[NOS2Go4Me]

You've got two things on the SPI that will specifically affect engine idle... the Idle Air Bypass on the throttle butterfly plate and the Intake Manifold Runner Control motor and butterflies. This is in addition to the tried-and-true "other stuff", such as vacuum and other more serious matters. If checking the stuff below results in nothing being found amiss, consult a mechanic.

There's no idle motor on a SPI. *goes hunting for a pic of a stock SPI TB* It has idle air bypass grooves, the same as the Zetec and SVT... to the best of my knowledge. The throttle body is cable-actuated, the same as any other 1st-gen Focus motor.

You might want to remove the worm gear hose clamp that secures the stock air intake pipe to the airbox and to the throttle body. From there, on a bright day preferably, inspect the TB itself to make sure there's no debris or buildup around the edges of the TB butterfly. Using a DAMP cloth (with water only to start... and I reiterate DAMP), wipe the interior of the TB inlet down and gently along the throttle plate.

Also, to check if the IMRC is functioning correctly, rev the engine to 3200 RPMs. If the IMRC motor and actuator don't engage (and you can visibly see this standing in front of the car with the hood popped, looking at the actuator as it sits behind the distributor), you might have a burnt out relay, motor, or cracked wiring.

EDIT - I'll also note that if the IMRC is stuck "open", as it was when I modded the car last fall for expressly that purpose, the car will stumble at idle and hesitate off the line. The reason for this is that the idle air grooves on the TB plate are designed to let enough air through to run the engine - but only feeding half the intake runners in doing so. When you're funnelling air through double the intake runners in the intake manifold, the airflow is "broken" and not enough to keep the engine from surging and bucking a tad... because the required air is taking double the paths to get there, and you're filling double the intake manifold volume with half the air required. Make sure you check the operation of the IMRC - much easier with a friend and a tachometer.