My idle stays up until I am not moving; why?

The idle in a 5-speed SHO will stay up while the vehicle is in motion while the transmission is disengaged, dropping down to normal only after the car comes to a complete stop. This has confused some SHO owners and caused them to suspect a malfunction when in fact this behavior is perfectly normal.

The reason for this behavior is that the Vehicle Speed Sensor (VSS) on the differential is telling the EEC that the car is still moving, but the computer doesn't know (and has no way of telling) that the transmission is in neutral. This is part of the throttle dashpot software strategy that improves driveability.

As you're driving along, with the throttle partly or fully open, the Idle Speed Control valve (ISC) is also 100% open, in anticipation of your closing the throttle. As EEC sees the throttle plate close via the Throttle Position Sensor (TPS), it starts to modulate the ISC to bring the engine smoothly back down to idle without stalling. If the engine dropped to idle too quickly, especially in a lower gear, the car would buck due to excess engine braking. This strategy also reduces the pulse of unburned hydrocarbons that occurs as the throttle plate closes and the mixture goes temporarily rich.

Always make sure to reset your idle after disconnecting the battery, as otherwise stalling and surging can occur.