This mod will allow you to adjust the air/fuel mixture of your 4AFE engine as you drive.
Locate the water temperature sender, it is a green plug located on the water neck at the rear of the engine (end closest to the air box). Disconnect the plug and cut the plastic shroud around the wires. On most 4AFE engines, there will be a brown wire and a white wire, the wire you want to tap into is the white one.
There is not a lot of wire exposed, so it is very important to be very careful when cutting and splicing the wires. It is possible, and also preferable (but not easier) to cut into the wire at the ECU.
Get your 0-5,000 ohm linear (not logarithmic) resistor (aka potentiometer) with a 1.25 watt capacity and wire it in series with the water temperature sensor. The resistor can be purchased at most electronics stores, I got mine from Jaycar for $2.95.
When wiring the resistor, there will be three prongs coming out of the resistor body. Attach one of the wires to the center prong, and the other wire to one of the side prongs (either side is fine).
You can mount the resistor in the engine bay, or in your dash. By mounting it in the dash (preferable) you can adjust the mixture as you drive. Use some nice thick wire if wiring into the dash. You can mark some sort of pointer on the resistor knob to show the optimum setting.
To determine the optimum position, adjust the resistor until RPM increases around 200rpm (may differ between engines), or you can use an air/fuel mixture meter to properly find the optimum point. If the "check engine" light comes on or the engine splutters or smokes, the mixture is too rich, just turn it back a little.