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The Mercedes M276 engine in our 2019 S450 twin turbo V-6 with 54K miles was making quite a racket after it warmed up at idle. Rev the engine just above idle & the rattle sound went away. Traced the noise to the Idler pulley assembly on the front of the engine with a stethoscope. Seems the Gates OEM brand internal spring rotary guide bushing wore out? The pulley bearing appeared good still. Good news is that I replaced the belt tensioner without having to remove or loosen the water cooled turbo inter-cooler.
Started by relieving tension on belt tensioner and stuck an 1/8" drill bit in the "holding hole." I tied back the rubber belt in two places with solid wire in such a way as to keep the belt out of the way yet still with some tension so it stays on the other accessory pulleys on the engine. Then removed the belt Idler pulley directly above the belt tensioner pulley to give even more room to wrench. Having many wrench & socket options on hand is key to fit into the tight area behind the inter-cooler.
AutoZone had the belt tensioner and idler pulleys in stock at my local store.
Duralast #305723 $44 USD "Automatic Belt Tensioner."
Duralast #231015 "Idler Pulley" $33 USD (both idlers on the engine are the same part; the one above the tensioner & the one behind the turbo cooler tank) I'll attempt to replace the latter next
Just wanted to let you know that this Belt Tensioner replacement procedure can be done without touching the inter-cooler that at first glance, looks formidable.
Good job fixing your belt system in one shot instead of piecemeal this system.
Its simple: everything must go!
hard tensioner
wobbly idlers
bold serpentine
No1 idler grease gets dried out
How long did your genuine factory set last:
(40 to 80kMi) ?
grease lost at 50kMi
> Plastic center ring... small shaft for a large bore with poor centering invites WOBBLING
It's amazing how a large bearing diameter is centered over a small shaft by a tiny piece of plastic.
engine side bearing surface
The M271/2/3/4 engine idlers used what looks like better centering solutions.
dual bearing centering rings, non-plastic
M271 uses an inner bearing holder
I wonder if this cup will mount over the M276 engine profile ?
Looks like a good close match
simplistic M276/8 center spacing ring
another solution to center idler steady
> WHY ?
I see my 15kMi idler runs a bit more wobbly that I'd like.
I want to preserve my A/C compressor bearings from this type of setup.
This loose pulley topic looks like an opportunity in the making... searching for good solutions.
#### existing knowledge...
MBW forum prompted me to read M272 related issues where Mercedes misaligned the pulleys that caused squeaking.
Keeping in mind that the pulleys line-up doesn't need to be extremely precise but can't be too far off either ie. be aware of positive or negative shim effect between pulley to engine ~ half a mm. centering cup is 0.63mm thick factory MB part
Last edited by CaliBenzDriver; Feb 6, 2026 at 09:54 PM.
Thanks for the tips. Our W222 factory belt tensioner (Gates brand) started to fail at 54,000 miles or 86904km. I did re-use the factory #1 idler pulley centering spacer ring. So far so good, but I agree, the plastic centering piece was a bit of an un-welcome find.
Both our USA spec '18 E400 Cabriolet & '19 S450 idlers vibrate normally as they do their job. Seems to be a high internal wear item always adjusting belt tension as it does. I could see that the old tensioner's two part aluminum castings were slightly off-center with each other while the new one's were centered.
As far as the intercoolers on the front of the engine go, I selected the MB V-6 & straight six engines so I could work on them DIY. IMO, the V-8's & V12's are just too cramped
Great job, those tensioners automatically adjust their tightness as far as I know but I see V12s are not easy to work on as their half way engine siblings lol
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