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I have noticed a new symptom that I am not sure about. On a cold start from sitting overnight or several hours, my M276 3.0 liter bi-turbo engine will sound like it stumbles for 1-2 seconds. There are no misfires or other DTCs and it goes away on subsequent warm starts if the car has not been sitting for several hours. Fuel pump pressure (LPFP in tank) is about 80 psi at idle and fuel rail (HPFP) pressure is around 1740 psi at idle, a little low.
An internet search returns a leaky fuel injector could be the culprit.
Anyone have had this experience before?
Thanks
Last edited by PotatoHeadII; Jun 14, 2026 at 08:34 AM.
I have noticed a new symptom that I am not sure about. On a cold start from sitting overnight or several hours, my M276 3.0 liter bi-turbo engine will sound like it stumbles for 1-2 seconds. There are no misfires or other DTCs and it goes away on subsequent warm starts if the car has not been sitting for several hours. Fuel pump pressure (LPFP in tank) is about 80 psi at idle and fuel rail (HPFP) pressure is around 1740 psi at idle, a little low.
An internet search returns a leaky fuel injector could be the culprit.
I have noticed a new symptom that I am not sure about. On a cold start from sitting overnight or several hours, my M276 3.0 liter bi-turbo engine will sound like it stumbles for 1-2 seconds. There are no misfires or other DTCs and it goes away on subsequent warm starts if the car has not been sitting for several hours. Fuel pump pressure (LPFP in tank) is about 80 psi at idle and fuel rail (HPFP) pressure is around 1740 psi at idle, a little low.
An internet search returns a leaky fuel injector could be the culprit.
Anyone have had this experience before?
Thanks
I definitely would not rush at replacing injectors besides NO single cyl is faulted here.
Good call on linking fuel to this stumble. Surya knows the expected rail pressure on the 3.0 TT. 1740.Psi meets minimum specs. We've seen bad rail sensor... I don't think this is a case for it.
What is your current mileage... 90 to 120k.Mi ??
May be time for a set of new Lambdas based on oil consumption + 100k Mileage...
-- The one thing I'd be interested in is your Longterm Fuel Trims at warm idle LTFT: 1+2... lean??
-- Look at your live misfire counts to help pick your weak cyl !
-- How old is your ignition tune-up: done or coming ??
-- Before committing the time to measure real cyl. compressions ... pls read your 4X camshaft correction factors.
--> I would not be surprised by a lean trim forcing random cold misfires. Let's confirm LTFT's corrections first.
Many items can contribute to misfire including low voltage. A missed combustion is a missed /weak contribution. We dont want that!
Right now there is no parts necessary until tested bad. Don't rush to coils & Injectors for random cold stumbling.
Can you pls record your 30sec cold start sequence to hear fast idles.
What other steps are you considering doing ?
One thing we can say is your engine is going to have more power once it's running well.
### Clean both of your MAF + MAP sensors.
Last edited by CaliBenzDriver; Jun 14, 2026 at 01:53 PM.
Rail pressure 119 bar and in-tank fuel pump (LPFP) at 75 psi at idle (updated using Xentry). I did wait until full operating temp (>176 F) to take the measurements.
Main battery SOH 99% and SOC 100%, but that was after running the engine a while. Aux batter SOH 100% and SOC 100%. I will check the main battery again in the morning before starting the car. I do have an ultrasonic rodent repeller running all the time off of the main battery.
Rail pressure 119 bar and in-tank fuel pump (LPFP) at 75 psi at idle (updated using Xentry). I did wait until full operating temp (>176 F) to take the measurements.
Main battery SOH 99% and SOC 100%, but that was after running the engine a while. Aux batter SOH 100% and SOC 100%. I will check the main battery again in the morning before starting the car. I do have an ultrasonic rodent repeller running all the time off of the main battery.
plugs probably have about 20k to 25k on them.
will be keeping an eye on things.
"stumbles" with out misfires... how ? missing cold Idle sound clip
65kMi is an early mileage...
