Thermostat Service and Testing

Thermostat Service

Effects of a Stuck Thermostat

  • Stuck Shut:
    • Restricts coolant circulation through the radiator.
    • Can cause engine overheating and coolant boil.
    • In hybrid systems, may lead to coolant and HV powertrain component overheating, though less severe than engine combustion heat.
  • Stuck Open:
    • Excessive coolant circulation through the radiator.
    • Engine may not reach proper operating temperature, especially in cold weather.
    • Reduces engine efficiency in terms of power, gas mileage, and driveability.

Thermostat Testing

  • Visual Inspection:
    • Observe coolant flow through the radiator neck opening (if available).
    • Cold engine: No coolant flow in the upper radiator area.
    • Warming engine: Thermostat should open, allowing coolant circulation.
    • If no circulation and overheating occur, the thermostat is likely faulty.
  • External Testing:
    • Remove the thermostat and test it in a container of water on a hot plate.
    • The thermostat should open when heated to its specified operating temperature.
  • Digital Thermometer Use:
    • Measure temperatures near the thermostat and at the thermostat outlet hose.
    • If the engine is at operating temperature but the outlet hose remains cool, the thermostat isn't opening.
    • A thermostat that doesn't open at the correct temperature is defective and needs replacement, as shown in Figure 48-10.

Thermostat Replacement

  • Thermostat Housing:
    • The thermostat is housed in a pot metal, aluminum, or plastic enclosure.
    • Typically located on the top or side of the engine.
    • Housing is clamped against the cylinder head, intake manifold, or cylinder block using bolts.
    • In newer vehicles, may be on the side of the mechanical water pump housing.
    • Refer to the service manual for part locations and torque values.
    • Avoid overtightening bolts.

Service Procedure for Thermostat Removal

  1. Preparation:
    • Ensure the cooling system is not under pressure.
    • Carefully remove the radiator cap.
    • Place a drain pan under the radiator to catch coolant.
  2. Radiator Hose Removal:
    • Loosen the clamp on the upper radiator hose.
    • Twist and pull the hose to detach it from the thermostat housing.
  3. Disconnect Components:
    • Remove any wiring connectors or vacuum hoses connected to the thermostat housing.
  4. Remove Housing Bolts:
    • Use a 3/8" air or hand ratchet to remove the bolts securing the thermostat housing to the engine (Figure 48-11A).
    • Note the bolt lengths for correct reinstallation to prevent engine damage.
  5. Housing Removal:
    • Lift off the thermostat housing.
    • If needed, gently tap the housing with a rubber or plastic mallet to loosen it.
  6. Thermostat Removal:
    • Use pliers to grasp and pull the old thermostat from the engine (Figure 48-11B).