2004 Mercedes-Benz ML500 Coolant Reservoir Replacement Cost

As the engine coolant temperature rises, the coolant expands, and the radiator cap allows excess pressure to bleed off. The reservoir is connected to the pressure relief valve on the radiator, and accepts coolant when the radiator cap opens the valve. When the engine coolant temperature falls, causing the coolant volume to decrease, the radiator sucks coolant back from the coolant reservoir, keeping the cooling system full, and free of air.

If the coolant reservoir becomes cracked or empty, the coolant level warning light will illuminate on the instrument cluster, alerting the driver that the coolant system is low on engine coolant. This is because the coolant level sensor is inside the coolant reservoir, so when the reservoir is empty, the sensor records coolant absence, and transmits it to the vehicle's computer. In the event of a cracked reservoir, there will be a very noticeable coolant leak on the ground, underneath the front of the vehicle.

Driving a vehicle with a faulty engine cooling system can result in overheating, cylinder head gasket failure, engine block failure, or cylinder head warping. It is never recommended to drive a vehicle with engine cooling issues, especially with modern engine casting materials.

Reservoir leaks are typical from the inlet hose, but breakage of the reservoir is not uncommon especially on those mounted directly on the radiator.

When an issue arises with the engine coolant reservoir, the coolant hose from the radiator, and the pressure relief valve will be inspected to ensure they have not become clogged. Also, the radiator cap should be tested to ensure it is releasing engine coolant into the reservoir at the proper system pressure. If the reservoir was noted to be leaking, it will be immediately diagnosed as failed, and replaced, unless the leak originates from the inlet hose.

Replacing the engine coolant reservoir is normally a quick and easy task, and does not require draining coolant from the vehicle in most cases. If the coolant is contaminated, or has not been changed or flushed per manufacturer scheduled maintenance intervals, the coolant should be flushed and refilled. The hose connecting the reservoir and radiator will be clamped, and disconnected from the reservoir. If the reservoir is empty due to cracking, it will not require draining. The old reservoir is removed from the engine bay, and the replacement coolant reservoir installed. The hose to the pressure relief valve at the top of the radiator will be installed, and the clamp removed to allow coolant flow. The reservoir is then filled, and the system tested to ensure the customer’s complaint is resolved.

Engine coolant flushes are often recommended needlessly, raising repair costs. However, there is a schedule for when these services should be performed. The factory scheduled maintenance section in the owner’s manual provides information for the customer concerning which maintenance items should be performed, and when they are needed. We always recommend following the factory scheduled maintenance intervals for preventive maintenance. If a leak is being repaired, and the coolant may be drained and replaced. Flushing the coolant system should be performed if excessive corrosion is present, or if the factory scheduled service interval has passed.

It is a good practice turn the heater onto its highest setting if the engine begins to overheat. The heater core can act as a secondary radiator, but temperatures inside the vehicle may become unbearable. Some coolant reservoirs are under constant pressure, and opening the reservoir or radiator while the engine is hot may result in severe scalding, requiring immediate medical attention.

The average DIYer can maintain the engine cooling system without much trouble. However, diagnosing the system can be a daunting task if the function of each individual component is not fully understood. If the cause of overheating is known, repairing this issue as a DIY is generally recommended for the average DIYer. If knowledge of the engine cooling system and diagnosing cooling issues are lacking, this repair should be handed to a certified technician.