Why do diesels shake when they idle?? Is it due to incomplete combustion, IP timing.

....bad motor mounts, rack dampener adj, low idle adjustment....

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20:1 compression ratio and 300+ lbs pressure in each cylinder seems the obvious answer to me. Thats roughly double the compression and combustion pressure of your typical gasoline engine.

because they can

why do transformers hum?





























becasue they don't know the words....yes I know....don't quit yer day job

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Rob M
Norwich, VT USA
1980 240D Euro delivery 4 speed manual silver/blue
~160K miles

I think that's the answer to a different question.

BTW, a well tuned diesel does not shake very much. I believe they tend to shake more at idle because the IP timing is optimized for higher rpms and the timing is somewhat advanced for the low engine speed. In other words, too much combustion takes place before the piston reaches TDC.

They don't unless there is something wrong. My 617 shakes a little sometimes because the rack dampener bolt is probably getting weak. My 603 is like a turbine, their is zero shaking or vibration at idle.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

The combination of the compression ratio and the fact that the 617 is a five cylinder is the reason that it can never be perfectly smooth. It's not possible to balance a five cylinder engine so that the internal forces are completely cancelled. A four cylinder engine is similar. A large four cylinder will shake your teeth out.......which is why there are so few of them.

A six cylinder in-line engine is perfectly balanced. Doesn't matter if it's gas or diesel, they run perfectly smooth due to the inherent design of the engine.

A V8 engine can also be perfectly balanced with the proper counterweights added to the crankshaft.......however.......it's not perfect by design.

That explains why the Camry's and their ilk with the 4 bangers vibrate at idle. I remember the motor mounts on mine were massive liquid filled things, it still vibrated at idle.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

Exactly. They need very good isolation mounts if the four cylinder is not going to be felt by the driver. Personally, I've observed that the newer Camry's and Accords do a fantastic job with this. The fact that the four cylinder is relatively small (2L) is a benefit. They could never properly isolate a larger four (3L or so).

Yes I have noticed that as well. Mine was older and had the 2.2 16 valve. Toyota seems to have gotten the vibration problem solved in the last couple of years. Although I have to give them credit even on mine at anything off idle it was pretty smooth.

Look at the 616 though it is 2.4L so a pretty good sized 4 cylinder. Did MB use engine shocks on them? I suspect a fresh set of mounts and engine shocks would help a bit. But remember this is 30 year old technoligy it isn't going to be as smooth as 2005 technoligy.

I'd right it off as the cars charm and live with it.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

Some 4 cylinders are smooth, like a lot of Subaru engines. But then again they aren't inline engines either, they're a boxer or oposing design.

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1985 300D Turbo ~225k
2000 F350 (Powerstroke) 4X4, SWB, CC, SRW, 6spd ~148k
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I'm not very familiar with them, but, intuitively, I'd say that an opposing flat four would be perfectly balanced by design.

In my TD it would shake all over the place. When I was doing my conversion I found out that the passenger side engine mounting bolt was GONE (and the drivers side bolt bent to a pretty good angle). It was only gravity and alignment from the tranny mount keeping the engine from falling out. Now that it's strapped into my 240 it does not shake/rock anymore, it just vibrates the marbles out of everything.

Items that produce 'shake'

1. Reactions from combution forces. An excellent example is the 'lugging' felt when a manual transmission is driven at low engine rpm...

Each time a cylinder 'fires' pressure on the piston produces a linear motion which acts on a lever (crank arm) to produce a rotation. The instantaneous torque applied to the crankshaft is complex sum of pressure on the piston area at that moment and the effective length of the lever as the crankshaft rotates.

The result is that a crankshaft does not want to turn at a constant rpm. Rotating mass of the crankshaft itself, and belt-driven auxiliaries like alternator and pumps adds rotational inertia or 'flywheel effect' to smooth out the impulses.

2. Reactions from rotating masses. The net effect of all of the rotating parts.

Piston motion is linear (up and down) but the connecting rod motion is a lot more complicated. The 'big end' is orbiting the crankshaft, the other end is linear.

3. Reactions from linear unbalanced motion. The net effect of all of the 'up and down' parts.

My 5... well, 5 sparkplug rotary RX-7 engine was smooth as silk.