Originally Posted by suginami

You must be the luckiest 350 SDL owner on the face of the earth to have your engine last this long. This is the worst engine Mercedes ever built.

Read this:

INTRODUCTION

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.

This is the first substantive bulletin to those interested in this engine. Hopefully the mailing list is relatively clean. If you know of someone that should be added, please advise.

Why this Group? Because this engine contains a design defect. It demonstrates significant wear quite early. It is expensive to repair. Mercedes will disclaim responsibility whenever possible.

Which Cars? S-Class TurboDiesel Mercedes model years 1990 through 1995. Includes W-126 with engine 603.970 (1990 350SDL, 1991 350SD, 1991 350SDL) and W-140 with engine 603.971 (1992 300SD, 1993 300SD, 1994 S350, 1995 S350.)

Symptoms? The most common symptom is excessive oil consumption - 1 quart in as few as 100 miles is not unheard of. Other symptoms include throbbing idle, smoke, mechanical noise. Many owners report an episode of the engine 'bogging down,' 'grunting,' or similar such description, followed by a puff of smoke.

Diagnostic Steps? Monitor oil consumption very carefully. Opinions vary on what is 'normal,' but certainly an increase in consumption needs to be investigated. Most people consider 1 quart per 1,000 miles to be acceptable; 1 quart per 500 miles is probably not.

Next step is a compression test. A wet and dry leakage test will help determine whether problems are related to rings, or to valves. Engine warm, all injectors removed. Design specifications call for compression of x-x psi, with a range between highest and lowest of no more than x.x psi. (Source: )

Assuming further investigation is indicated, the head will be removed. May reveal carbon buildup in combustion chamber; worn valve guides or seals; scored cylinder walls. Most critical: deck height, i.e., whether all pistons rise to the same level. Differences indicate bent connecting rods and/or pistons. Further disassembly may reveal worn or broken rings, deformed pistons and/or out-of-round cylinders.

Likely cause? Mercedes diesels have traditionally been considered high mileage engines. These include the 4-cylinder 616 engine in the 240D, the 5-cylinder 617.95x in the 300D and early 300SD, and the 6 cylinder 3.0 litre 603.961 in the later year 300SD models. None of these experienced the frequency of problems that the 3.5 has. The 3.5 litre 603 is simply a 'bored out' version of the 3.0 litre block in search of greater displacement and power. In this writer's opinion, the problems likely stem entirely from this boring out resulting in insufficient material between adjacent cylinders which may compromise cooling or lubrication or the integrity of the head gasket. In late 1990 the design of the head bolts was changed, and as of the 1992 model year the head gasket was modified. However, problems persisted.

This investigation is in its infancy, and so I am certain that someone out there has more and better information. Please share it with me so that I can share it with the group. Future bulletins will discuss strategies for getting MBNA assistance for out-of-warranty repairs.

Stay tuned.
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Here's my painfully acute understanding of "the problem" with the 350 diesel engine. I take no pride of authorship and may very well have one or more statements wrong. So the rest of you guys and gals PLEASE jump right in and correct me. Here's what I understand led to my $6,000 bath with my 91 350SDL
MB increased the displacement of the engine from ~3000 cc to ~3,500 cc.
To do this they changed the bore and/or stroke.
They switched to lighter weight (read weaker) piston rods.
Due to the now lighter than before piston rods combined with some (unknown to me) massive force, the number 1 rod bends. I've seen and held one of these bent rods. It's a HUGE thing. The forces required to bend it must be massive.
Number 1 piston, still attached to now-bent number 1 rod, is still merrily churning away. But it is no longer churning in perfecto alignment with its mate, number 1 cylinder.
Number 1 cylinder now begins wearing in an elliptical fashion, much like an egg.
Oil goes up past the rings, unburned fuel goes down past the rings. Bad boogie. Oil consumption increases, engine oil becomes diluted with diesel, further exacerbating the entire process.
If there is any good news in this sordid tale, it's that MB has "worked" with owners 350's that have less than 100k miles. I personally know of an offer to one list member to replace the parts if he'll go for the labor. Another lister, Jackie Mason, recently shared that his S350 that's for sale has a new engine courtesy of MB. His may have been covered by a Starmark warranty, now that I recall his e-mail.
But regardless, as Pete's earlier message pointed out, MB is painfully aware of the problem. I believe Richard Easley may know the name and/or email of one person who has been organizing disgruntled 350 owners. That might be a starting point on any crusade for justice.

The only other thing I can add is that we really, really miss our 350. Sweet car. I just couldn't afford the $8-000 to $10,000 rebuild at that time. I couldn't sell it my list friends if I suspected a problem, eh? So I gave it away to a dealer in trade. Back to the same one I bought it from. Seemed fair.

I hope your situation works out better than ours did.

Mark Duckworth
Austin, TX
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I can give you SOME information on 3.5 liter OM603 engines, but
it does NOT refer to the engine number, only the chassis numbers of the
cars the engines were used in! Model W140s from chassis numbers A092142
to A202313 had the engines that were to be inspected IF the owner
complained. This is according to TSB 05/93 dated Nov '95

In the past, MB had a stamped or plate affixed to new engines (long or
short block). I don't know about rebuilt engines. In my '67 200D the new
short block came with a metal plate that was to be affixed to the block
by pins that were driven in. I attached the plate to the block. I don't
recall any number cast or punched into the block, but that doesn't mean
it wasn't there. It's been a lonmg time (almost 30 years!).

Marshall
August M. Booth, Jr. Ph.D.
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The S350 was the subject of the TSB 05/93 dated Nov '95 about stretched
timing chains. If the car was dealer serviced, then that dealer did the
owner a BIG disservice by not picking up on the problem. The owner
didn't do him/herself any favors by remaining oblivious of the symptoms
that accompany extensive chain stretch. If the the chain stretch WAS
addressed at some earlier time and a new chain fitted, then I would
question the repair.

I'll pass this on to John Blazer and Stu Ritter.

Marshall
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From: David Stagner (stagnerd1@texoma.net)

I have a 1993 300SD with 70K miles. It has a bad engine. Most of
these 3.5 L diesel engines will fail prematurely. MB is unwilling to
accept responsibility for their flawed design. I have a law license,
and intend to obtain the data, through litigation, to ascertain whether
MB committed fraud and other torts in conjunction with the design,
manufacturing, and marketing of the vehicles sold with the subject
engine.

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All the 3.5 engines (they stopped production in '95 TO MY
KNOWLEDGE) had the problem. The rebuilds (there are NO NEW
ENGINES) with modified rods and SOME with modified pistons,
have to date have not failed (to my knowledge), but few of
them have even 100kmi on them. Failures tended to be MUCH
higher on cars that were almost exclusively city driven and
MUCH lower on cars that were largely highway driven. While
MB has said that the problem was confined to US cars, at
least one European resident reported that it's well known
there that the 3.5 engine is one to stay away from as it was
troublesome in Europe as well.

August M. Booth, Jr. Ph.D.