Mercedes Benz GL350 Water Pump Replacement

Written By Adam Clune

Cooling system leak diagnosis and repair on the OM642 engine

When this Mercedes Benz GL350 arrived at our Perth workshop, the customer reported that the coolant warning light was appearing on the dash. After inspecting the vehicle, we confirmed a coolant leak at the water pump, a known issue on Mercedes vehicles fitted with the OM642 3.0 V6 diesel engine.

At EU Auto, we specialise in Mercedes servicing and repairs and see this failure often across GL-Class, ML-Class, and other Mercedes models using the same engine platform.

 

Customer Concern: Coolant Warning Light Appearing

The coolant warning light usually appears when the system detects low coolant level or a pressure drop. In this case, the customer noted no obvious signs of leaking on the ground, which is typical for OM642 water pump failures because many leaks occur higher up on the engine and evaporate before reaching the ground.

To confirm the source, we performed a full cooling system pressure test, which immediately identified the leak from the water pump housing.

 

What We Found: Water Pump Seal Failure

Once the engine covers and pipework were removed, we could see:

  • Coolant residue and staining around the water pump body

  • Corrosion around the pump housing

  • Evidence of coolant weeping from the pump's internal seal

On the OM642 engine, the water pump is driven by the main auxiliary belt. When the internal seal begins to fail, coolant escapes through the weep hole and collects around the housing. This also risks coolant contamination of the belt system, which can lead to belt noise or premature belt wear.

 

Why OM642 Water Pumps Commonly Fail

Mercedes’ OM642 V6 diesel is robust, but its cooling system has a few well-known weak points:

  • The water pump sits low in the V, where heat is concentrated

  • The internal seals deteriorate over time, especially at higher mileage

  • Leaks often start slowly and only show under pressure

  • Access is tight, making visual detection difficult without stripping surrounding components

Regular servicing and prompt investigation of coolant warnings helps prevent overheating and more serious damage.

 

Repair Procedure: Accessing and Replacing the Water Pump

Replacing the water pump on the GL350 requires more labour than on many other vehicles because the pump cannot be accessed directly. For this repair, we completed the following steps:

1. Pressure test and fault confirmation

We confirmed the water pump leak through pressure testing, ruling out other potential sources such as the thermostat housing, crossover pipes, or radiators.

2. Remove EGR cooler and intake assembly

To gain access to the water pump on the OM642, the EGR cooler, associated coolant lines, and the air intake assembly must be removed. This gives enough clearance to reach the mounting bolts and replace the unit safely.

3. Drain coolant and disconnect hoses

We drained the cooling system and removed all hoses attached to the pump. Each sealing surface was cleaned thoroughly.

4. Remove the old water pump

The old pump showed significant corrosion and coolant staining. Removing it also allowed us to inspect the surrounding components for secondary damage.

5. Install the new pump

A new OEM-spec water pump was installed, with all bolts torqued to Mercedes specifications. We also inspected the belt for coolant contamination.

6. Reassemble the intake and EGR components

All removed components were reinstalled with new gaskets where required.

7. Refill, bleed, and test

We refilled the cooling system, bled all air from the circuit, and then performed a full leak check. Once complete, we ran the engine up to temperature and carried out a road test.

The cooling system stabilised correctly, and no further coolant loss was detected.

 

Old vs New Water Pump

The difference between the failed unit and the replacement was clear:

  • The old pump showed corrosion around the flange

  • Coolant residue was visible around the weep hole

  • The internal seal had degraded to the point of constant leakage

Replacing the pump restored proper coolant pressure and circulation.

 

Common Symptoms for Mercedes Owners to Watch For

If you own a Mercedes with the OM642 engine, look out for:

  • Coolant light appearing intermittently

  • Pink or red residue around the belt area

  • A sweet smell from the engine bay

  • Rising engine temperatures, especially under load

  • Coolant loss without visible drips on the ground

Early diagnosis prevents overheating and more expensive repairs.

 

Why Choose EU Auto for Mercedes Cooling System Repairs?

At EU Auto, we specialise in European vehicles and have extensive experience with Mercedes cooling system faults, including:

  • Water pump failures

  • Thermostat issues

  • Coolant crossover pipe leaks

  • Radiator and hose failures

Our workshop uses genuine or OEM-equivalent parts and follows Mercedes repair procedures to ensure reliability and longevity.

To learn more about how we support Mercedes owners, visit our
Mercedes Servicing and Repairs page.

 

Need a Cooling System Check for Your Mercedes?

If a coolant warning light has appeared on your Mercedes, or if you suspect a leak, our team can help. Prompt attention prevents larger and more expensive problems so call today on (08) 6150 5245

Adam Clune
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