I do not know if a "perfect" idle is possible in any 14 year old car such as yours, but here's a thread that offers a 50rpm increase that may be of value. It may or may not apply to your vehicle. Look at the words presented by stevebfl:
'87 300E Stalling
1991 300-SEL - Model 126
M103 - SOHC
James Madison - Oppressors can tyrannize only when they achieve a standing army, an enslaved press, and a disarmed populace.
Liberty Firearms Training - After years of trying to regulate California criminals with a never ending stream of gun control laws they have still not given up with the belief that if they make something a crime that criminals will miraculously start obeying the law.