Fast Idle Air Valve bypass and/or Block-Off Plate Kits
The FIAV is attached to the lower portion of the throttle body and its function is to provide additional air to the throttle body during cold engine idling. To do this, it incorporates a valve similar to the one used in your cooling system's thermostat which opens and closes in response to temperature changes in the coolant. Herein lies one of the big drawbacks of the FIAV: it has to be plumbed into the cooling system.
The FIAV coolant feed and return hoses are sources of potential coolant leaks. Also, there's a risk (a small risk, to be sure) that coolant from the FIAV can leak into your throttle body and as you can guess, this would not be good.
These units come in two different styles:
1) FIAV Bypass plate
2) FIAV Blockoff plate
The bypass plate fits between the FIAV and the throttle body and blocks off the air passages used by the FIAV. However, this plate does NOT block off the air passages used by the ISC (the ISC is bolted to the FIAV). This allows you to retain full use of your ISC. When installing the bypass plate you can choose to re-use the stock FIAV bolts, or for your convenience, a kit is available that includes new stainless bolts and lock washers (these are especially useful since it's not uncommon for your stock bolts to be seized in place and require desperate measures to remove).
The blockoff plate completely blocks off the entire FIAV (including the air passages used by the ISC). Therefore, this plate should only be used by those who wish to remove both their ISC and FIAV. When using this plate, you can choose to keep the FIAV in place (but not functioning) or remove it completely. The blockoff plate includes a set of bolts. These are to be used if you choose to remove the FIAV completely (since the stock bolts will be too long), or if the FIAV is retained, you should re-use the stock bolts.