Rawliii

Since 2-3 weeks now I've been experiencing rough idles on start up with the car. Normally on a cold start, the car would jump to around 1200-1300 r.p.m. and then slowly come down.Now the r.p.m. kinda fluctuates and jumps giving a rough idle on start. It would stop after about 30-40 secs or so.

My mother and I(Since it's her car, I just drive it) have been to the dealer and they kept it overnight to reproduce the problem.They were able to reproduce it and they said it was okay and they test drove the car and they said the car works perfectly.

I cannot take that this is 'normal' for an answer.The car is well taken care of and goes into dealer for scheduled services when they are due. Car is 1 and 1/2 years with about 17.4k miles (We do use it quite a bit) and works perfectly except for this issue.
All feedback is greatly appreciated

MDMercedesGuy

On occasion I will notice that my C250 runs a it rough on a cold start - I'm not sure why because it doesn't seem weather related.

I've noticed that this car seems more sensitive to fuel than other cars I have had - on occasion it will drink through a tank in 200 miles. I almost wonder if the roughness is related to fuel?

Rawliii

I think fuel is a big factor, the dealers even recommended we take gas from ONLY ONE station.

Glyn M Ruck

The M271 has never been the smoothest engine at initial full open loop (fully enriched) cold start. Some of this is normal & I don't know whether this tendency has carried over to the EVO turbo models. I don't have much experience of them yet.

Things that can make matters worse are bad temperature info to the ECU from the temp sensor & throttle body actuators that oscillate slightly. The only fix for either is replacement.

I have always felt with the 4 cylinder models that they should remove parasitic load from the engine while cold & until stable ~ in particular the alternator after cold cranking.

I would only really worry about it if it persists at closed loop hot start.

Rawliii

Okay thanks Glyn! Never does it on hot starts, just cold starts on mornings. Maybe not a big issue, thanks again everyone!

michail71

I've been experiencing this a bit more frequently now on warm starts. Colds starts are fine.

I had torque hooked up on one of the starts and I could see about a 40 RPM variation. It's enough to make the car shake. It reminds me of starting up a 1970s era car that wants to stall.

michail71

I caught it again with torque monitoring. It was rough on a warm start. The temp gauge was still in the normal operating range after the car had been sitting an hour and the idle speed was normal.

While the fuel status was "Open loop (low temp)" the car felt like it was about 30 to 40 years old on start. When it went into "Closed loop using O2" it smoothed out.

yazankinany