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I recently acquired a 2012 S350 Bluetec and just noticed this "knock" (toc toc toc), it sounds like it comes from the back of the engine, not necessarily more at the bottom. is that normal? Is this a rod starting to go?
this one sounds like its coming from the top of the heads, maybe lifters.
mine sounds like a lower frequency "tec" sound, doesnt come from the top, sounds like coming from the back/center of engine
Yours does sound a little deeper/louder, but I don't really hear anything that is cause for alarm, especially since it quiets down when you rev it. What's the mileage and maintenance history on this car?
Hard to tell from a video, but, if I had to guess, I'd say it sounds like a leaky injector. You can probably turn off individual injectors with a good scan tool.
Yours does sound a little deeper/louder, but I don't really hear anything that is cause for alarm, especially since it quiets down when you rev it. What's the mileage and maintenance history on this car?
It seems to reduce when I rev but at one point its just getting masked by by overall engine operating noise.
odometer says around 90.000mi. Not a lot of history available.
Also did an oil change, running the engine with about 500ml of seafoam (in the crankcase) for about 15 minutes before draining the oil, no change. No change in the sound when the engine is either hot or cold (even starting from freezing temp) either.
Originally Posted by John CC
Hard to tell from a video, but, if I had to guess, I'd say it sounds like a leaky injector. You can probably turn off individual injectors with a good scan tool.
I looked around the top and didn't see any fuel stains around injectors or heard any "pff pff" common with leaking injectors. Will look again.
I noticed there was a slow engine oil leak around the top oil pain and CV shaft seals. (car is 4matic)
Last edited by RobertC350; Jan 6, 2025 at 03:48 PM.
I looked around the top and didn't see any fuel stains around injectors or heard any "pff pff" common with leaking injectors.
Not that kind of leak; an injector that doesn't close properly, dribbling fuel into the combustion chamber. It causes pre-ignition, hence the knocking. The amount of fuel leaked per revolution goes down as engine speed goes up, so the knock subsides.
Come to think of it, turning the injector off might not identify the bad one as it would probably keep leaking... My troubleshooting skills predate common rail systems...
Not that kind of leak; an injector that doesn't close properly, dribbling fuel into the combustion chamber. It causes pre-ignition, hence the knocking. The amount of fuel leaked per revolution goes down as engine speed goes up, so the knock subsides.
Come to think of it, turning the injector off might not identify the bad one as it would probably keep leaking... My troubleshooting skills predate common rail systems...
ah, I see.
What are my options in this case? pull all injectors?
I'd still try shutting them off one by one. Doesn't cost anything even if it doesn't help. Otherwise you'll have to take them out if you want to test them.
I'd still try shutting them off one by one. Doesn't cost anything even if it doesn't help. Otherwise you'll have to take them out if you want to test them.
Just did that today, shut off one by one. When I deactivate injector/cylinder 6 the noise completely disappears.
My mechanic has a seized engine from a ML350 Bluetec in the shop that still has all injectors. Next step is trying to swap the injector #6 e see if there's a difference.
1. Replaced temporarily with another injector from a 2010 ML, different code, but coding updated on the PCM, no change, same knock. (but looking at the parts catalog today, it seems that 2010 injector doesn't work on the 2012 so that might be the issue).
2. I scanned the fuel flow/adjustment after installing the used injector, got the screen below, looks like the 6th cylinder is way off.
3. I took the original injector to an injector shop. They claim injector is electronically bad.
Next I will see if I can refurb the injector, apparently you might be able to. I'm debating if I should try other injectors from that ML 2010 (I have 6) in case that one happened to be off as well.
Injector replaced with a correct one for the year from a junkyard, no change on noise, but fuel trim is within limits now, and not like above.
Injector swapped between cylinders 4 & 6, no change, still noisy. When I deactivate cylinder 6 noise goes away still, so definitely cant be injector.
What could be causing this noise on cyl 6? Oil building so bad due to faulty injector that cylinder compression has increased to the point of causing a noise?
One thing I noticed in both injectors when removed was a lot of carbon on them.
Last edited by RobertC350; Jan 27, 2025 at 12:41 AM.
Could be a piston problem, or, I suppose it could be wrist pin or even big end bushing problem. When was the last time you changed the oil and filter? If it was before the problem, it might be worthwhile to drain and strain the oil and inspect the oil filter. Also, boroscope the cylinder.
I recently acquired a 2012 S350 Bluetec and just noticed this "knock" (toc toc toc), it sounds like it comes from the back of the engine, not necessarily more at the bottom. is that normal? Is this a rod starting to go?
I have a 2013 GL350 with north of 260K miles on it that has made that noise since I got it at 96K miles. I have spent hours with a mechanics stethoscope poking and listening even just today. drives me nuts!! It is going to the shop next week for a bunch of work: Right hand exhaust manifold, belt, idlers, tensioner, alternator, a front strut, fuel filter, transmission service, transfer case chain replacement & his opinion on that dang sound. I'd be surprised if I get out of for under $475 on this one; LOL. My mechanic was employed by MB corporate as the tech that went out in the field when the dealership tech's needed a major warranty ok'd or when they couldn't figure out a problem. I will post what he tells me.
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