Subaru STI: Radiator and Cooling System Neglect Leading to Overheating

All EJ-series WRX engines are sensitive to cooling system condition. Radiators on GC8/GD/GH era cars commonly develop electrolytic corrosion between the aluminium core and plastic end tanks, causing internal coolant passage blockage or end tank cracking. Subaru's coolant change intervals are frequently ignored on JDM imports with unknown service histories. A marginal cooling system combined with NZ summer temperatures or sustained motorway driving causes overheating that triggers or accelerates head gasket failure.

• Temperature gauge climbing above normal operating range
• Coolant overflow bottle frequently needing top-up
• Coolant appearing brown, rusty, or with sediment
• Steam or coolant smell from engine bay
• Heater output reduced (blocked heater core)

1. 1

   Check coolant colour and condition - it should be clean, not brown or rusty.

2. 2

   Inspect radiator for corrosion at end tanks and core damage.

3. 3

   Check all hoses for softness, cracking, or swelling.

4. 4

   With engine warm, verify the thermostat opens properly (upper hose should become hot).

5. 5

   Look for any weeping from radiator end tanks.

Parts involved: Radiator.

$500 to $1,800 at New Zealand independent workshop rates. That is for this job on its own, not for the car.

An estimate, in NZD, for the repair as normally carried out. What a specific car costs depends on which part has failed and what it damaged on the way — treat this as the floor for a negotiation, not a quote.

• “When was the coolant last flushed and replaced?”
• “Has the radiator ever been replaced or repaired?”
• “Has the car ever run hot or overheated?”
• “Do you have service records showing cooling system maintenance?”

The failure described above is documented in published sources — owner forums, technical service bulletins, recall notices and workshop write-ups — held in our fault database. The inspection instructions and the cost estimate are KickTyres’ own interpretation of them, and it is a starting point for an inspection rather than a manufacturer statement.