TL;DR
Mercedes-Benz OM642 BlueTEC diesel engines commonly develop oil cooler seal leaks — the fix is replacing worn rubber seals with heat-resistant Viton seals, a solution Mercedes adopted as the factory standard in 2010.
Bottom of oil cooler. You can see the location for the seals on each end of the 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…
What’s Actually Going On With the OM642 BlueTEC Oil Cooler?
Location in engine “V” where the oil cooler is mounted.
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.
“The most common oil leak we see is the oil cooler on the OM642. The oil leaks in the valley of the engine and ends up on the side/back of the motor and front of the transmission. This is frequently misdiagnosed as a rear main seal, another costly repair.” – Matt Novy, Technician, Fifth Gear Automotive Repair
Why the Leak Is So Often Misdiagnosed
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.
Location of weep hole where leaking oil drains, eventually down through the bell housing.
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.
The Correct Fix: Viton Rubber Seals — Not a Band-Aid
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-Benz adopted and are now the factory standard part.
Comparison of old and new oil cooler seals.
“In general, Mercedes-Benz vehicles are more complex and more difficult to work on than domestic vehicles. There are many more computers, wires, and components to fail. A good understanding of these systems and how they function is required to diagnose and repair them accurately.” – Matt Novy, Technician, Fifth Gear Automotive Repair
Quick Facts: OM642 BlueTEC Oil Cooler Leak
Here are the quick facts you need to know…
OM642 Oil Cooler Leak — At a Glance
Quick Facts: OM642 BlueTEC Oil Cooler Leak
Here are the quick facts you need to know…
OM642 Oil Cooler Leak — At a Glance
| Issue / Field | Details |
|---|---|
| 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-Class, R-Class, ML-Class; select Sprinter vans; Euro-Australian spec Chrysler 300C with this engine. |
| Model-Years | Post-2005 until pre-2010 |
Mercedes-Benz Repair and Maintenance You Can Count On at Fifth Gear Automotive
When it comes to Mercedes-Benz BlueTEC diesel repair, having a specialist you can trust makes all the difference. Misdiagnosing an oil cooler leak as a rear main seal issue can result in an unnecessary repair that costs thousands of dollars and still leaves the problem unsolved. The right shop — one that knows these engines — will check the oil cooler first.
Stop by any of our locations in Lewisville, Argyle, McKinney, Aubrey – Crossroads, Frisco, Allen, Southlake, Castle Hills, or North Fort Worth for expert German auto care.
Frequently Asked Questions
How is an OM642 oil cooler leak different from a rear main seal leak?
Both present as oil visible beneath the rear of the engine, but the OM642 leak originates at the oil cooler seals in the engine's center V. Oil travels backward through weep holes and down the bell housing, mimicking a rear main seal leak. A Mercedes specialist will test for the oil cooler source first to avoid an unnecessary and costly repair.
Why did the original oil cooler seals fail in the first place?
The oil cooler sits in a thermally insulated location in the middle of the engine's V, surrounded by intake plumbing that traps heat. The original rubber seals degraded prematurely under that sustained heat exposure — a design shortcoming Mercedes-Benz formally acknowledged by issuing TSB LI18.30-P-055434.
What is Viton rubber and why is it used for the replacement seals?
Viton is a fluoroelastomer material that offers significantly greater heat resistance and durability compared to the original rubber. Mercedes-Benz adopted Viton seals as the factory-standard replacement in 2010. The new seals are purple in color, compared to the original red/orange rubber.
Will replacing the seals once solve the problem permanently?
Yes — provided the updated Viton seals are used. Replacing the failed seals with another set of the original rubber seals would simply repeat the cycle. Using the Mercedes-approved Viton rubber seals, which are now the factory standard, is the only effective, long-term solution.
Methodology
This post reflects John Miller’s professional experience, the technical expertise of Fifth Gear technician Matt Novy, and the broader Fifth Gear Automotive Repair team, which has been diagnosing and repairing Mercedes-Benz BlueTEC diesel vehicles across the DFW metroplex since 2003. Failure patterns referenced are based on in-shop experience with OM642 oil cooler repairs. Reference is made to Mercedes-Benz TSB LI18.30-P-055434.
Sources & Further Reading
- Fifth Gear Automotive Repair — fifthgear.biz
- Mercedes-Benz TSB LI18.30-P-055434 — “Traces of Oil in Area of Oil Cooler (Inner V) on Engine 642”
- Matt Novy, Technician, Fifth Gear Automotive Repair — Technical contributor and subject matter expert on Mercedes-Benz OM642 BlueTEC diagnosis and repair
- Chemours (DuPont) — Viton Fluoroelastomer: Heat and Chemical Resistance Properties, chemours.com/viton
John Miller is the Director of Business Development at Fifth Gear Automotive.
Fifth Gear Automotive Repair provides automotive service and repair. We service all makes and models – both domestic and import. We specialize in European auto repair for BMW, VW, Mercedes, Porsche, Audi, Volvo, Jaguar, Land Rover and most other popular European marques.
Fifth Gear Automotive has 10 locations – Allen, Argyle, Aubrey-Crossroads, Castle Hills, Frisco, Lewisville, McKinney, North Forth Worth, and Southlake, Texas.
