I'd have expected an indy shop to do more than listening to an engine for $260. I guess sooner or later we'll find out if this is indeed rod knock which is what the used car lot I bought mine from thought when they sold it for cheap. Meanwhile everything I've analyzed points to something on the top end. How does your noise sound when you accelerate under load? Does it disappear when you take your foot off the gas pedal while coasting or decelerating? Also, how is your oil level? Mine is definitely louder as the oil lever drops which to me also points to something in the valvetrain. If you're within reasonable driving distance from the Philly PA area then we could meet to compare notes/symptoms.

 

Mine is there under load or not, but when under load its a bit more “tin” like. From inside the car you dont really notice it unless under load.

There’s an engine machine shop local to me with a mechanic connected to their building. I’m going to run it by them to see what their opinion is.

At this point I’m considering doing a drivetrain rebuild and a quick restoration on this thing. For $10k I can pretty easily get this looking like new and last me another 150k miles. That’s less than the cost of depreciation on a new or used one.

 

You description under load sounds just like mine. If you are considering a rebuild check with member @arsupisemnet to see if you can work something out. You can see some of his work he’s shared on this forum. I contacted him a while ago because I’m sure a rebuild is in my future due to cylinder scoring but I haven’t pulled the plug yet

 

Took mine to a local import mechanic tied to an engine builder/machine shop and they suspect piston slap. I will probably have hime pull the motor and then I’ll send it off to have sleeved..

 

please do share what they find. thanks.

 

Yes, please share any findings.

 

Mine is still doing it at around 140k, but only when cold. It's gone after 2 min of idling.

One odd thing is that it is much quieter if the car is on an decline with the engine lower than the rest of vehicle. Cam shaft maybe and quieter when more oil on it? At this point I don't really care unless it gets worse and my true diagnosis will be when something fails.

I really need to check my cam mags and sensors for oil....

 

It's interesting that yours disappear after 2 min of idle. Mine is there all the time. I took note of your incline observation a while back and checked on mine with no difference. Mine is definitely more prominent when oil level is low. These are part of the reason why I don't believe it's rod knock or piston slap or any related crank/piston/cylinder issue. It looks more like valve train and oil delivery related but it's a question of what? I think I've ruled out some obvious ones like weak oil pump, lifters, rockers, worn cam lobes, and may be even cam adjusters and tensioner. For cost reasons I checked the cam shaft, adjusters, tensioner by swapping with used ones I'm sure didn't have any prior noise. Rocker and lifters were brand new. I often get the question of whether my GL is diesel due to the noise.

 

rod bearing? if i remember correctly, that used to be common on the porsche v8s or was it v6 engines a few years back. interesting they can hear it out. would be surprised if they were correct.

 

Does your motor have the updated tensioners and check valves?

 

Hey all- M278 here in a '15 S550. Came across this thread re: knocking. I have a deeper sounding knock only on a hot restart, but after the car sits for 10 mins or so. It instantly goes away on startup after 1-2 seconds which I presume is the oil pressure building up. It does not knock if I stop and restart quickly.

Guess my question is, w piston slap, would it go away instantly once oil pressure builds? I'm guessing I may have some sloppy rod bearings perhaps that clears once pressure builds.

Car has 63k on it. I bought it w 45k 2 years ago. And noise has not changed/ worsened. Oil changes at 3-5K since I've owned and seems well maintained by other owner. Once the knocks clear after couple seconds the engine is whisper quiet, no taps etc.

 

What you're describing sounds like the timing chain tensioner/check valve symptoms (i.e. it goes away 1-2 seconds after startup). Are you sure it's knocking and not more like rattling? Share audio if possible.

 

Thanks--- definitely knocking, deep sounding. Not a tap. I actually haven't even listened outside the car. I can hear it inside on start up w doors closed . The car has such a low idle on hot restart, wondering if it's literally the oil pressure building up. Interesting you mention check valve. It only makes the noise after it sits for 10 mins then restart- ie the oil pressure must bleed off. On an immediate, hot restart it makes no noise.

If you've ever restarted after an oil change, and get a low end clinking before the pressure builds, it's like that. I will definitely get an audio of it and share. Thank you!

 

That's exactly the kind of noise I'm referring to. I've heard it many times especially after frontend timing/tensioner/cam actuator work before the oil pressure is primed. It's very likely the tensioner hammering due to lack of oil from ineffective check valve. If you were to see this in action you'll see the tensioner piston pulsating in and out as the chain rolls. I've listened to MB service audio that claims a defective cam actuator can cause similar noise. Do you know if yours have the updated tensioner and check valve? BTW I'll trade my knocking for this one any day if that's what it is...

 

Actuator has pin which keep it from turning till the oil pressure buildup in upper area. That pin wears out letting actuator fly around.

 

Did you ever figure out what was causing this sound in your GL? Reading your description was like reading my own 2014 GL 450's story. I have this same loud knock. I had the oil exchanger replaced due to a leak about 6 months ago. While there, the independent mechanic said the "tappets" were worn and needed to be replaced. (I think these are the valve lifters??). So he replaced those. Not long after that, the knocking started and grew steadily worse. The dealer looked at it and said it was probably a rod about to go and I needed a new engine. The mechanic who did the tappets also said he didn't know the exact cause of the knocking but thought it would need a new engine soon. Meanwhile, the GL w/ 147k miles runs just fine other than that very annoying knocking sound. The knocking sounds just like the videos posted on his thread. The timing of when the knocking started could be a coincidence, but I can't help thinking it's related to the tappets replacement somehow. Tried LiquiMolli, but no change. I doubt this noise is the valve lifters themselves, since they've been recently replaced, but wonder if something else in that part of the engine is to blame. I have now driven about 8,000 miles with this sound, even though when you listen to it it sounds like the engine is about to go at any moment!

