Debunking old Mercedes "Myths"

Ever come across a Craigslist ad featuring an old rusty diesel w123 or w126 Mercedes and they're asking an exorbitant amount of money based on a "bulletproof, million mile engine", extreme mileage, "runs on any oil", etc?  Well I'm here to set the record straight.

Despite the main aim of this article being the w123 and w126, the logic outlined in this post pretty much applies to any Mercedes diesel up until the late 1990s so.


Myth #1: This engine will go a million miles!
Will it?  Well, the answer is yes and no.  First of all,  its important to establish which engine you're talking about.  As far as Mercedes diesels go, there have been a ton of engines over the years.  Wikipedia is actually a pretty good source for getting a high level overview of these engines.  Here are a number of Wikipedia links where you can read up on each one:

• Mercedes-Benz OM601 engine
• Mercedes-Benz OM602 engine
• Mercedes-Benz OM603
• Mercedes-Benz OM604 engine
• Mercedes-Benz OM605 engine
• Mercedes-Benz OM606 engine
• Mercedes-Benz OM611 engine
• Mercedes-Benz OM612 engine
• Mercedes-Benz OM615
• Mercedes-Benz OM616
• Mercedes-Benz OM617 engine
• Mercedes-Benz OM642
• Mercedes-Benz OM647 engine
• Mercedes-Benz OM648 engine

The engine most commonly referred to as "bulletproof" is the OM617 (and pretty much anything earlier).  This came in the w123 and w126 chassis.  So the nagging question: is it as bullet proof as it's reputation?  The answer comes down to maintenance and care.  Theoretically, many engines can last for hundreds of thousands of miles without a rebuild as long as proper maintenance is followed.  The reason (in my experience) that the OM617 has such a great reputation is because it can put up with a ton of abuse.  Now I am NOT condoning redline launches off of every stop sign, or never changing your oil again!  The OM617 simply has a very stout block and very stout internal components.  I have seen original engines with 400k on the odometer run like clockwork with minimal blow-by--which leads me to the #1 layman's way of quickly assessing engine health:

Blow-by - oil vapor that escapes past the piston rings and into the head.  You check blow-by by opening the oil cap while running after the engine warms up.  Just be careful, oil is hot and its often splashing under that cap!  Most older engines will have some blow-by.
Here is an example from the wonderful interwebz:
http://www.youtube.com/watch?v=BnXaMkyA0hw

Blow-by is a decent way of assessing the health of the engine because it is related to the compression of the cylinders which is essential in maintaining diesel ignition.  If you have concerns over blow-by before you buy a car, have the seller conduct a compression test.  A good rule of thumb is that no cylinder should be below 300psi, and they should all be within 10% of each other.  Good compression usually  means a well maintained engine.  Keep changing that oil in the correct intervals, keep your valves adjusted, and keep an eye on the timing chain and you could easily go to a few hundred thousand miles or well beyond.  I mentioned timing chain stretch - this one is also important as it will determine its replacement, and is also a good indicator of maintenance performed on the engine over its lifetime.

Here is a good DIY for checking chain stretch: http://www.peachparts.com/Wikka/OM617TimingChainStretch

Third tell-tale sign of engine health is the fuel system.  Is the see-through plastic fuel filter all gunked up?  Is it black or really dark brown?  That is a sign of fungi in the fuel tank.  Not necessarily a deal-breaker but it will give you an indication as to how well the car has been cared for.

If you don't believe me, there is also an awesome wikipedia site on longevity:
http://en.wikipedia.org/wiki/Car_longevity


Note that a 240D (OM616, basically a 4 cyl OM617) hit 2.8 million miles!  So yes, they are million mile cars when it boils down to it!

Myth #2: 50 MILES PER GALLON!  100 MPG!

No.  Won't happen.   Diesel engines in cars DO perform rather efficiently (sad that the U.S. hasn't really picked up on it).  For example, the new VW and Audi diesels get excellent mileage in the 40s--older models like the Mk4 Jetta TDI were able to achieve 50mpg (for real, not a myth!).  The OM617 in both the w123 and w126 are unable to achieve 35mpg unless you're going downhill with the  wind at your back, etc etc.  The w126 is a bit more efficient on the highway (lower drag coefficient) and I have personally hit 33mpg in mine.  The later 190D (w201) and 300D (w124) have better mileage due to more efficient and more powerful engines, but most report getting numbers in the 30's.  If you see an OM617 powered car for sale claiming 40mpg or higher its a bunch of BS.

Myth #3 Runs on anything you can dump in the tank!  Great for veggie oil conversion!
Technically true for anything combustible but its not too good for the engine.  Be very wary of existing WVO (Waste Veggie Oil) systems.  When all parts work, they are nothing to shy away from, BUT I have seen many, many WVO-equipped diesel MBs in the junkyard with crapped out engines.  Anecdotal?  A bit, but there is science behind it.  The reason that WVO systems really make me wary is that longevity via WVO is entirely contingent upon the ability to filter and process the oil initially.  Therefore, if the owner is a bit lazy or simply makes a mistake, it can seriously damage the fuel injection system (death to motor, basically).  Furthermore, the temperature of WVO is much more sensitive than normal diesel fuel...not a problem in the sunny southwest, but up here in the Northeast, an improperly engineered oil heating system can also cause the aforementioned fuel system woes.  Last, there are a number of systems on the market.  Some are better and some are worse (you get what you pay for!)...and some are even backyard-engineered.  If a seller is asking for higher price based on the ability to convert to WVO, just be aware that the "craze" has sort of died down in recent years.  While the OM617 is a fine candidate for those interested in WVO conversion, there is no reason to pay extra for some hype.