Looking at a 1975 300d with "low compression" - Page 3 - PeachParts Mercedes-Benz Forum

sailorbenjamin · #1 01-02-2016, 09:10 PM

Well, big thanks to everyone for the advice. I've agreed to purchase this tired old beast Monday morning when my bank opens. I'm getting a good solid body and nice upholstery and if I can make this engine run, that'd be awesome. If not, I don't mind putting another one together. Should be a great learning experience. I'll start a new thread when it's actually here and I own it.
Here are the Craigslist pictures;

t walgamuth · #2 01-06-2016, 11:00 AM

Quote:

Originally Posted by sailorbenjamin

Well, big thanks to everyone for the advice. I've agreed to purchase this tired old beast Monday morning when my bank opens. I'm getting a good solid body and nice upholstery and if I can make this engine run, that'd be awesome. If not, I don't mind putting another one together. Should be a great learning experience. I'll start a new thread when it's actually here and I own it.
Here are the Craigslist pictures;

That engine looks very clean. I bet if the glows work and you can get fuel to the injectors it will start right up.

barry12345 · #3 01-06-2016, 01:56 PM

I might want to set the valves as many of these old cars have never been touched for eons. Or at least check that some clearances are still present.

Also gives you an opportunity to check the general condition of the timing and components.

I can see someone owning one of these and by pure neglect letting the valve adjustment go undone not even realizing they are not hydralic. Until the point the engine may not even start or only with severe difficulty.

When someone buys a really old car nothing should be just assumed. Another common case for discontinued use could be the glow plug circuit. Or just total neglect of fuel filter changes.

Actually these things should be checked prior to getting into a frustrating experience. The claim of low compression could be simple valve adjustments lacking for almost forever.

As Tom says though it just might just start up. Depends a lot on why usage of the car was discontinued. Diesel engines seem to stand long periods of inactivity far better than gas engines. That's if they were starting and running well until when last shut down.

.Now on those cars that were using vegatable oil and the car has sat unused for a long period because it had developed low compression. The vegetable oil residue in the ring lands will turn into a very strong glue with time. Although nobody as old as the last owner is likely to have used it. So it would be off my radar on this car.

mannys9130 · #4 01-02-2016, 09:41 PM

It looks like it was cared for...

I think the valve adjust and MMO soak I described early in the thread will bring the compression back to spec.

freebenz84 · #5 01-02-2016, 09:47 PM

The valve seat in the old 200d ws a very soft metal that the valve would slam into to form a good seat. when you ground the valves to the three angles ( listed in minutes of degrees) the main angle of the seat was ground to an angle just a few minutes off. this was very hard to do in a traditional auto shop but i did my best. it required a bunch of adjustments as the valve seated in. the gaps would grow closed.

duxthe1 · #6 01-03-2016, 11:59 PM

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.

mannys9130 · #7 01-04-2016, 05:00 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.

Uhhhhh, no.

Connecting rods don't stretch. This is a myth.

funola · #8 01-07-2016, 10:07 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.

duxthe1 · #9 01-05-2016, 02:15 AM

Uhh yeah they do. The largest load a rod experiences is tensile load and all metals have some degree of elasticity. That non-turbo rod wasn't Mercedes best feat of engineering. I've only seen the turbo rod fail at the big end but all of the non turbos break high up on the beam. Seen plenty of non turbos with a rod poking through the block and a few with broken top rings. You don't find broken top rings in these engines if they always been driven hard. If somebody babies it long enough it wont pretty when the next guy doesn't.

mannys9130 · #10 01-05-2016, 02:27 AM

That's the thing though, rods break because they are not elastic. What is WAYYYYY more likely is that the bearing gets worn down and the rod magically "stretches" in an old engine.

Connecting rods don't stretch. Again, that's a myth.

t walgamuth · #11 01-05-2016, 11:51 AM

I'd have to agree the rods don't stretch....much. I'd find it very hard to imagine one stretching enough to break a ring.

But if you run an engine low on oil or with very dirty oil various things can happen and usually do.

sailorbenjamin · #12 01-05-2016, 08:01 PM

I'm a newb here but I now that bicycle chains "stretch" when the holes in the links and/or the link pins wear and allow the links to pull further and further apart. 1/8" per foot is the standard wear limit but I've seen them almost 3/8" to the foot to long.
Whether due to bearing wear or rod stretch, it's a good theory. I've heard it applied to "Gramma" cars of other makes. I'll look into that and if I do get it running, I'll drive it gently when I first get it going.
I have the car in my driveway now. I'll post pictures when I get a chance. Of course there was a discrepency between the title and the bill of sale so I'm waiting on the post office to bring me the corrected forms. Fortunately, the guy is friendly and cooperative.

mannys9130 · #13 01-06-2016, 02:05 AM

Yes, chains "stretch" due to wear, but it isn't that the metal literally stretches.

Honestly, I can't even see grandma driving an old 240 gently... You really need to demand maximum power from the engine in order to drive normally. Thinking about old granny trying to merge into traffic staying below 2k rpm is laughable.

sailorbenjamin · #14 01-06-2016, 10:56 PM

Quote:

Originally Posted by mannys9130

I can't even see grandma driving an old 240 gently...

My Dad is the most conservative driver I've ever met. He let me drive his diesel Vanagon a while back and when I hit one particularly steep hill he actually told me, "really gun it here cause if you drop into second, you'll never get out."
The guy says he adjusted the valves. I'll check his work. i'l also flush the fuel system and all the other fluids.
Step by step. No hurry.

dude99 · #15 01-06-2016, 11:45 PM

Quote:

Uhhhhh, no.

Connecting rods don't stretch. This is a myth.

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