Lambdas should be ok.
> Preliminary answers rule out:
ok good battery voltage
ok recent ignition plugs
ok fuel pressure delivery
I will get the LTFT, but may have to wait a while. I had the batteries disconnected for a while this morning while I ran some tests on them with my battery checker. If there is a leaking injector, I should see some negative values.
I will record that startup sound tomorrow morning and post it. The “stumble” can be heard in the exhaust. The exhaust on the SL400 has always been a little throaty compared to my C350. Like I said, no DTCs and the misfire count across all cylinders is zero.
I double-checked my records and my plugs have about 18k miles on them.
I will get the LTFT, but may have to wait a while. I had the batteries disconnected for a while this morning while I ran some tests on them with my battery checker. If there is a leaking injector, I should see some negative values.
I will record that startup sound tomorrow morning and post it. The “stumble” can be heard in the exhaust. The exhaust on the SL400 has always been a little throaty compared to my C350. Like I said, no DTCs and the misfire count across all cylinders is zero.
I double-checked my records and my plugs have about 18k miles on them.
based on that...
isn't "the stumbling" in synch with the ECU ajusting the throttle lower as engine warm up ??
Right after cold start up there are many rapid Rpm transitions.
(I wouldn't even consider the single throttle plate being dirty).
Last edited by CaliBenzDriver; Jun 14, 2026 at 03:01 PM.
STFT Bank 1: 0.0%
STFT Bank 2: 0.0%
LTFT Bank 1: 3.1%
LTFT Bank 2: -0.8%
Based on these data: your engine Cyl. 1,2,3 want more fuel where 4,5,6 are satisfied.
You don't have a leaking injector nor a bad ignition.
This is consistent with unmetered air inlet.
Check your charge air inlet ducts are sealed.
camshafts positionings
Intake B1; B2: = (32.5°) ; (35.0°) .... 2.5°!!
Exhaust B; B2 = (-23.3°) ; (-22.6°).... 0.7°
Your intake cam positions are significantly apart. I'm not sure if this is phaser or chain and unlikely shifted reluctor. It's still within specs so not calling a fault while ECU is applying correction.
This 2.5° timing difference can upset your fuel trims.
Last edited by CaliBenzDriver; Jun 14, 2026 at 06:13 PM.
I made a video of my cold start by placing my phone by the exhaust, but cannot upload it. I will try to figure something out.
In the mean time, the video shows very slight puffs of smoke (or water vapor) whenever you hear the stumble. It happens three or four times. Very interesting….
I made a video of my cold start by placing my phone by the exhaust, but cannot upload it. I will try to figure something out.
In the mean time, the video shows very slight puffs of smoke (or water vapor) whenever you hear the stumble. It happens three or four times. Very interesting….
interesting.... that is consistent with pressure differences - Something's happening: is it always reproducible like clockwork or more like random, once a while ??
I was able to upload a video and I hope you can see the puffs of wispy white smoke (or vapor) about ½ second after you can hear a change in the exhaust.
I was able to upload a video and I hope you can see the puffs of wispy white smoke (or vapor) about ½ second after you can hear a change in the exhaust.
I watched and listened to your video twice.
Exhaust sounds excellent
I failed to see any smoke
Startup idle ramps down quickly... great!
Can you capture the sound from the engine itself when acting up.
Last edited by CaliBenzDriver; Jun 21, 2026 at 01:29 PM.
This is a higher resolution copy. The puff of smoke is around seconds 5, 6 and 10 seconds. You can actually see the exhaust pipe shake a little at the 10-11 second point.
This is a higher resolution copy. The puff of smoke is around seconds 5, 6 and 10 seconds. You can actually see the exhaust pipe shake a little at the 10-11 second point.
Hmmm, this might sound weird but how are the engine air filters?
This is a higher resolution copy. The puff of smoke is around seconds 5, 6 and 10 seconds. You can actually see the exhaust pipe shake a little at the 10-11 second point.
