Questions about the '80s models

Earlier I asked about a specific 300TD, but while I want to get a wagon and will probably when the time is right, my wife really doesn't want one and we'll likely get her a car to do the veggie conversion before
mine. (The '92 Corolla is in worse shape than the '98 Grand Prix, let's just say.)
Anyhow, I'm curious about a few things based on the tidbits that I've picked up here and there...
Wikipedia says the W123 was built between 1976 and 1986, and then says that pre-'85 engines are "known to be indestructable".
http://en.wikipedia.org/wiki/Mercedes-Benz_W123 http://en.wikipedia.org/wiki/Mercedes-Benz_300D
I remember reading here that MBZ changed to aluminum heads at some point, so now I'm curious and want to avoid the aluminum, was aluminum on the '85 model, or did that start in '86?
I also understand that '82s got the driver airbag, did it ever appear on the passenger side?
How do the W126s compare, insofar as reliability goes especially, with the W123s? How do the engines seem to last? ...and how much bigger is the 300SDL over the 300SD?
What questions should I be asking, that I'm not?
Oh- and others have said that I should check compression on any of these cars that I go to see. Not knowing jack-all about diesels, I only know that there's no spark plug to pull. How do I connect my compression gauge?
thx, -tom!
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Tom Plunket wrote:

The W123 and W126 '85 models (300d and 300SD respectively) are all iron. Infact, the engines are the same on both chasis. If you want an "indestructable" engine, you want OM617 turbo. I am only familiar with 1985 300D and 1985 300SD as I have owned/own both of them. W123/W126 OM617 Turbo was produced from 81-85 if memory serves me right. '86 and later models had aluminum heads. 1985 was the last year of the 617 with some refinements to almost perfect the design.

Based on my ownership experience of 1985 300D and 300SD, they both are equally reliable. Besides being an entirely different chasis, the W126 has some more "electronics" (power seats, electric components for locks, etc). Other than that they seem pretty much the same. However, I prefer the W126 ride much better. W126 is a little more roomy comapred to W123.

Don't let anyone fool you into thinking that diesels last forever. Nothing lasts forever. However, with proper maintanence, these engines have known to easily go 500K miles. Compression is not the only thing to look at. I would look for all maint records. Diesels require that the engine oil was changed religiously. The oil is what keeps the soot in suspention. If oil changes are ignored, it is well know that the engine will wear much quicker.
check out http://www.mercedessource.com/ and look at "maint alert" tab.
good luck.
Anil Bharucha 1985 300SD 217K mi
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In reverse order, one connects a compression tester to the diesel's glow plug bores which are M 12 x 1.25. The specification for a 617.95X (5 cylinder all iron engine) is 319 - 348 psi, 218 psi minimum and 44 psi allowable difference between cylinders. The engine's valves ought to be in adjustment or the compression test is meaningless. The 617.95X turbodiesel was manufactured for MY '78 through '85 and is a very tough durable machine. But has mechanical valve adjustment that ought to be done every 15 K miles. These motors made 120 HP and about 170 ft lbs torque at 2,400 rpm.
The next newer engines ('86 & '87 MY) were straight sixes with hydraulic valves; they produced 148 HP and are smoother, more modern designs but not as tough due to their aluminum heads. These engines cannot be overheated. These sixes were installed in the 300D (124) and 300SDL.
The cars you are considering are old and most have done a zillion miles by now. Of course you and others will find exceptions but most cars are quite worn after 20 years use. So rather than focusing on certain types 123 vs 126 etc. I suggest you look for a car that's in good condition. I would personally avoid old cars with twenty to twenty four year old air bags. These old airbags - if they even still deploy - were contemporary in 1980s but are crude by today's dual stage standards. Remember, airbags were mandated because seat belts were not being used, not because seat belts failed to restrain their drivers and passengers.
Finally, don't chase after diesel cars just because they're diesels. Sellers know this too and raise the asking price accordingly. Look at a model's gasoline version's asking prices, figure the annual fuel use and added gasoline cost and whether it even makes sense to buy an old diesel just because it's diesel powered. A minor collision often "totals" an old car because insurance companies will only pay book value and a car's market at that age is not very high (plus, insurers' "independent surveys" find the lousiest old dogs, cite them as "market sales" and those sales are 2 to 6 months old so one can no longer inspect those cars' condition to challenge the price - they're long gone - and that ends the negotiation).
Biodiesel seems to be driving the choice of car. Should it?
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T.G. Lambach wrote:

Following Richard Sexton's advice, perhaps I shouldn't do that myself. 300psi is good for washing my house, probably not so good blowing toward my face. ;)

I'm guessing that an experienced mechanic could probably tell by ear if the valve adjustment is decent, though? I know on my '83 Civic I could tell by listening if one of the valves was too far out, and it was time to pop the valve cover off for an hour or so.

Sure- I've done old cars and know the TLC that goes into them. ...however, with what I /do/ know about maintenance, it's stuff that I actually can work on successfully.

Ah, but you picked up on my driving motivation below.

Heh good point. ...images of playground balls shooting out of the steering wheel have entered my mind...

That's exactly why I'm chasing after them. It's not the money I'm worried about, it's being vehemently in favor of weaning our world off of fossil fuels. Change starts at home, yadda yadda.

Sure, but then there's negotiation and sometimes you just find that a place has got a lot of them.

I'm not getting the car so I can wreck it. ;) (The airbags weren't my concern, honest!)

How many soldiers need to die to keep those Hummers on the road? I live near a marine base, and it fills me with so much sadness every time I hear of another group getting deployed to secure "our" oil in the Middle East. I'm going to do what I can to bring our boys home and keep them here.
In the end, these are questions that we answer for ourselves. My mind is made up for reasons that do not include financial ones.
-tom!
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300SDL is 4-5" stretched than 300SD. All the extra space is allocated to rear passenger so you can imagine it is like a limo ride back there. 300SDL was made for 86 and 87 only and that was when aluminum head started. If you are looking for iron head (which outlasts everything else on the car :-), stay with 300SD (or W123) and no later than 85.
If you want a diesel wagon, that is W123 only. W124 has wagon too but that is aluminum head.
In fact, aluminum head is OK as long as it is not overheated. Of course, iron head can withstand mistake which means bullet proof reliability.
Tom Plunket wrote:

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The 6 cylinder engines were the ones with aluminum heads, the 5's had iron heads.
The 300SDL had an aluminum head, the 300SD didn't. The 350SDL did.
The 190D 2.5 did.

No.
The 126's are less reliable as they have more stuff. Electric everything; more parts means more things can fail.
4" more rear legroom. Same as the difference between a 500SE and a 500SEL.

Diesel compression tests are not a DIY thing. Diesel fuel at that pressure can cause gangrene. You take it to a diesel shop.
--
Need Mercedes parts? http://parts.mbz.org
Richard Sexton | Mercedes stuff: http://mbz.org
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