misfire, rough idle disappears when engine is turned off and back on
 

Hi - new poster but long time reader. Here is my issue with a relatively new to me w208 clk55 m113. Car has the following codes and CEL is on:
P0400 EGR
P0170 fuel trim
P0173 fuel trim
P0100 MAF circuit
and the following issue/s:

- rough/fluctuating idle, engine sometimes dies when coasting to a stop
- occasional misfire, CEL light blinks and car runs rough, no power, smells of unburned fuel
- also when putting tranny in park after a drive the rpms rise to 1200-1500

the misfire occurs maybe on 3 out of 10 drives, if I stop, turn engine off and immediately back on the misfire disappears and the car runs perfectly fine

the rough idle is also somewhat intermittent - it's never "glass smooth" like I find Mercedes V8s normally to be but it's reasonably stable when it's not acting up

the car has new plugs, wires and MAF including elbow seals, I replaced the upper breather hoses and cleaned the inside of the valve covers.
The vacuum lines that I can see look ok and since the issue disappears after an engine restart I am thinking my problem is electrical and not vacuum?

Any ideas where to look next ?

thx much All

2002 CLK 55 w208

Try to find a more detailed description of the codes, this will offer more clues.

RE EGR what type of failure ?

Fuel trim, long or short term ? Lean or rich limit?

MAF, high or low signal output ? shorted to ground or voltage? Open circuit?

I'd hang a volt meter or scope on the MAF output and watch it for an out of the ordinary voltage.

A hidden gem. On my 97 SL320 and 97 C280 With a M104 inline 6 and Bosch ME 2.0 / 2.1 compuer, if the computer detects a miss fire, it shuts fuel off to the miss firing cylinder. Your car might do the same. This slightly complicates diagnosis as an ignition fault will look like a fuel injector driver fault.

This is exactly what also happens on M112 and 113 - the misfiring cylinder will be shut down in both fuel and spark to prevent cat converter damage.

to OP.

have you checked the spark plugs - my old W210 used to do this when I bought it, the plugs were bad, once I replaced all 12 it never misfired again.

The first 3 codes can be caused by a variety of things as has been suggested. The P0100 is a little odd (I've never heard of it). Did you replace the MAF yourself? Is it OEM? There are a lot of cra* MAF's being sold for cheap. A lot of cheap parts work perfectly fine, but often not cheap sensors of any kind.