I'm getting tired of the rumbling and/or rough idle on my car at a stop. I put some Biodiesel B-20 last night, hopefully that will settle it down a bit. But I was in a 1992 300d once, and it was very quiet for a diesel, and
STONE STABLE. Vibrationless.

I wonder if I need new motor mounts?

Jeff 1991 300d, 114k

Excessive vibration and a inspection are surefire way to check your motor mounts, if they show surface cracks, they are dry and brittle and time to change, if your engine is in proper tune with valve clearance in spec and you get excessive vibration at idle, time to change the mounts.

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99 Gurkha with OM616 IDI turbo

2015 Gurkha with OM616 DI turbo

2014 Rexton W with OM612 VGT

The '91 doesn't have adjustable valves, so forget the "tune"
Also the '91 motor mounts don't usually show cracks as they are the jelly filled type, unlike the 123 you are accustomed to.
The mounts are a rather short lived thing on the later diesels too. They act like an engine shock which is not used on the later models either. I grab hold of the crossover pipe and shake the engine as hard as you can side to side and up and down. Good mounts will feel tight and the engine will not move much if at all. A shop uses a floor jack under the engine to see how much it raises.
Another test is shutting the engine off manually and watch to see how much side play it has just as it "dies"

Greasybenz had bad mounts on his '87 300D and the fan was starting to scrape the cowling the engine was so loose

I don't know what the rumbling jbach36 is referring to. You might have a bad injector or something, an experienced mechanic can tell by listening and cracking injectors to see if its cylinder related but that engine should just purr at idle

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!

When the engine tries to jump through the hood. you know they are bad

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1979 240D with a 617 turbo diesel.

If you feel any vibration in your seat you need new mounts. If you have no record of them being done in the last few years you need new mounts.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

and I would add - When you can put THREE fingers on the intake and rock the whole engine left and right by four inches with the same force that would easily move an empty shopping buggy.

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1987 300D (230,000 mi on a #14 head-watching the temp gauge and keeping the ghost in the machine)
Raleigh NC - Home of deep fried sushi!

I have a better one...

When a friend pulls up behind you at a red light, and later asks what song you were bopping along side-to-side to, even though there is no radio in your car, it's time to replace the mounts. If I had a dashboard hula dancer, she'd be violently bashing her head on the dash at idle.

This post reminds me that I need to buy motor mounts before the car shakes a part off.

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- K.C.Adams

'77 300D Euro Delivery
OM617 turbo / 4-speed swap
404 Milanbraun Metallic / 134 Dattel MB-Tex

Current status:
* Undergoing body work

My '77 300D progress thread

gsxr has a rule-of-thumb measurement between the engine support arm and the frame. Something like 13mm but ask him or check the archives to be sure. If there's less of a gap, the mount has collapsed.

Sixto
87 300D

The other thing is how much the engine bucks when you turn it off. If it shakes like it just finished using the men's room, pardon my French, the mounts are worn or the damping fluid has leaked out.

Sixto
87 300D

What about that center mount? (W123, 240D) They dont seem too do a whole lot, I havent seen much difference with or without/worn out. One supplier I call didn't even carry them.

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1985 Euro 240D 5 spd 140K
1979 240D 5 spd, 40K on engine rebuild
1994 Dodge/Cummins, 5 spd, 121K
1964 Allice Chalmers D15 tractor
2014 Kubota L3800 tractor
1964 VW bug

"Lifes too short to drive a boring car"

When you pull your car into a parking spot and don't want to shut it off and get out because that massage you're getting from your seat feels so darn good ... you need new mounts.

Mine started vibrating pretty intensely at idle after a while, and upon cursory inspection of the motor mount that was easiest to see, it was pretty obvious it needed a fresh set ... some of the inner goo had made its way to outer goo, from what I could see. New mounts and the vibration was gone (well, relatively speaking ... it is a diesel).

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1987 300D, arctic white/palomino--314,000 miles
1978 240D 4-speed, Euro Delivery, light ivory/bamboo--370,000 miles
2005 Jeep Liberty CRD Limited, light khaki/slate--140,000 miles
2018 Chevy Cruze diesel, 6-speed manual, satin steel metallic/kalahari--19,000 miles
1982 Peugeot 505 diesel, 4-speed manual, blue/blue, 130,000 miles
1995 S320, black/parchment--34,000 miles (Dad's car)

'kinda like wondering if the "shuddering" upon braking details the need for new
rotors and pads.

If you question the old ones (especially at your mileage)...inspect [for due
diligence] , Then replace...You'll be AMAZED!

You mean MBs DON'T acually "shake the dew off the lily?" Hmmm, guess I'd better go check MY motor mounts tomorrow. Not that I need anything else to fix tright now . . .

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" We have nothing to fear but the main stream media itself . . . ."- Adapted from Franklin D Roosevelt for the 21st century

OBK #55

1998 Lincoln Continental - Sold
Max 1984 300TD 285,000 miles - Sold
The Dee8gonator 1987 560SEC 196,000 miles - Sold
Orgasmatron - 2006 CLS500 90,000 miles
2002 C320 Wagon 122,000 miles
2016 AMG GTS 12,000 miles

teeth chatter?

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1980 300TD-T (82 Turbo and Trans) 159,000 Miles "Jackie-O"
1983 300SD 272,000 Miles "Aristotle"
1987 Jeep Wagoneer Limited - keeps the MB's off the ice and out of the snow
1994 BMW 530it

. . . . and it's NOT cold outside.

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" We have nothing to fear but the main stream media itself . . . ."- Adapted from Franklin D Roosevelt for the 21st century

OBK #55

1998 Lincoln Continental - Sold
Max 1984 300TD 285,000 miles - Sold
The Dee8gonator 1987 560SEC 196,000 miles - Sold
Orgasmatron - 2006 CLS500 90,000 miles
2002 C320 Wagon 122,000 miles
2016 AMG GTS 12,000 miles