welcome to the gas powered cars. i had the same question on my 88 124. my car has always idled at 500rpm. with a rough idle , worse during the summer. as far as i could find out this is a normal idle on a lot of these cars. what finally fixed the roughness was motor mounts(cap,rotor and wires made a small change) and what a diferance it is. this car runs like a brand new car.
plus the parts were cheap and very easy to put in. did all three in one hour.
best money i have spent on this car
good luck
rich

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88 300E White, sold 9-04
88 190E Black 2.3 sold
90 300CE White, Grey sold 12-24-04
92 400E White,Grey Sold 5-06
90 300CE 91K Signal Red,Black Sold 2007
1989 300CE Smoke silver,Tan Sold 1-08
1993 300E 3.2 M104 motor
1991 300CE White,Grey
1994 E420 beautiful car,gone to new home
1994 E320 Smoke an Parchment.Beautiful car