Q: Rough slow before 40

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My ML430 drives very rough until it get’s up to about 40 mph to 45 mph. It drives alot better but still doesn't have the power that it should. I replaced the catalytic converters and fuel pump, but it doesn’t improve the car's performance. The new converters are causing sulfur like smell, too. The car was driving great for a while, but then it went back to its rough, slow self. Do you have any ideas on this?

Thanks for writing in about your 2000 Mercedes Benz ML430. The fault codes need to be read out of the engine computer. It sounds like there could be a mixture issue causing the rough running and power loss. The MAF sensor helps the engine computer adjust the base timing for the fuel injectors while the oxygen sensors adjust the base timing by examining the amount of oxygen and fuel in the exhaust.

If either of these sensors are reading incorrectly, then rough running may result. Either of these sensors can become contaminated or degrade over time. Another thing to look for is vacuum leaks. Unmetered air can also cause lean misfire conditions (rough running and power loss) due to too much air in the combustion chamber compared to fuel.

The fault codes may be able to help the direction of diagnosis. I recommend enlisting the help of a certified mechanic, such as one from YourMechanic, who will have the equipment and expertise to diagnose why the vehicle is running rough and let you know what repairs are needed.

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