Black coolant: likely causes and risk to the engine

Fresh coolant is pink, yellow, green or blue, and stays clear. Black points to an internal problem. Each cause has its own fix: changing the fluid without stopping the leak does nothing.

The serious path is oil in the circuit: a head gasket, or a porous oil-to-water cooler (the French article cites some 1.6 HDi coolers). An iridescent film, an oily look, sometimes a mayonnaise. Oil kills the additives, thickens the fluid, and can block the radiator, thermostat or water pump.

With no oil, it can be corrosion, hoses shedding rubber, mixing coolant families (OAT / HOAT / IAT), or fluid that is simply too old. The exact interval is the manufacturer’s.

Overheating, a fan that never stops, level dropping with no puddle on the ground, foam in the tank, a sweet smell. Driving on oily black coolant can seize the pump and warp the head.

Cost ranges quoted in the French article (parts + labour, not a quote): head gasket often €800–1,500 (sometimes more), water pump €150–400, oil cooler €300–800.

• Engine cold. Never the cap when hot.
• Tank and cap: colour, film, mayonnaise, level.
• CO₂ test (combustion gas in the coolant) and a pressure test on the circuit.
• Confirm before you open the engine.

Repair the cause, then a double flush, then the coolant in the book. Not a random mix.