Hey all,

About 6 months ago I picked up this really beautiful 350SDL with 190K. I've been daily driving it without any issue since. We're at about 204K now.

Recently there's been a 'rumbling' coming from what seems to be the front of the engine. It's only apparent at idle, slightly raising the engine speed seems to make it go away. It behaves sort of like a bent catalytic converter. (It's not). The car does use some oil, but when I had the belly pan off earlier today to change the oil, I was (sort of) relieved to see that it's been leaking, and not necessarily consuming it.

I read some things about fuel 'nailing'. There's also a 'tick' coming from the top side of the engine, which I have read could indicate a bad injector.

I've taken a video to try to point out the sound. The car runs beautifully and idles smooth. This noise is starting to bug me. Any thoughts are appreciated.

https://youtu.be/_2sHX-dPBUA

Take a small diameter hose and hold it to your ear with one hand and move it around the different locations on the engine. It will quickly tell you where the noise is originating. Don't get caught in the moving parts.

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84 300SD
85 380SE
83 528e
95 318ic

Sounds pretty normal to me. I will let you if I think of anything. Look forward to the comments.

Great advice! Both parts!

I think it's most noticeable when I rev the engine by hand in the engine bay. It's a very low pitched rattle sound. It's actually been mentioned by friends in the last week, definitely noticeable.

Sounds like a similar sound I had, turned out the belt tensioner dampener had to be replaced as in my case the rubber bushings had worn slightly.

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OBJECTS IN MIRROR ARE CLOSER THAN THEY APPEAR, ONLY IN A BENZ

89 560SEC
90 350SDL (Rudolph)
91 350SDL (300SDL drivetrain)
87 BMW 635CSi (Swanhild)
89 BMW M6 (euro)
99 Suzuki Hayabusa (Only the fastest thing ever)

"Looking for help identifying engine noise. 1990 350SDL"

It is, after all a 350, aka "rod bender".

I like the idea about the serpentine belt tensioner. Another test: while the engine is at idle, using a long piece of wood (broom handle) or long screwdriver, push on the belt tensioner to make it a little tighter, and if you can make the noise change at all, that's the source of your noise.

Other observations:

(1) The plastic fuel line clip at the front of your engine is broken. Left alone, your hard fuel lines are at risk of cracking and breaking. Time to replace all those clips and the little rubber shims under them.

(2) Your little vacuum line at the front of the engine is routed incorrectly, jumping all over. It should be going between the serp. belt and the engine, not on the fan side of the belt.

(3) Other noises: I could not detect anything unusual, but my ear is notoriously bad. These engines have a bad reputation for developing click/clack noises from the hydraulic compensators (aka "lifters"). Mobil1 synthetic oil has a good reputation for quieting that noise, but it may take hundreds or thousands of miles of driving to clear that up.

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Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC

+1 belt tensioner or tensioner pulley. The way you go about identifying a noise like this is to remove the serpentine belt, then run the engine a bit. If the sound is gone, it's a rotating component or the belt tensioner, and so you inspect them one by one.

X2

I have an 86 SDL pushing 430,000 miles. Twice now I've replaced the tensioner shock & spring, tensioner pulley bracket, and tensioner pulley wheel. Most recently about 10,000 miles ago. Much quieter post renew.

I'm thinking they are good for 200K or so.

Each had a different sound that initially sounded really expensive.

Worn shock sounded like some kind of rod knock. (try the above mentioned wood stick pressure on the tensioner bracket (arm))

Tensioner bracket (arm) pivot bearing started failing making a rattling noise (you can grab it and rock it, or it will also set off align with other pulleys when viewed from above). I was sure it was a water pump going bad.

Belt tensioner wheel - bearing started whining. (sounded just like a transmission whine)

Crazy I know but these belt tensioning systems can make some odd noises but are easy and affordable to fix.

My favorite was the belt tensioner wheel - bearing whine. I drove around for months thinking the trans was going. But alas pulled the belt and no whine...just a bad $25 tensioner wheel (I now like Ina brand)

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1986 300SDL 440,xxx

Thanks to all of you! Great advice, and if it's the issue being discussed then I'll feel very relieved.

Now, at the risk of sounding like an idiot (I'm no mechanic, especially for diesels), could someone point out which arm should I be looking at? There are one left and right of each other. On the left, it seems to bounce a little more than the other. I placed the handle of a wooden hammer on what I assume is the pulley arm bracket. I wasn't to noticing any distinct difference, but I also had the business end rattling in my ear. Is there anything special about removing/replacing the serpentine belt on these? I inspected mine and it does seem to have seen better days. Not dangerous, but definitely a candidate to be replaced. I've done a few before so if they're nothing special I can do it myself.

I took another video of the engine bay while running, and one of he exhaust. In the exhaust I can hear the distinct puffs as the engine breathes. Not something I noticed in my 300D. Normal?

https://youtu.be/Lr1BMKl8SeE

https://youtu.be/nan1jxupdDU

Maxbumpo, I assume:

This is the clip you're referring to?:



This is the vacuum line you're referring to?:

"I took another video of the engine bay while running, and one of he exhaust. In the exhaust I can hear the distinct puffs as the engine breathes. Not something I noticed in my 300D. Normal?"

Sounds just like mine...kind of like an old motor boat..bloop, bloop, bloop
at idle.

Google "350sdl drive belt tensioner" and you'll see what we're talking about. You lean on it with the hammer handle to see if it gets quite. If so replace the shock.

Google 350sdl idler pulley to see the tensioner pulley wheel.

Belts can make odd noises too. The photo below shows the belt route (disregard hand drawn circle in photo)

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1986 300SDL 440,xxx

On my 95 E300 when the tensioner was shot the engine sounded like a Dodge truck. In your photo the shock absorber device on the lower left, push down on it with a long screw driver while idling. You will notice a reduction in noise if that is it.

Also you should consider replacing your vac pump at your mileage for good measure.

THIS.
I just changed the crunchy old belt on my 91 350SD and the squeaking noise that I got coming off throttle and back to idle went away.

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91 350SD
14 F150 Eco
19 Fusion Hybrid
11 GT500

The noise is the belt flopping around.

Replace the tensioner shock. While you have the shock and belt off, feel how the tensioner arm feels. It should have no play and the bearing should move smoothly. Replacing it is not difficult.

I'd also recommend installing a clutched alternator pulley, as fitted to the later 606 cars. This will drastically increase tensioner shock life, and make the belt last longer too.

https://www.youtube.com/watch?v=DGI9XLuhaYY

https://www.youtube.com/watch?v=fB_r4odE9hI

-J

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.