Skip the auto shop - Our mechanics make house calls

Q: Rough slow before 40

asked by on

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?

A: Thanks for writing in about your 2000 Merce...

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.

Was this answer helpful?

Need advice from certified mechanic? Get help now!

Over 1000 mechanics are ready to answer your question.
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

Ask a Mechanic
(100% Free)

Have a car question? Get free advice from our top-rated mechanics.

Ask A Mechanic
Over 10,000 questions answered!

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Save up to 30%

Get a quote

What others are asking

Q: How do I fix smoke coming out my engine

Hi there. The cooling system should be inspected in order to determine why the engine is overheating. There are simple possibilities, such as low coolant, a bad engine cooling thermostat, a partially plugged radiator, and a non-functioning electric cooling fan,...

Q: Bad ball joints

Hi There, There is a possibility if the upper and lower ball joints are bad for the suspension to not old the wheel spindle in place. You would need to have the upper, lower and tie rod end to let...

Q: Scan vehicle Code P0430 Cataylic system efficiency below threshold bank 2

The code is meant to indicate your catalytic converter is not performing up to par with the EPA (environmental protection agency) standards. Your car uses the oxygen sensors to monitor the catalytic converter (cat) efficiency. There is an O2 sensor...

Related articles

What Causes Hoses to Leak?
While the largest part of your engine is mechanical, hydraulics plays a significant role. You’ll find fluids at work in a number of different areas. Your car's fluids include: Engine oil Transmission...
How Long Does a Distributor O Ring Last?
The distributor is part of the ignition system in your vehicle and its purpose is to route high voltage from the ignition coil to the spark plug. The spark plug then...
How to Renew Your Car Registration in Oklahoma(DELETED)
Having your car registered with the Oklahoma Tax Commission is an important part of driving legally in this state. You will need to do this within 30 days of moving to Oklahoma...