Hey everyone,

I have a 2014 350 (M276 engine), and I’m dealing with a slightly rough idle that smooths out when RPM goes above 1000. The car accelerates strongly and drives fine, except at idle.

🔹 No check engine light (CEL), No other light

Symptoms:

• Intermittent shaking at idle (shakes for 1 second, idles normally for 2 seconds, then shakes again).
• When I shift it to Drive (D) and I release the brake padel, the car starts to move forward, but with each shake, it feels like it's briefly braking.
• The shaking happens in all gears (P, R, N, D), but I feel it more in D and R at lower RPM the shake gets a bit stronger.
• Idle smooths out completely above 1500 RPM.
• with each shake, I see the RPM move down and go up slightly, it idles at around 600 - 550, and with the shake, it drops to 480 and goes back
• interior light dim slightle with each shake
• At a Cold Start, The car idles fine for the first 1 minute, but once gets warmer, the shaking happens more frequently.

What’s Been Replaced So Far:

✅ PCV valve (was leaking oil)
✅ Spark plugs
✅ Fuel pump
✅fuel filter
✅ Air filter
✅ Engine mounts & transmission mount
✅ Driveshaft mount
✅ fresh oil & filter 5w30

Recent Workshop Diagnosis:

• After replacing all the above, the car triggered a check engine light (misfire in cylinder 6).
• They replaced:
  • Cylinder 6 fuel injector
  • Cylinder 6 ignition coil

The issue remained 100% the same, with no improvement.

What Should I Check Next?

I'm really puzzled at this point and unsure what else to diagnose or try.

The workshop can't confirm any specific fault, yet they suspect a possible internal engine failure and suggest an engine rebuild or replacement—but without solid evidence.

Before going down that route, I’d really appreciate any advice or suggestions on what else to check.

Thanks in advance!

 

I would make a diagnose, with Xentry. No need to search Fault codes as there is a lot of tests regarding air leak, injectors, carbon canister, misfires, etc

 

What fuel pump was replaced?

 

ROFL! Of course they did! Why didn't they just suggest replacing the entire car?

Was the shop a Mercedes dealer or an independent?

Have you checked for a vacuum leak or done a smoke test?

Recently, I was working on my Audi TT and had to remove the hose from the turbo to the intercooler and forgot to tighten it down when I put things back together. After revving the engine a little, the car would stumble and shake like crazy at idle, but it would behave just like you say. I immediately got out and checked under the hood, and there was the hose blown off the pipe. So, the car was stumbling because the amount of air going to the engine was less than the amount measured by the MAF. Put the hose back on and tightened the clamp and all was better.

So, you may have a vacuum leak or a bad MAP or MAF, or something else going on. But you're not done yet.

 

--1-- To be your own man you're gonna need to buy a $150 scanner to read your chassis live or let shops do as they please.

--2-- You're USING THE WRONG OIL. Dump that W30 out and buy yourself any "W40 MB Approved".
The wrong oil screws up engine timings... there's a chance your issue is related to dry hot cylinders.
With W30 oil your pistons do not get cooled until near 3000.Rpm in best case scenario else 3500.R.

--3-- What's current mileage??

--4-- Plugs replaced...
How about moving coils around?
How about new boots ??

--5-- Oil in harness: have your shop check the ECU for oil swamp.
Using the wrong oil guarantees limited cooling for extreme heat. That waste the plastic cam sensors...that provide oil into harness.

--->> Keep in mind... your Cyl#6 may be terminally damaged by W30 low oil pressure.
Get it bore scoped or pressure tested.

-- Parts Canon: No one is fixing your car!
Somebody needs to TROUBLESHOOT YOUR ISSUE instead of browsing parts catalog.
🤞

 

Hope you have the old parts in a box. Likely none of them needed change, and you could use them in the future.

Do not change anything else until properly diagnosed. Installing new parts can cause more issues than fixing anything when not done properly.

Get a MB aware scanner, for the price of the coil and the sparks plugs you would have had a very good one already.

Interior light dims when shaking? Loose ground?

