The cooling system is probably the most underestimated component of a car. Many owners never give it a thought until the temperature needle swings into the red zone — and by that point, it is often already too late. An overheated engine can suffer catastrophic damage within a few minutes of driving: cylinder-head warping, gasket failure, piston seizure. And yet, cooling-circuit maintenance is either ignored or handled with a worrying carelessness.
Not All Coolants Are the Same
The Two Main Families
Coolants fall into two main categories, based on the corrosion-inhibitor technology they use, and these cannot be mixed with one another:
Inorganic-Based Coolants (IAT — Inorganic Acid Technology)
- Contain corrosion inhibitors based on silicates, phosphates, nitrites and borates
- Form a physical protective layer over the metal surfaces
- Shorter service life (change every 2-3 years / 50,000 km)
- Typically green or blue in color
- Older technology, used on pre-2000 vehicles
Organic-Based Coolants (OAT — Organic Acid Technology)
- Use inhibitors based on organic acids (sebacates, 2-ethylhexanoates)
- Protect only where needed, forming a molecular protective film solely over the corroded areas
- Much longer service life (up to 5 years / 150,000 km)
- Typically orange, pink or red in color
- Standard on almost all modern vehicles
There is also an intermediate category, HOAT (Hybrid OAT), which combines organic and inorganic inhibitors — typically yellow or turquoise in color.
The Problem of Mixing
Mixing IAT and OAT coolants is a serious mistake. The two chemistries react with one another, producing:
- Gelling of the inhibitors → obstruction of the cooling passages
- Loss of the anti-corrosion properties of both products
- Formation of deposits that reduce heat exchange
The color of the coolant can help identify the family, but it is not always reliable: some manufacturers use dyes that differ from the standard. The only safe way is to know the vehicle manufacturer's specification (e.g. VW G12++, BMW HT-12, MB 325.0) and to comply with it.
Tap Water: A Common and Devastating Mistake
One of the most widespread and most damaging practices is topping up the circuit with tap water. It seems harmless — "it's just water" — but the consequences are serious:
Limescale
Mains water contains calcium, magnesium and other minerals which, at the circuit's operating temperatures (80-110°C), precipitate and form limescale deposits:
- They settle on the radiator walls → reduce heat exchange
- They build up in the cylinder-head passages → create localized hot spots
- They encrust the thermostat → it can become stuck (open or closed)
- They clog the cabin heater tubes → the heating stops working
Bacteria and Algae
Mains water also contains microorganisms which, in a warm, humid closed system, find an ideal environment to proliferate:
- Formation of biofilm on the internal walls
- Microbiologically influenced corrosion (MIC) of the metal surfaces
- Clogging of the coolant filter (where fitted)
- Production of organic acids that attack gaskets and O-rings
The rule is simple: use only demineralized water (distilled or deionized), mixed in the correct proportions with the antifreeze/anti-corrosion coolant specified by the manufacturer.
The Water Pump and the Phenomenon of Cavitation
What Cavitation Is
The water pump is the heart of the cooling system: it circulates the coolant through the engine, the radiator and the cabin heater. Its impeller spins at thousands of revolutions per minute, creating a constant flow.
When the coolant is dirty, degraded or non-homogeneous, a phenomenon called cavitation occurs: in the low-pressure zone of the impeller, microscopic vapor bubbles form which, when they collapse, generate microscopic shock waves that are nonetheless extremely violent against the metal surfaces.
The Consequences of Cavitation
Over time, cavitation can literally pit and pierce the impeller blades:
- The blades thin out progressively → the pump loses efficiency
- Craters form on the surface of the blades → further turbulence → more cavitation (a vicious circle)
- In advanced cases, the blades break → fragments in the circuit + insufficient circulation → overheating
The factors that promote cavitation:
- Suspended debris in the coolant (limescale, rust, gasket residue)
- Incorrect concentration of the antifreeze (too diluted or too concentrated)
- Non-homogeneous mixture after makeshift top-ups with different products
- Air bubbles trapped in the circuit after a poorly performed service
Cleaning the Circuit: Essential Maintenance
Chemical Flushing
The cooling circuit must be cleaned periodically with specific products. It is not enough to drain and refill — the deposits accumulated over the years must be removed:
- Acidic detergents: Dissolve limescale deposits and mineral scaling
- Surfactants: Emulsify and remove traces of oil that inevitably seep into the circuit through the head gasket (even in microscopic amounts on a healthy engine)
- Anti-rust agents: Passivate the metal surfaces after cleaning
The process involves:
- Complete draining of the old coolant
- Filling with detergent solution + demineralized water
- Running the engine at operating temperature for 15-20 minutes
- Complete draining and rinsing with demineralized water only
- Filling with new coolant at the correct concentration
- Bleeding the air from the circuit
External Cleaning of the Radiator
An aspect that is often forgotten: the radiator must be cleaned on the outside as well. The radiator fins gradually clog up with:
- Insects (especially after motorway journeys)
- Leaves, dust and road debris
- Pollen and organic residue
- Anti-freeze salt (in winter)
When the fins are obstructed, the airflow through the radiator is reduced → heat exchange worsens → the engine temperature rises. Cleaning must be carried out with:
- A jet of compressed air (from the inside outwards, so as not to bend the fins)
- Low-pressure water with a specific product
- Never a high-pressure washer, which irreversibly deforms the aluminum fins
Our Protocol
At AutoImport Lab, the cooling system receives a level of attention that is rarely found elsewhere:
- Density test with a refractometer: verifies antifreeze protection (minimum -35°C guaranteed)
- pH test of the coolant: a pH outside the range indicates active corrosion in the circuit
- Identification of the correct specification for the make and model
- Complete chemical flush of the circuit with professional products
- Water-pump inspection: shaft play, leaks from the seal, noise
- External cleaning of the radiator and the A/C condenser
- Thermostat check with an infrared thermometer during warm-up
- Circuit pressure test with a digital pressure gauge: detects invisible leaks
- Filling with OEM coolant and demineralized water in the correct proportion
- Complete air bleeding and verification that no bubbles remain
The cooling system protects the engine from its most lethal enemy: heat. An engine can run on degraded oil for a while. But with insufficient cooling, just a few minutes are enough to cause tens of thousands of euros in damage.