house67

Hi everyone,

I have an issue with my W204, M272 engine that occurs only at startup. Sometimes while starting it feels like one of the cylinders isn't firing and the car is "hiccuping". This clears up after about 20 seconds and the car is perfectly fine thereafter. Some specifics:

- It only happens when the car sits for a few hours and the coolant temparture is between 40-60 deg. It is fine in the morning.
- When starting, the RPMs will start high as if its a cold start, but then quickly drop down to normal idle rpm which is when the hiccuping starts.
- While is it hiccuping you can hear and feel the secondary air pump going off like a jet. After about 20 seconds it changes to a more normal tone and right after it turns off it is fine.

No codes, no CEL. 88K km/55K miles. Plugs are relatively new. Battery is very old but don't think its related since if it was, it would be worse in the morning. Any ideas? Thanks!

sege

I have the same sort of issue only at idle and upon first start in morning drives fine but misses at idle... Got new plugs at last service but still there.

Ronin_Dax

I have the same issue but the M271 engine. Never been able to figure it out. Due to other issues the Plugs, Battery and timing components have all been replaced but it still feels like it misses on cold starts.

house67

Thanks for the responses. I think it could be a leaking fuel injector which fouls one of the spark plugs, so that cylinder doesn't fire on startup.

Jaimemb

My rough idle started after i replaced the high pressure fuel pump. I'm thinking it could be a vacuum leak coming from the line that connects to the purge valve as the rubber elbow on it ripped. I replaced plugs and packs, valve cover gasket, and the intake gasket.Other than that still trying to rule out the fuel injectors and intake timing sprocket being too advance in time.

house67

If it's a consant rough idle with no CEL it's probably engine mounts

Jaimemb

Not constant only for 45 seconds then it'll rev up once like it burned off something then smooth out. The CEL is on with codes for misfires in all cylinders and random but not the p0340 code just misfires.

sege

I am wondering if it is an intentional EMS thing at start up when cold to advance/retard timing to get engine to operating temperature quicker.

FutureAmgOwner

Did you ever get this fixed?