 

Replace cam sensors

replace cam magnets

replace ignition coils

clean ecu harness from oil

report back

 

Again, what fuel pump was replaced, the transfer pump in the tank or the REAL PUMP at the top of the engine. This is the high-pressure pump that is critical for correct fuel burning. Checking the fuel rail pressure with a cheap scanner like iCarsoft MB II or V3 or whatever would tell if there were a pressure issue.

 

Definitely sounds like a vacuum leak. A smoke test is in order.

 

I will take the car to another workshop. This shop uses an Autel scanner, not Xentry.

The fuel pump under the seats was replaced. Before, the car took around 5 seconds to start, but after the replacement, it now starts in 1 second. However, the idle shake remains the same.



It was an independent shop, not a Mercedes dealer.

I visually checked all the vacuum hoses without a smoke test and found a cracked tip on a small rubber hose with a yellow stripe at the rear of the intake. I got it fixed, but the idle shake is still the same.

1. I have an iCarsoft MB, so I can run live data, diagnostics, and clear code but I'm not that expert.
   
2. Regarding the Oil , sorry for the confusion – I’m actually using 5W40 Helix Ultra, which is MB approved.
   
3. The car has 128k miles.
   
4. I've already replaced the spark plugs and swapped the coils around. I even replaced one coil that had a stored code with a new one, but the issue is still the same. all boots seem fine as well.
   
5. As for the ECU, I’ve checked for any oil in the harness or camshaft sensors, and there are no signs of oil contamination all-around the engine

6. "The shop checked Cylinders with a borescope, and it’s in good shape. There is some carbon buildup on the valves, but nothing excessive or chunky

The shop will cover both the labor and parts costs for the fuel injectors and ignition coils, and they will return the original parts that were on the car due to the improper diagnosis. However, the costs for the PVC leak, spark plugs, engine mount, and transmission mounts are my responsibility, as I have already purchased these parts and requested the shop to replace them first.

I've checked all the engine grounds, and everything is good and tight. No issues there.

I'll check with Xentry before replacing more parts at this point.




-------------------------------------------------------------

and this is the live data I tested today
Test Conditions:

• Idle in Drive (D) while on hold brake
• Test duration: 3 minutes

Live Data Readings:

• Engine Coolant Temperature: 93°C
• Short Term Fuel Trim - Bank 1: -0.8% to -3.9%
• Long Term Fuel Trim - Bank 1: -2.3% to -3.1%
• Short Term Fuel Trim - Bank 2: 2.3% to -0.8%
• Long Term Fuel Trim - Bank 2: -12.5% to -14.6%
• Intake Manifold Absolute Pressure: 36-37 kPa
• Engine RPM: 495-560
• Fuel Rail Pressure: 14,860-14,930 kPa
• Ignition Timing Advance for Cylinder #1: 1.5° - 4.5°
• O2 Sensor B1S2: 0.355V - 0.800V
• O2 Sensor B2S2: 0.170V - 0.815V

I APPRECIATE EVERYONE'S INPUT, ADVICE, AND EFFORT IN HELPING ME DIAGNOSE THIS ISSUE.
YOUR INSIGHTS HAVE BEEN INVALUABLE, AND I’LL KEEP UPDATING AS I GO THROUGH FURTHER TESTING.
THANKS AGAIN!

 

Have a TRUE compression test and Leak Down Test been done ?

 

the Bank2 trim vs. Bank1 confirms the misfire, yes?
I understand plug is new

Exchange coil+boot with the No1: other bank side!

Then see what LTFT says at idle, closed loop.

 

Hey OP, any update on the root cause & solution?

 

Update

Based on an initial check and diagnosis from another workshop, it appears that the engine may require a valve job.
There are also a few minor scratches on the cylinder wall, although we did not perform a full teardown investigation to confirm this with 100% certainty.

Temporary Solution

To improve drivability in the meantime, I slightly increased the engine's idle RPM using Xentry.

• Previous idle speed: ~470–500 RPM
  
• Current idle speed: ~700 RPM
  

This adjustment has resulted in a much smoother idle, and the engine now runs and drives well.
A valve job typically includes:

• Resurfacing the valve seats
  
• Replacing valve seals
  
• Correcting any wear or damage that affects compression
  

 

Have you done smoke test as suggested? How about the condition of your intake seals? I suggest performing another diagnostics.