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Discussion Starter #1
Several recent issues with my 1999 SLK 230 include (to me) excessive gasoline usage--about 20 MPG on Super premium, a blackening of the exhaust pipe and a fast idle--about 1K rpm after warmed up. Any place to start looking for the source of these issues before I take it to my local $ervice $hop.
 

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Black exhaust and low mileage sounds like a rich fuel mixture.

Start with the easy stuff. Air filters clean? Intake tubes unrestricted? Fuel filter clean and undamaged? Fuel filter change recently? Defective filter? A bottle of Techron can't hurt. But that will generally increase the amount of fuel and the efficient distribution within the cylinder. That doesn't sound like your problem.

Have you had the car performance tuned on or off a dyno? Generally, a tuner will be leaning out the air fuel ratio not enriching it but they could have screwed up the fuel maps.

Your car is 14 years old. Plugs worn or fouled? Pull them and check the electrodes. Gaps still OK? Any oil around the electrode? Excessive carbon buildup? How about your plug wires? Any cracks in the insulation?

Emission systems are another place to check on an older car but I'd expect a code from the ECU if a sensor goes bad or the exhaust gases are outside the normal operating range.

Fast idle could be the throttle position sensor (TPS). Not sure if your 1999 SLK230 is drive by wire or a mechanical throttle linkage. If it's mechanical, it may be a simple adjustment problem.

At any rate, with the age of the car it may be time for a complete engine checkup and tune including gaskets and seals.
 

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Discussion Starter #5
Just got the car back from my favorite independent shop. Here is what they did: Checked all codes for faults, replaced plugs and wires, replaced air and fuel filters, cleaned the IAC, the Mass Air Sensor, and throttle body, flushed the injectors, reset everything to spec.

Car runs like a champ. All it takes is parts and money. They also replaced a faulty windshield washer pump and put new wiper blades.

I think I now have a new 14 year old car.
 

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Glad you got it sorted!
 
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