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Howdy yall - trying to identify what's going on here. GF's daughter was about to make a 9 hour drive and noticed a screeching sound - asked them to take a look at the belt as it clearly sounded like it was slipping.
This looks like it may have been squeaking for a while... can anyone help me quickly ID it? " class="post_inline_image" data-size="1200x1600" data-src="https://cimg3.ibsrv.net/gimg/www.mbworld.org-vbulletin/1200x1600/whatsapp_image_2025_08_05_at_18_05_14_7c82c8a7_e7cceda7beabf7497d369d7e025849b0ef999ce6.jpg" loading="lazy" />
They thought it may be a plastic bag I think its a melted pulley - after the bearings failed..
I just replaced this idler pulley and in my car it is made of steel. All other pulleys are plastic, and I was wondering why this one is metal. Perhaps it is so hot area it needs to be steel but it is right next to the plastic tensioner though...
Good call - I actually just replaced the one Idler and the belt late last night. But I didn't replace the tensioner nor the other idler pulley - as seems there's another on the top left. I know I needed to replace the tensioner but was having a hard time seeing in the dark at 11PM.... brought my tools - but forgot a flashlight!
So I'm going to try and get that done this morning before she goes off on a 6+ hour drive. What's funny... scary... is that her daughter didn't even seem to notice the squealing.... music blasting....
No, that is not the tensioner. This is the idler pulley in the corner.
Tensioner is the one with the 180 degree belt wrap next to it in the picture.
I just went to double check and in my V8 this is a steel idler pulley in this location.
For V6 class engines MB uses plastic idler pulleys... steel is too heavy - It uses "more gas".
Jokes aside... plastic may help shield heat transfer from belt into bearing.
We've seen upgraded metal idlers go to waste due to heat transfer and perhaps failed tensioner keeping belt loose and hot...
What idlers really need is a quality ball bearing with sealed grease. As soon as it starts wobbling hot grease leaks out of seal to become dry.
50 to 70kMi life span.
Last edited by CaliBenzDriver; Aug 6, 2025 at 02:13 PM.
For V6 class engines MB uses plastic idler pulleys... steel is too heavy - It uses "more gas".
Jokes aside... plastic may help shield heat transfer from belt into bearing.
We've seen upgraded metal idlers go to waste due to heat transfer and perhaps failed tensioner keeping belt loose and hot...
What idlers really need is a quality ball bearing with sealed grease. As soon as it starts wobbling hot grease leaks out of seal to become dry.
50 to 70kMi life span.
My V8 engine has all plastic idlers and tensioner pulleys but this one in the corner next to A/C compressor. It has total of 3 plastic and one steel pulley.
Thanks all - I swapped out the other idler pulley and the tensioner this morning... and she's off to college. Next visit she will need to get some seals replaced... its definitely seeping oil from the magnetic cam adjusters and more...
My V8 engine has all plastic idlers and tensioner pulleys but this one in the corner next to A/C compressor. It has total of 3 plastic and one steel pulley.
That idler pulley right before the A/C compressor is the one that gets wobbly first.
Do you think your steel puley is OE for V8 or is an aftermarket "upgrade" idler when original became noisy ?
Some of us had question about placement of a washer that comes with new idlers to secure parts.
It's a dumb pulley that's well worth attention... we all seeking trouble-free performance.
Last edited by CaliBenzDriver; Aug 6, 2025 at 11:49 PM.
I appreciate your skill OP .
Sometimes cheap parts cause expensive repairs . Well, even if approved by Mercedes-Benz , I have to admit that I didn't like the plastic stuff in engine components
Happy Driving
That idler pulley right before the A/C compressor is the one that gets wobbly first.
Do you think your steel puley is OE for V8 or is an aftermarket "upgrade" idler when original became noisy ?
Some of us had question about placement of a washer that comes with new idlers to secure parts.
It's a dumb pulley that's well worth attention... we all seeking trouble-free performance.
I do not know if that pulley is OE. The old one that I removed was steel and for some reason I managed to find this new pulley also made of steel. I dug in some parts pictures I found in the net and ordered part number accordingly and got it.
I’m starting to think that it is OE steel for some reason but wonder why?
Belt load on the pulley is a fraction of what the tensioner sees due to so little wrap around it so it cannot be about force from the belt.
I do not know if that pulley is OE. The old one that I removed was steel and for some reason I managed to find this new pulley also made of steel. I dug in some parts pictures I found in the net and ordered part number accordingly and got it.
I’m starting to think that it is OE steel for some reason but wonder why?
Belt load on the pulley is a fraction of what the tensioner sees due to so little wrap around it so it cannot be about force from the belt.
Is it temperature then?
Engine extreme heat may be factor affecting bearing grease loss.
The other idler on Bank1 shates the same belt heat.
The one thing that's different is the distance from and to that pulley: it's a long distance reach.
It really needs to guide that belt. Any amount of lateral motion from previous idler is amplified by the long distance to Bank2 idler.
So one idler kills the other.
Again really appreciate the post and glad I asked because she would have taken off and that other pulley or the tensioner would have failed. I could actually feel the stiffness and jerky motion of the old tensioner when removing it.
Regarding the considerations for material on the pulleys:
Inertia is a consideration for steel vs plastic/nylon/etc. Plastic wins from this perspective.
Noise is another consideration. As any imbalance with steel will produce more sound.
Corrosion resistance is yet another win for plastic.
Plastic seems to be the better option IF there is not a need for greater strength alone.
Good call - I actually just replaced the one Idler and the belt late last night. But I didn't replace the tensioner nor the other idler pulley - as seems there's another on the top left. I know I needed to replace the tensioner but was having a hard time seeing in the dark at 11PM.... brought my tools - but forgot a flashlight!
So I'm going to try and get that done this morning before she goes off on a 6+ hour drive. What's funny... scary... is that her daughter didn't even seem to notice the squealing.... music blasting....
Most people don't speak "car". It can be talking but if there's nobody listening then there's nobody to notice.
Cars are talking to us all the time, be it a happy purr, a glorious scream of RPM's, a rattling control arm or a squeaking idler pully. It is up to us to hear it and know what it is trying to tell us.
That said, take off the serpentine belt and spin everything the belt touches. If it makes noise, why? Pulleys should be dead silent; sound = heat = failure.
Definitely replace:
-the tensioner
-ALL pulleys
-the belt
Actually, I see you already did some of it and sent her off. Next time you're in there, do the other pulleys. And please tell me you didn't re-use that belt...
Initially I replaced just the failed pulley and the belt... at 11PM... the next morning and after reading the post about the need to replace all... we did replace the other pulley AND the tensioner as well. She's good to go for now but I did notice oil seeping from the magnetic cam adjusters and likely the valve covers... so next trip she'll need to get that fixed up as well.
The front valve covers are probably indeed leaking. The risk from this is oil flowing down into the alternator which can start to wreak havoc eventually.
The leaking in the cam adjusters and sensors can also lead to the infamous "oil in harness" catastrophe.
Hopefully her next visit isn't too far in the future.
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2014 e350 Sedan - melting pulley
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