This is an observation, but maybe some of you encountered the same symptoms. I have a 94 S500, but lately I've noticed, if I don't let the engine warms up for a good 10 minutes, a rough idle will occur during driving and the throtle does not respond properly, however if I let the engine run and the temperature is at the 70 to 80 C, it runs like new. It's been a cold winter in the Northeast, is there any direct relation with the rough idle and the cold engine? I don't have an engine block warmer, wish I did.

Neil

 

You shouldn't need to warm up a '94 car for 10 minutes. It should start and go.

It sounds like your throttle body (TB) may be icing up when you turn your car off. Then the TB blade is covered with ice particles when you start it cold. An iced up TB blade will cause a rough idle and all kinds of goofy non-responsive throttle pedal issues.

I'm not sure about your model and year car. Does the TB have a small coolant hose connected to it? If not, can you tell if was bypassed? Racing freaks bypass this connect to gain 3-4 hp.

Do MBs maybe have an electrically warmed TB that may be defective on yours?

 

When you start it, it runs wells, however, 5 minutes later that is when the symptons shows, but if I let it run for 10 minutes, the rough idle is not an issue. I had the spraks plugs and air filter changed recently, I also changed the pollution pump and added a brand new one. The ignitions wires were also changed, I'm not sure of any direct connection as you mentioned, how do I look for it?

Neil

 

It's something else. You initially said the car starts with a rough idle when it's extremely cold outside. Now, you're saying it starts with a smooth idle, then the idle gets rough, then gets smooth again after 10 minutes.

 

Put it this way, it runs fine during a cold start, nevertheless, if you don't let the engine warms up for 10 minutes and I attempt to take it for a drive, the rough idle will occur while driving. but If it runs for 10 minutes and you take it for a drive, you will never know that the rough idle is an issue, but this only happen when the weather is extremely cold.

 

Pull off your dist. caps and check for signs of condensation, usually a greenish flake, you probably need caps and rotors.

 

I have a 2000 S430. Recently, when the car is cold, it is very hard to start (it starts, but dies right away). After starting it10-15 times, it stays running, but idles very rough. If you attempt to drive the car, it runs no faster that 20 MPH. all of these problems clear up when the temprature guage reaches 80 degrees. Any idea what could be wrong? I appreciate any help.