Diagnose a cooling system fault
Start from what you can actually see — the symptom — and work through the likely causes, the checks worth doing in order, and the action that fixes it. Urgency is flagged so you know what not to drive on.
Select a symptom on the left and we will walk through the likely causes, the checks worth doing and the right next action — in order.
What each part of the system is actually doing
Most cooling faults trace back to one component losing the ability to do its job. This diagram shows where coolant quality matters and what typically fails first in each place.
Select a numbered component to see why coolant quality matters there.
Radiator core
- What it does
- Rejects engine heat to the air as coolant flows through its tubes and fins.
- Why coolant quality matters here
- Deposits insulate the metal. A layer of scale or corrosion product only 1.6 mm thick can cut heat rejection by roughly 40 %, so the engine runs hot even with a full coolant level.
- Typical failure
- Clogged tubes from hard-water scale, depleted inhibitors or mixed incompatible coolants. Often the first place sediment collects.
Check the cap before the coolant
A pressure cap that no longer holds its rating removes roughly 25 °C of boil-over margin. It is the cheapest component in the system and the most commonly missed cause of unexplained coolant loss.
How cap pressure worksMilky coolant is not a coolant problem
If the coolant is milky, creamy or oily, oil or combustion gas is entering the system. Repair the mechanical fault first — flushing before then simply wastes new coolant.
Read the full diagnosis guideDo not just top up a brown system
Brown coolant means the inhibitor package is spent or the mix is diluted. Draining and refilling leaves corrosion product in the block and heater matrix, so the fresh coolant starts cleaning and blocking passages.
Full flush procedure