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Discussion Starter #1
The car runs well most of the time, but every now and then when sitting at a light the engine RPM drops for a moment and it shudders before picking back up to normal. As soon as the pedal is moved off idle the engine runs properly.

The car has 130,000 miles.

In the last year I have replaced all the vacuum hoses, replaced the intake manifold (the vacuum tubing was deteriorated), replaced the brake booster line, taken apart the throttle actuator and cleaned the carbon element and wiper, replaced the accelerator pedal as its wiper cannot be accessed, replaced the spark plugs, replaced the fuel injectors, replaced the fuel filter, replaced the AC dryer (unrelated but time for it). The plug wires were replaced within the last 5000 miles. All this was done for normal maintenance and the occurrences of the rough idle have decreased, but not completely eliminated.

I have not replaced the MAF sensor nor the EGR components nor plug coils. The alternator hold 13.9 volts with almost no variation.

Any suggestions?

Thanks
 

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Indeed I would go for the MAF an clean the EGR tube. Do you have a check engine light? Even if you don't, there may be codes so if you can get it read you may find further clues and other things that may need your attention. Also you may want to confirm you don't have a degrading catalytic converter in combination.


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Discussion Starter #3
There are no codes. The MAF is inexpensive, that is easy. I'll look at the EGR components. How does the catalytic converter indicate it is failing?

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Did you check the throttle plate to see if it's gunked up?

These are electrically operated, so never manually open by hand. With the key in the on position, have a friend press the gas pedal all the way down and take a look with a light.
 

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Discussion Starter #5
I had cleaned the throttle assembly last year when I replaced all the rubber.

Update on the idling.

I changed the MAF sensor and I am going to say that the problem is now 99.9% fixed.

With all the maintenance I listed above the most significant items were cleaning the throttle actuator wiper and replacing the MAF sensor.

I will change the EGR assembly this winter when it is a bit cooler. 110 degrees yesterday here in Vegas.

Before my wife's 2003 C320 I had only owned Japanese cars and have been very successful at keeping them going for over 300,000 miles. My overall assessment of working this problem over the last year and a half is the Mercedes engineering, which is quite good, requires every item in the system to be working very close to optimum to deliver the designed in performance, whereas Japanese cars are much more tolerant of degradation with time of all the ancillary items on the engine. Since her car is 2003, even though it has been well taken care throughout its life, after 16 years the various items degraded more than Mercedes allows.
 
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