My wife's 2003 E320 has been idling rough at times. On startup or cold it idles rough then warms up and runs smooth. Every now and then, during a stop the engine idles rough again. It's got 98K miles.


Could it be the MAF sensor?


I've never cleaned one before and I'm always leary to do something for the first time on her car.

MAF sensor on your car has self-cleaning cycle, every time you turn off the engine, so it's not necessary to clean it with MAF cleaner as a preventive maintenance. OTOH if it is out of specs cleaning it rarely does anything, you could try that as a last resort before replacing but, it never worked for me on M112 or M113 engines.

You really need to determine if it is MAF problem in the first place. Is your CEL on, any fault codes? Do you have scan tool capable of reading Air Mass values, idle speed and partial load self-adaptations?

No Malfunctions on dash display

I don't think it's MAF, not yet anyway. I my experience with E320 and SL500 when MAF needs attention there would be fuel trim fault codes and possibly misfire code in addition. Do you have any issues under load? Like 40 mph and put accelerator to the floor… any hesitation, stifling?

If the issues are only experienced at idle I would start looking at possible vacuum/ air leaks. Visual inspection first to see if there is something obvious. If not then smoke test.

Has it ever had new plugs? As was said, a scanner that can read the fuel trims could provide some insight. Those can be had fairly cheap these days.

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra

Check the breather vents from the valve covers.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)