the box is all cleaned out, apply a 1/4" bead of the sensor safe silicone to the deck plate and insert it into the box following up with the screws. There is no need to drill pilot holes for the screws, just let them make their own holes for a tighter seal. Let the silicone dry over night before putting the air box back in the vehicle to insure the fumes from the silicone do not cause problems to the MAF sensor.
I dyno tested this modification on my 1999 3.4L V6 Toyota Tacoma 5 speed 4x4 with Flow master 40 muffler. Below is the result from the dyno run with the deck plate closed vs. open. I also tested an Amsoil and K&N filter as well. With the Amsoil and K&N filters installed and the deck plate closed the result was about equal to having the deck plate opened with the stock filter. With the deck plate opened and the Amsoil or K&N installed there was only an additional 1-2 HP gained. Note that the majority of the power is gained between 2000 and 4000 RPM which is where power counts.
Once this modification is complete you can run the car without the deck plate while on the highway for maximum performance, and close it up if you go off road to keep water or dirt out. Be aware, that you WILL hear more intake noise with this modification, but most find the more powerful sound pleasant. Combine this modification with the Intake Silencer Removal for optimal intake performance.