Mercedes ML & GL OM642 Engine
Oil Cooler Seal Repair – Fixing a Well-Known Weak Point Properly
The Mercedes ML and GL platforms equipped with the OM642 V6 diesel engine are known for their torque, longevity, and effortless cruising ability. These engines have powered everything from luxury SUVs to commercial vehicles, earning a reputation for strong performance and durability.
However, like any complex piece of engineering, the OM642 has a known weak point that almost every owner will face at some stage – the oil cooler seals, and in some cases, the oil cooler itself.
When this issue appears, it is not something that can be ignored or patched over. It requires a thorough, labour-intensive repair done the right way.
The OM642 V6 Diesel – Brilliant Design, One Critical Flaw
The OM642 is a compact, high-output V6 diesel that places the oil cooler deep within the engine valley. From an engineering perspective, this keeps the engine compact and efficient. From a servicing perspective, it means access is difficult and time-consuming.
Over time, the oil cooler seals harden and lose elasticity due to heat cycles and age. Once this happens, oil and coolant are no longer properly separated, leading to internal leaks that slowly make their way out of the engine valley.
This is not a rare fault. It is a known and well-documented issue across ML and GL models running the OM642.
Common Symptoms of Oil Cooler Seal Failure
Owners usually notice one or more of the following signs:
Oil residue visible in the coolant expansion tank
Oil mixed with coolant, creating sludge
Coolant leaks with no obvious external source
Coolant running down the rear of the engine or gearbox housing
Strong oil smell and buildup trapped in the V of the engine
In many cases, the leak is not immediately visible from above or below, which is why these engines are often misdiagnosed until the problem becomes severe.
Why This Repair Is Labour-Intensive
The oil cooler sits beneath the intake system, directly in the engine valley. To access it correctly, significant disassembly is required.
On this job, the repair process included:
Intake System Removal
The complete intake assembly must be removed to expose the engine valley and oil cooler housing. This step alone requires care, precision, and proper reassembly to avoid boost or vacuum leaks.
Accessing the Oil Cooler
Once the intake is removed, the oil cooler becomes visible within the V of the engine. By this stage, oil residue and contamination are often clearly present.
Oil Cooler Seal Replacement
The old seals are removed and replaced with updated, high-quality components. In some cases, the oil cooler itself is also replaced if wear or damage is found.
Valley Cleaning
The engine valley is thoroughly cleaned to remove oil, sludge, and debris. This step is critical to prevent lingering smells, heat-related issues, and future contamination.
Reassembly and Leak Testing
Everything is reassembled to factory specifications, followed by a full system pressure test and leak check to confirm the repair has fully resolved the issue.
Why Shortcuts Do Not Work on OM642 Repairs
There is no shortcut repair for oil cooler seal failure on the OM642. Additives, partial disassembly, or external resealing attempts simply delay the inevitable and often make the job more expensive later.
Because of the labour involved, this repair needs to be done once and done properly. When completed correctly, it restores long-term reliability and prevents further damage to cooling and lubrication systems.
A Long-Term Fix
Once the oil cooler seals are replaced and the engine is cleaned and reassembled correctly, the issue is resolved for the long term.
No more oil in the coolant.
No more hidden leaks.
No more contamination slowly damaging the engine.
This is one of those repairs that separates proper specialist work from general servicing, and it is exactly why experience with European diesel engines matters.
Mercedes Diesel Specialists
The OM642 is a strong engine when maintained correctly, and repairs like this allow these vehicles to continue delivering the comfort, torque, and reliability they were designed for.
If your Mercedes ML or GL is showing signs of oil cooler seal failure, early diagnosis and correct repair can save significant time and expense in the long run.
Call (08) 6280 2087 to speak with our team, or book in your Mercedes service online.
