I have noticed for the past year or so, idle speed in my 1989 300TE drop by like 150rpm when I shift the gear from neutral to drive. It drops down to 550rpm and in neutral it is at about 700rpm. The car does not die but in South Texas especially in the summer months, it is quite hard on the alternator and battery if I am stuck in traffic where the car idles at low speed (550rpm) and with so much load like A/C, radio etc.
I remember my idle speed never dropped before. My 300CE des not drop in idle speed when I change gears.
Any suggestions what I need to do.
Thanks

May be a place to start:


89 300 TE hard start


‘88 300TE: Rough idle, low idle speed

I am having a peculiar stalling problem with my 1988 300E?