I've got a 2017 C63S AMG with 38k miles that I bought in 2018 from the dealership. About a year and a half ago I bought FI catless downpipes and DME stage 2 tune. When i got the downpipes I installed them right away but didn't install the tune until about a month ago. Car ran great with the downpipes and ran great when I put the tune on. I sold the car to my wife's cousin (I know rookie mistake) and he brought it back to me and said its not running right. He had removed the mufflers and then drove the car 1200 miles in 6 days. The car started to idle weirdly so he took the tune off and there was a misfire on cylinder 4 but then when I took it to the mechanic they weren't finding any misfires. Essentially what is happening is that in park or neutral the car will rev up and down a bit. In gear though it runs just fine. My mechanic wasn't able to find any misfire even with the tune but they checked the fuel and said that it had this sweet smell to it and that the color was off and the fuel looks like a mixed fuel. When i got the car back it was almost empty so I added 91. Compression on cylinder's 3,4, and 6 were between 130 - 150 psi. My wife's cousin's mechanic heard we had a tune and just said that ohh the tune has put holes in your cylinder and didn't do any further testing and told him it would be 25k to rebuild the engine. Could this idle issue be due to bad fuel?

I guess my question is has anyone else experienced rpm going up and down when in park or neutral? Has anyone had any similar problems to this. I'm getting desperate now as we're having a hard time pin pointing the issue.

All possible issues I've heard from the mechanics:
- Hole in cyclinder assumed due to stage 2 tune that I had on the car for only a month. No leak down test or borescope to confirm this.
- Removal of muffler might be an issue since there is no back pressure anymore? Idk if that actually makes sense or they were just trying to give me a diagnosis for anything to call it a day
- Bad spark plugs/coil packs but scanner is not picking up a misfire anymore
- Bad/mixed fuel - Mechanic said it looks mixed. It has a sweet smell to it and the color doesn't like consistent with 91

 

I don't know where you are located, but I'd find new mechanic.

As for the rough idle, is there a check engine light? If not, you can start by just pulling and checking the spark plugs. I had an intermittent issue with a rough idle a while back and was not getting any CEL. I pulled the plugs and found one to be bad. Replaced all 8 and the problem never returned.

 

Please post the mechanic’s photos indicating mixed fuel. Thanks.

 

Stages 2 tuned are notorious for fouling the spark plugs. Start there by changing the plugs.

 

LMAO, get a real mechanic!!!


Plugs
Coils
Leaking injectors!!!!

 

Get an hptuners and datalog it.

 

There was for a misfire on cylinder 4 but that went away and didn't come back. I have a new mechanic and they are taking a look at the car. I told them I want new spark plugs at the very least so hopefully that resolved the issue.

 

Here is a picture of the fuel. On the left is what they pulled out of the car and on the right is fresh 91 from the gas station.

 

There was for a misfire on cylinder 4 but that went away and didn't come back. I have a new mechanic and they are taking a look at the car. I told them I want new spark plugs at the very least so hopefully that resolved the issue.

 

Thanks. I don't see anything concerning. Slight color difference, but does anyone know for certain why? Fuel is not the cause of the rough idle.

You have an early build M177 4.0TT and this engine is known for misfires which can lead to rough idle. Search this site and you will find that coils and software (coding) are the dealer field fixes.

On a car this new, sensors should be assumed OK (cam, crank, cam solenoids, O2, etc.). I would put new spark plugs and ignition coils in. If the problem persists a software fix from the dealer would be the next course of action. Need to find a campaign number or TSB. I think the W213 AMG guys know about this.

Here is one to read: https://static.nhtsa.gov/odi/tsbs/20...58383-9999.pdf

- intake manifold leaks
- cracked intake manifold
- faulty ignition coils
- faulty chassis grounds

 

Hey so a couple days ago my car had an obvious misfire - jerky acceleration and rough idle followed by a check engine light.

parked it for a couple days and on starting there was mo CEL but still took it to my mechanic however he says they cant find any stored codes.

i thought misfire related codes get stored but is that not the case?

my car is at 90k kms and I had the spark plugs replaced 10k kms ago so i think the issue might be ignition coils but dont want to start throwing money at parts without confirmation

 

u may want to gap and index spark plugs. with ignition coils you may see if they are bad visually

 

I installed new coil packs and spark plugs and a month later my car is starting to make a weird noise and it has like a pulse/ hard shake that lasts about a second multiple times