Just to add a note to Jarrod's post. The idle speed control system is basically a servo controlled valve that controls the amount of idle air to assure a constant idle. However, this valve only covers a certain amount of range, so it is very important that the throttle body be set up properly so that the 'base' idle speed is within the range of control of the idle valve. To do this, grab your Robert Bently or factory manuals for the 240 or appropriate manuals for the 700/900 series and follow the brocedures under 'Base Idle Adjsustment'. Just be sure the throttle body is spotless (clean it first) before your start. This procedure will show you how to ground the idle control circuit at the test lead under the hood causing the idle valve to close and take the idle control out of the loop while you adjust the rest of the system. THis will also tell you whether the idle circuit is working or not. Note that there are a couple of temp sensors that are used to control the fuel mixture during warmup. Failure of these can affect the idle beyond the range of control of the idle valve.