Back again asking for help and any information that could guide me away from the main dealer and/or scrap yard! Thanks in advance!

Quick history.. had timing chain, cam gears etc replaced in January this year. Still get a very short rattle/grind from cold start but car was running fine for a few months and now I have a rough idle/engine hunting. Sometimes it goes away completely when the car has fully warmed up, basically after about 20-30 minutes of driving.

Rpm DROPS DOWN then back up, car shakes when it does this. Happens randomly but regular but theres no rhythm to it.

Rpm bouncing is causing the car to jerk down the road if I'm driving slowly or crawling in traffic etc.

I have a guy who has an m271 in his shop so I've had access to some spare parts, here's what's been done in the last couple of months.

Brand new mercedes spark plugs very recently. Air filter checked and still rough when removed. 2 clean mercedes mafs installed. New coolant temperature sensor. 2nd set of injectors and a fuel rail from another m271 have been installed last week(this actually seemed to help it run a bit smoother for a week and car was starting first crank when warm!!) New(Bosch remanufactured) alternator becuase pulley was seized. Battery is less than 12 months old. Idle and tensioner pulleys are new. Power steering pump is new. Belt is new.

Not to mention my mercedes guy has had it on star and drove it around town numerous times and says he can't find what's wrong?! Even reset throttle/adaptations with star. Also had it smoked tested for vacuum leaks and tested myself with a smoke tester with no leaks.

Any ideas I can try are appreciated before I either go pay through the nose to have it looked at or send it to the scrap yard.

Thanks!

 

Some screenshots of live data. Negative short term fuel trim pretty much all the time. Closed loop with feedback fault??

 

When my w203 had this issue, it was caused by a cam sensor. So i would recommend trying a new cam sensor, and then possibly the crank shaft sensor.
Either of these items can cause your symptoms.. but there can be any number of causes!
Good luck!

 

Does it stop if the a/c is turned on, assuming your a/c works?

 

ac doesn't work as the condenser is leaking so it cant be gassed up. turning the ac on doesn't change anything though. im going to take the belt off when i get time to see if its something belt driven.

 

is this normal? took the air filter out to figure out where this noise is coming from. sounds like the supercharger as its directly underneath the airbox.

 

Blower sounds ok to my untrained ear. Ah, the sound of angry bees in the engine. An aphrodisiac to any car enthusiast and what so many yearn for.

Where I was going with the a/c inquiry was just a hunch. Does the 271 have an idle stabilizer? Seems like a vacuum leak issue.

https://mbworld.org/forums/c-class-w...-fix-free.html

 

i have the same problem, and the jerking stops when i turn ac off, i drive c200 diesel

 

i get the a/c thing, but its never had gas in the ac since i got the car so i never use it, never even switched it on.

i could give that tb strip and clean a go but id have to be careful so i dont have to buy another haha.

been smoke tested for vacuum leaks by a mechanic and by myself!

the problem seems to be less obvious now the weather and temperatures have dropped! its been pretty cold in the uk recently. it is quite obvious when im in traffic moving slowly around 10mph as you can feel the car accelerate and slow down repeatedly even though im steady on the pedal. like its got a mind of its own.

 

after all this time.. it turns out it DOES stop the rpm bounce when the AC is on! not long had my ac working. original thread issue still stands. figured out if i turn the ac on with blowers on it smooths the rpm out but the engine vibrates like mad.

what was your hunch? if you're still active!

thanks.

 

over one year later and i still have this problem. sometimes the car runs fine and seems to smooth out when hot.

basically when i slow down and stop the rpm drops so harshly to under 500 rpm the whole car jerks and the rpm bounces back up, over and over again. feels like someone is slamming into the back of my car or feels like the engine is trying to do a backflip. sometimes i have to turn the engine off whilst in traffic. if i put it in neutral or park it isn't as bad but you can still see the rpm twitching. seems to run ok for the first few minutes of driving and then just i basically cant stop or it starts going mental.

car has been with my local MB Indy who is ex mb mechanic, he has driven the car and said he couldn't find anything wrong with it. m271 engine has had new timing chain, gears, tensioner etc. new engine and trans mounts. was running ok after all the engine work for about 6 months.

 

update. it was the throttle body! it was fully closing shut every second over and over again.

 

you have a faulty ground somewhere.