My 1989 420 has started running rough once it warms up for 4-5 minutes. Generally, when I first crank it up it is smooth, both at idle and when I accelerate. After it gets warm, the idle gets real rough, and it sputters when first starting to accelerate. Once you get 5-7 MPH it seems to smooth out and acceleration is fine. I noticed if I run it on the interstate and then get back in 'stop 'n go' traffic, it does not have the sputtering problem - it accelerates fine from a standing start. The plugs appear fine, no apparent hoses loose under the air filter shroud, the motor mounts were replaced not to long ago so they should be ok. Any thoughts would be appreciated. Thanks.

What are your RPM at idle once warm up?
When you say rough idle, do you mean it shakes the engine or RPM goes up and down.

There could be numorous reasons for rough idle at warm up, coolant temp. sensor, Oxygen sensor, distributor rotor & cap to name few.
I will start with good cleaning the ICV, it is known to cause all kind of issues with idle.
What is the history of the car, how many miles? regularly maintained ?
Did you tried "Fuel injection cleaner" of any kind, that might help.

Hope it helps

__________________
87 420 SEL
Looking for a nice mid 80's MB diesel.
87 BMW 528e (Sold)
99 TOYOTA 4Runner
1989 Honda Accord (Sold)
1988 Honda Accord (Total)

Rough idle - the RPM's are constant, engine basically shakes. RPM's are around 750-800. It's got 93k miles, and has always been maintained by a competent mechanic. Have used fuel system cleaners, but no impact on performance.

The RPM are correct and if they do not flactuate up and down, it seems like you do not have rough idle problem rather have an engine shake at low RPM. If your motor mounts are not bad, check your engine shocks, Front flex disc, Tranny mount, and finally differential mounts. All of these effect the engine shake at idle.

Keep us posted.
Thanks

__________________
87 420 SEL
Looking for a nice mid 80's MB diesel.
87 BMW 528e (Sold)
99 TOYOTA 4Runner
1989 Honda Accord (Sold)
1988 Honda Accord (Total)