A few observations: Like some others here, the knocking speed corresponds to RPMs. When I'm driving and take my foot off the gas to coast, the knocking disappears completely and only comes back when the car slows way down to nearly a stop. The knocking starts immediately upon ignition and is loud and obvious at idle. It gets both faster and louder during acceleration. However, if I park the GL and put it in neutral and rev it a bit, the knocking disappears completely around 1600 RPM! I don't know a ton about cars, but this is such a mystery, and I would love to know if someone has figured it out. I don't know if I should bail on this vehicle while it still runs, or if there is still hope for it. Would love to have the advice of any knowledgable folks on this forum.

 

Bro, mine also same, my mechanic said its a common issue with this model. i sent an email to German head office requesting for recall hehehehe, i know this is not going to happen at all. i have driven 3k miles with the noice.

 

Speeding like 50 no noice, vehicle is perfect only the noice

 

Welcome to the mystery. Hopefully we can compare enough notes and get to the bottom of this. As I think about this some more and consider everything I've checked or changed, I keep wondering if it's the timing chain tensioner. Perhaps not the tensioner itself but oil delivery to the tensioner. My reason for this is one time I replaced the camshaft and tensioner on a GL 450 (not mine) and observed the following:

• When I started the engine after new tensioner installation, it rattle/knocked badly for quite sometime. I shut off the engine immediately wondering what the issue was.
• When I inspected it I discovered the noise was coming from the brand new tensioner. I removed it, put the old one back and it too made the same noise. There was no such noise before the repair (car only needed camshaft but I also replaced the tensioner while in there).
• I later realized that it's normal behavior until the tensioner is primed with oil pressure so I put the new tensioner back, let the car run for about 30 seconds and then the noise disappeared.

The noise I hear from my GL seems similar to such tensioner noise. I replaced my tensioner with the old one from that GL 450 and that didn't make a difference so I assumed it's not the tensioner (because the old one didn't make a noise before being removed). However, if I leave the upper timing chain cover open with the engine running, the tensioner piston moves in and out in what seems to be in sync with the noise. I don't know if that piston movement is normal or it's supposed to be completely steady when applying tension to the chain. I don't have a good running GL to compare with. If someone knows for sure please chime in

So, is it possible that the oil passage to the tensioner is clogged or defective or has defective check valve that prevents full hydraulic pressure on the tensioner? My noise get much worse when oil level is low. I have scored cylinder walls and oil burning problem which I'm not sure is related to the knocking noise. I can tell when my oil level is getting low by how prominent the noise becomes at idle and low speed.

 

Guys I am going to check mine this week and I will update

 

The knocking noise saga. comes to the end. Two major factors contributing to it
1) Wear of tensioner (internally) - usually stuck due to wear and easy as temperature goes up
2) Wear of camshaft lobes - both or exhaust camshaft on driver side goes bad causing knocking noise as it goes due to grooves on lobes - basically rocker arms jumping in and out of grooves unable to compensate for lash.
Service adviser will advise to change timing chain and adjusters were real reasons mentioned above.

 

When I cold start mine later I'm going to try to deactivate a few drivers side cylinders one at a time to see if that is of any help here (with iCarsoft). Mine definitely sounds like its from top of engine near Cylinder 5-6 area.

@tadiguy did you replace check valves with updated design or just tensioner?

 

In my case:
1) I replaced the tensioner with one I had pulled from another GL that wasn't making any noise and that didn't help.
2) My camshaft lobes showed no visible sign of wear. However, I replaced both the intake and exhaust camshafts and cam adjusters and that didn't make a difference.
I also replaced all lifters and rocker arms with brand new OE parts as well as the tensioner.
The intake cam adjuster was new aftermarket, the intake shaft was pulled from a GL due to bad tone ring, the tensioner was pulled from a working GL during camshaft replacement, and the exhaust camshaft/adjuster were used from the Bay. There is a small chance that one of these used parts had the exact same problem I'm trying eliminate but as you know, new ones cost and arm and a leg and so I wasn't ready to throw more money and parts at the problem if I'm not confident it will resolve it.

 

@tadiguy did you replace check valves with updated design or just tensioner?[/QUOTE]
No, I didn't replace the check valves. I bought it used so I don't know if the check valve was ever updated. However, the bolts holding the upper timing chain cover had blue paint on them which I understand from reading MB service bulletin means they've been updated (someone correct me if that's wrong).

BTW: Selectively shutting down cylinders with Xentry, disconnecting coils, fuel injector, etc. didn't help in my case, but your symptoms are a little different (i.e. noise disappears after 2 min) so it will be an interesting experiments. Let's know.

 

I shut off the injector after the car had been sitting (wouldn't let me on cold start because coolant temp), and it still knocked.

Accessed 'Smooth Operation of Combustion Engine' menu and Cylinder 5 jumps to +/- 3.5 and others hover around +/- 0.5. Not sure if related to knocking, but going to check injector connection etc for oil since I just found my cam sensors to be wet with oil.