I don't see any smoke but this time I can hear hiccups during the fast idle ramping down.
Is this cold only or warm also?
Hmmm, this might sound weird but how are the engine air filters?
Filters a good and new, MAHLE, I believe.
Originally Posted by CaliBenzDriver
I don't see any smoke but this time I can hear hiccups during the fast idle ramping down.
Is this cold only or warm also?
Cold start only. The smoke or vapor is very brief and dissipates immediately.
There are no misfire codes or counts. Coolant level is good.
The puffs of smoke come out of either side exhaust. I have not watch for the same coming out of both sides at the same time. I will try that next. The SL R231 exhaust has two H style cross-over pipes, one in the front and one towards the back.That may or may not tell me if the cause is coming from one bank or the other or both. This is quite the mystery. If it continues, however, I may take the car in to my local Mercedes dealer.
Cold start only. The smoke or vapor is very brief and dissipates immediately.
There are no misfire codes or counts. Coolant level is good.
The puffs of smoke come out of either side exhaust. I have not watch for the same coming out of both sides at the same time. I will try that next. The SL R231 exhaust has two H style cross-over pipes, one in the front and one towards the back.That may or may not tell me if the cause is coming from one bank or the other or both. This is quite the mystery. If it continues, however, I may take the car in to my local Mercedes dealer.
It will be interesting to read what dealer tie that sound to... probably set you up with genuine new set of parts in the process.
Based on these data: your engine Cyl. 1,2,3 want more fuel where 4,5,6 are satisfied.
You don't have a leaking injector nor a bad ignition.
This is consistent with unmetered air inlet.
Check your charge air inlet ducts are sealed.
camshafts positionings
Intake B1; B2: = (32.5°) ; (35.0°) .... 2.5°!!
Exhaust B; B2 = (-23.3°) ; (-22.6°).... 0.7°
Your intake cam positions are significantly apart. I'm not sure if this is phaser or chain and unlikely shifted reluctor. It's still within specs so not calling a fault while ECU is applying correction.
This 2.5° timing difference can upset your fuel trims.
Based on these differences of trim and timings between banks 1+2 :
Get the dealer to check your intake phasers are ok or offset at 53°.
Last edited by CaliBenzDriver; Jun 21, 2026 at 09:01 PM.
Cold start only. The smoke or vapor is very brief and dissipates immediately.
There are no misfire codes or counts. Coolant level is good.
The puffs of smoke come out of either side exhaust. I have not watch for the same coming out of both sides at the same time. I will try that next. The SL R231 exhaust has two H style cross-over pipes, one in the front and one towards the back.That may or may not tell me if the cause is coming from one bank or the other or both. This is quite the mystery. If it continues, however, I may take the car in to my local Mercedes dealer.
I see.... Reason I mentioned it because I have stutter issues when I swap the engine air filter before and it slowly got better, it instantly got better if I put the old one back in, I remember certain MBs have a reset option in XENTRY to reset the values after changing engine air filters but that was never mentioned in WIS I don't think? Either way, it should auto adapt itself.
Ya no worries, it was a few years ago and I never had an issue changing filters since. A few years ago I first changed the filters for the first time and it was dirty as expected, construction near my area also contributed to it, when the behaviour started, I tried using oem filters bought from the dealership, it says it was made by MAHLE or Hengst. I then tried a pair of Hengst branded filters then I went to MAHLE, etc. I forgot the sequence but it eventually worked out, I wonder if I messed up the sensors near the airbox or something.
Before starting the car today, I did open the air boxes and checked the filters. I also vacuumed the filters (which were virtually new) and any debris on the pre-filter side of the air box. I did notice that the seal on the driver side filter was twisted a little bit and I fixed that. In the process, I disconnected and reconnected what I believe are the intake air temperature sensors. Started up with no detectable “hiccups”. Also, I had the ignition on for probably a minute or so before starting the engine to detect any misfires (fault counts) — there were none. The verification will be tomorrow.
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I will try to figure something out.
