Mercedes-Benz BlueTEC – Leaking Oil Cooler
If you own a Mercedes Benz with a BlueTEC diesel engine, chances are you may have heard quite a bit about the oil cooler leaks that have plagued some models. It is, in fact, a common issue with many of the BlueTEC diesel Mercedes’ but if you’re like many of the partially informed owners out there, you are probably left with a lot of questions and wondering things like, “Which BlueTEC engines have oil cooler leaks?”, “What Mercedes diesel models and model-years are affected?”, and “What are the signs of a leaking oil cooler?” That’s where we come in. We’re here to clear up that confusion for you in this no-nonsense, straightforward format that you can reference. First, let’s cover a little basic information about what’s going on…

Bottom of oil cooler. You can see the location for the seals on each end of the cooler.
As we stated above, engine oil cooler leaks are a common issue with certain Mercedes BlueTEC diesel cars. Specifically, the Mercedes “3.0 liter V6 diesel” engines, designated with the engine code “OM642”, are the afflicted units. These engines can be found in a wide range of models over several years and, in fact, Mercedes issued a “TSB”, or “technical service bulletin”, relating to this exact issue and what to do to correct it. Essentially, it was discovered that an inordinate amount of OM642 engines developed leaks from the oil cooler seals which had degraded prematurely from the significant amount of heat they are exposed to. The oil cooler itself is positioned right in the center of the “V” of the engine, towards the rear of the engine compartment. All engines create heat while running but, due to the unique way these engines operate, they create a LOT of heat. Add to that the oil cooler’s thermally insulated location on the engine beneath myriad intake plumbing and so forth, the oil cooler seals simply can’t take it.

Location in engine “V” where the oil cooler is mounted.
To complicate matters further, the oil that leaks from those seals flows backwards, through a pair of weep holes (pictured below), and down through the transmission bell housing (this is the large metal housing that surrounds where the transmission is coupled to the rear of the engine) where it finally becomes visible from underneath the vehicle during oil changes or other routine service. When these services or maintenance tasks are being done at a shop that doesn’t specialize in Mercedes-Benz repair, it is very common for the technician or do-it-yourselfer to incorrectly identify the source as a leaky rear main seal (which typically presents in an apparently similar way), leading to costly and unnecessary repairs that don’t solve the problem in the slightest. An experienced Mercedes specialist shop would immediately recognize the true culprit, or at the very least know to check the oil cooler for leaks prior to making any recommendations to the customer, potentially saving them thousands of dollars.

Location of weep hole where leaking oil drains, eventually down through the bell housing.
So what is the fix for keeping those oil leaks from reappearing as quickly as they occurred the first time? If the problem is seals that can’t stand the heat that they are exposed to everyday, is there any point to replacing them for another inadequate set of seals? Fortunately, Mercedes-Benz solved the issue in 2010 by updating those parts and now the only approved, effective solution is to replace those old seals with the new ones, made of an entirely different material called “Viton” rubber that is far better suited to the application, resists heat degradation better than the original rubber, and last far longer than the original seals. You can see the two parts in the image below, comparing an old, red/orange rubber seal with the new, superior Viton rubber seals which are purple. It is these purple seals that Mercedes-Bens adopted and are now the factory standard part.

Comparison of old and new oil cooler seals.
Here are the quick facts you need to know…

Issue: 3L BlueTEC V6 diesel engine oil cooler seal leaking

Related TSB: LI18.30-P-055434 “Traces of Oil in Area of Oil Cooler (Inner V) on Engine 642” (Source)

Engine Code: OM642

Affected BlueTEC Models: Various C-Class, S-Class, B-Class, GL/G-Clsss, R-Class, ML-Class, as well as a handful of Sprinter vans, and the Euro-Australian spec Chrysler 300C equipped with this engine.

post-2005 until pre-2010