300 SDL or 350SDL, that is the question!
 

Am actively looking to buy one of these models, looking for 87-89 but ran into a 350SDL,91 model...have had some folks telling me to stay away from the 350and was wanting more backround from some experts...other owners. I am a former heavy truck mechanic and farmer and have been around diesels all my life.. currently own cummins diesel in dodge. please give as much info as possible so i can make an informed decision. technicians opinions welcome. This is my first post here, so thanks in advance to all who reply. Username is bigjoe, but my friends call me Shrek! see ya!

Do a search on `350SDL'.

You will find all the information you need. There has been extensive discussion, in depth on this forum.

Good luck!

The 3.5L OM603.97x engine was used from 1990-91 (on the W126 chasis) and 1992-95 (W140 chasis). About half of the engines would "oval out" the cylinders and/or bend a rod. It usually happened with low miles (say, under 100kmi) but not always. One of the warning signs is increased oil consumption that keeps getting worse and worse. Usually by the time you notice it's too late. The fix is either a new bottom end or a new engine, which will run $6k-$12k plus labor.

I copied the following from a newsletter on the problems with this engine:

This newsletter is being made available via the world wide web to all
those interested in the problems with the 3.5 liter turbodiesel engine
used in S-Class Mercedes between 1990 and 1995. It incorporates
significant new research and information and the input of several list
members since my last communication some time ago.

Briefly, this engine has problems. With distressing frequency these
engines show significant, grossly premature wear, requiring expensive
rebuilding. Symptoms are excessive oil consumption, mechanical noise,
smoke, and a throbbing idle. Diagnostic procedures usually reveal
that one or more cylinders have compression that is below
specification, with excessive variation in the compression among all
cylinders. Mechanical teardown frequently reveals cylinders that are
out of round and connecting rods that are bent. Catastrophic engine
failure has occurred quite often. These problems are clearly due to a
design defect that Mercedes refuses to acknowledge. These problems
can not be prevented with any sort of routine maintenance, nor can
they be forestalled with changes in driving behavior. In essence, the
engine self-destructs during normal operation. Symptoms have appeared
as early as 50,000 miles. Due to the expense of individual repairs,
Mercedes honors warranty repairs only after considerable pressure is
applied. Out-of-warranty claims are summarily rejected.

So after reading the previous post, by all means, find a nice '86 or '87 300 SDL. The 300 SDL was not sold in '88 or '89.

They are very nice cars, and there are many members on this site who happily own them.

The 86-87 SDLs have problems of their own in the way of cylinder heads that develop cracks. I don't have data to suggest that the 603.96 head problem is more or less prevalent than the 603.97 cylinder and rod problem but there is more negative discussion around the 603.97 considering how few you see on the road.

Replacing a 603.96 head is a reasonable DIY job.

Sixto
91 300SE
87 300SDL
83 300SD

You might also include in your search the '81-'85 300 SD.

They have an all iron diesel engine, and are not known to have any problems.

I don't consider this an issue worth the keystrokes considering many many cars today have aluminum heads with the same "chances." My car went up to 120dg a few times for various reasons and no problems here. Cast iron heads do crack as well, they are not bulletproof.

Mine has also seen close to 120c on several occasions (hot hot days with AC up running a 5% grade at full throttle for 5 minutes).

My head is original as far as I can tell and I have no issues right now (knock on the zebrano wood).

Mercedes redesigned the head on the 603.960 engine three times and redesigned the head gasket 5-6 times. These engines are known to have head cracking problems more frequently than other aluminium head engines. However, it seems that 20% or so of these engines had head problems, whereas 50% or more of the 350's had bottom end problems. Fortunately, head replacement is much easier than a new bottom end. Most of these head problems occurred when excessive heat was introduced from the trap oxidizer (since removed) or from broken rad necks, etc.

How about....
 

So what you really want is a 92 -95 140 car with a blown engine into which you can put a 603.960 powerplant! All the 140's latest available bells and whistles and the second most dependable engine.

SLOW. The W140 weighs in at ~4700lbs.

The 140 cars may look nice and ride well, but they are very complicated and MB has cut costs on some of those critical things like WIRING which are everywhere - to control all the electronics. Evaps fail faster on 140's that on previous models, etc.

I'd recommed a 86-87 300SDL for a balance of power, simplicity, and ride.

Is the emphasis on the 'seems?' 20% and 50% failure rates are astronomic even by Edsel standards. There's no way to avoid a class action suit when it gets that bad.

I'd expect about as much flap as were hear now with 2% and 5% failure rates.

Sixto
91 300SE
87 300SDL
83 300SD
... 1 in 5 or 1 in 50... either way, I have a new head :(

Being a farmer and a diesel mechanic you need to get a 1981-1985 300SD in very good to excellent condition. You will not regret it :).

Fortunately, head replacement is much easier than a new bottom end. Most of these head problems occurred when excessive heat was introduced from the trap oxidizer (since removed) or from broken rad necks, etc.

This must explain a good part of it I would imagine. My car never even came with the trap oxidizer. They should have called it the head crakidizor/turbokiller instead. I wouldn't buy an sd over an aluminum head issue though. The sd doesn't seem to have much more interior room than an accord! It's all body! The sdl however, is a drastic difference and tons of room even by today's standards. SD's sure are great cars, don't get me wrong, but I looked at an 83' and it had minimal room inside. I was dis-appointed. Are all sd's created equal in this regard?? Why such a drastic difference anyway?