Looking at a 1975 300d with "low compression" - Page 4 - PeachParts Mercedes-Benz Forum

mannys9130 · #46 01-06-2016, 11:12 PM

Quote:

Originally Posted by Shortsguy1

[Aside: I sometimes tire of these arguments where only opinions are shared, instead of any facts or support of a position]

Okay, so Mr. Newton helped us figure out that to accelerate a mass you need a force (F=ma). Clearly a change in speed of any part of the engine requires a force on that object (like the force of a connecting rod on a piston). A force per unit area is a stress, so there is some stress inside the connecting rod. For materials like metals, stress is proportional to strain (the proportionality constant is called the Young's Modulus), so any time you have a stress in a metal, you also have a strain. Strain is a change in length (per unit length), so by simple logic, a connecting rod must change its length as it accelerates the piston. So can we please raise the level of this argument beyond, "Yes it does" or "No it doesn't." The question should be can a connecting rod stretch enough to matter? And if so or not, please provide some discussion or analysis or even an anecdote to support your position. Thanks.

Go ahead and provide those things that support the claim that rods stretch.

mannys9130 · #47 01-06-2016, 11:17 PM

Quote:

Originally Posted by dude99

No its not, at higher engine speeds the rods stretch as the piston reaches the upper apex.

So after 200k miles of being driven hard, how is it that the piston top never touches the cylinder head? I drive my 190D to redline very often, and by the stretchy rods logic, they should have let the two meet by now.

Look, the statement isn't "rods NEVER stretch no matter what." The statement is "rods don't stretch because of the forces they see in normal engine operation." The engineers decide what the redline is. They then design the rods and valve train and crankshaft to perform well up to and perhaps slightly past that point. The rods will HANDLE THE MAXIMUM STRESS the engine will ever see (redline) without any deformation. They were designed to be that way, and that is always the case. Rev the engine above redline, and you're on your own.

I'll say it again for the sake of reading comprehension:

Connecting rods don't stretch.

vstech · #48 01-07-2016, 12:04 AM

I think you all are arguing about different things...

Yes, the rods do stretch out on each exhaust stroke, and compress on each power stroke... They do not significantly alter in overall dimension.

ALL MATTER UNDER STRESS REACTS TO SAID STRESS...

WHEN YOU JUMP INTO THE AIR... THE EARTH MOVES AWAY FROM YOU... THE ENTIRE EARTH. Not much, mind you... Likely less than the width of a proton... But it DOES MOVE.

funola · #49 01-07-2016, 09:07 AM

Quote:

Originally Posted by duxthe1

I've seen broken rings in these engines before. It happens something like this.... Grandma puts it around for years and years. The engine develops a bit or ridge at the top of the cylinder. Car gets passed on and being as they are slow molasas the next owner runs it quite a bit harder than Grandma did. Higher revs stretch the rods and slams the rings into the ridge worn into the block. Result is a hard starting engine that no amount of tinkering is ever going to fix.

The greatest force on the rod is when the fuel ignites during the compression stroke, when the piston is forced downwards. Compressive forces makes things shorter while tensile forces makes things longer in general, if the parts are malleable enough for the forces involved. Please explain when the tensile forces are great enough to stretch the rods.

Shortsguy1 · #50 01-07-2016, 09:12 AM

Pick up any physics book. Look up Newton's 2nd Law and Hooke's Law.

You simply cannot accelerate or decelerate an object without a force. And any such force will cause a stress and a strain. This isn't rocket science people. If you want to remain ignorant and try to reject basic physics, you are welcome to do so.

Shortsguy1 · #51 01-07-2016, 09:15 AM

Quote:

Originally Posted by DenverW123

Can you please cite a source that has done physical testing to back that opinion with fact?

I found your request very amusing. If you want to see physical testing that proves Newton's 2nd law, go to college and take a physics lab.

Let me know how that turns out.

Shortsguy1 · #52 01-07-2016, 09:18 AM

Rather than have you continue to make a ???? of yourself by making clearly incorrect statements, I have modified your opinion to make it vaguely more accurate.

Quote:

Originally Posted by mannys9130

...

I'll say it again for the sake of reading comprehension:

Connecting rods don't stretch [enough to matter, in my opinion].

t walgamuth · #53 01-07-2016, 10:01 AM

Come on guys, let it go!

mbolton1990 · #54 01-07-2016, 10:06 AM

Quote:

Originally Posted by t walgamuth

Come on guys, let it go!

sailorbenjamin · #55 01-07-2016, 07:35 PM

Umm.... I was reading the instructions that were in the glove compartment and they say that the "preglow indicator light" should come on when I turn the ignition switch to position number two.
What does it mean when the light doesn't come on?

vwnate1 · #56 01-07-2016, 08:41 PM

On one of my five cylinder Mercedes Diesels it meant three glow plugs were failed .

sailorbenjamin · #57 01-07-2016, 10:22 PM

And then there are several faded glow plug diagrams in the glove compartment......

97 SL320 · #58 01-07-2016, 10:48 PM

The salt shaker type pre glow indicator is in series with the glow plug feed. This performs 2 functions.:

1) After a period of time it also glows from the high current draw to inform the operator that the plugs have reached max heating.

2) It acts as a ballast resistor to prevent burning out the engine glow plugs. When the indicator is cold it's resistance is low allowing the engine plugs to heat up faster. As the indicator heats up, resistance rises limiting current. ( and dropping voltage )

And, when a glow plug burns out, overall current draw drops and the indicator no longer performs voltage regulation. This will stress the remaining good plugs. Many glow plugs are rated at a voltage less than 12V to allow for rapid heat up and the ability to continue to operate while cranking.

mannys9130 · #59 01-08-2016, 12:00 AM

Does the engine still have the series glow plugs?

Shortsguy1 · #60 01-08-2016, 07:22 AM

Quote:

Originally Posted by 97 SL320

The salt shaker type pre glow indicator is in series with the glow plug feed. ....

I am pretty sure his car doesn't have the "salt shaker" indicator. It has a more common glow plug indicator light, like on the w123.

As mentioned above, if the light doesn't come on when you turn the key, it indicates a problem with the GP circuit. Probably it means that more than one GP isn't working. But I suspect a broken fuse or relay would give the same indication.

If you weren't sure how to answer Mannys9130's question, you have to look at the GP wiring on the engine. Take note of what happens electrically at the Cylinder 1 glow plug (front of engine). Perhaps the electricity will simply end at GP1 (which indicates parallel wiring) or perhaps the electricity will continue on via a woven wire grounding strap to the block (series wiring). The car originally had series wiring for the GP circuit, with 12V distributed across all the glow plugs and wires. Later in MB diesel evolution, series GPs were developed where each plug handled the full 12V, resulting in faster preglow starts. A common retrofit in older MBs is to change out the series GPs for parallel, for easier and faster starting, particularly on cold mornings.

The fact that you found faded GP diagrams in the glove compartment suggests that someone has had problems with the GPs or that they bought a retrofit kit to change from the original series GPs to parallel.