Overheating problems with Subaru

Maintenance/Repairs
1

Has anyone faced a similar overheating problem with a Subaru Legacy?

Symptoms:

  • Started ~9 months, 5-10k miles ago
  • Enging overheating at higher engine loads (either speeds > 60mph or longer uphill stretches)
  • temperature gauge goes beyond center towards the red area, ECU reading confirms temperature going beyond 105C (I never went beyond 110C before taking action)
  • in such situations, reducing speed and putting heating on full blast cools down the engine in a few minutes to nominal
  • behaviour strongly dependent on outside temperature,
    e.g. on a cold winter day going 90-100mph is no issue,
    on hot days with AC on, 45mph can already be a problem.
    This makes it difficult to decide whether problem is becoming worse over time.

Problems considered so far:

Fans:

  • ECU reports fans switching on
  • visual check confirms both fans are running
    Head Gasket:
  • no obvious oil in coolant
  • no obvious coolant in oil
  • mechanic tested twice with sniffer for exhaust in coolant (negative)
  • no significant loss of coolant
    Radiator Cap:
  • cleaned and shortened reservoir hose according to Subaru TSB -> no change
  • replaced radiator cap -> no change
    Thermostat:
  • mechanic replaced thermostat -> no change
    Collapsing hose:
  • hoses appear firm and in good condition
    System not properly “burped”:
  • radiator flushed and refilled by mechanic -> no change.
    Thermal sensor broken:
  • measured engine block temperature near thermal sensor location and it is close to ECU read-out
    Water pump:
  • how to check best?
    Radiator (currently my prime suspect):
  • lower hose is hot
  • difficult to check radiator temperature by hand since fans and AC heat exchanger block access.
  • checked the temperature on the left and right edge of radiator and it appears substantially coolr at bottom end
    actually it feels cooler than lower radiator hose
  • how can one check whether the cooling capability of the radiator is degraded?
    Crack in engine block:
  • hairline crack that only opens when engine is hot?
  • how can one check this?

Any recommendations or suggestions would be great.

2

Year, mileage, and engine size/type please. Not all Subaru Legacys are the same.