PINPOINT TEST B : The Engine Overheats: Notes

Normal operation and Fault Conditions 

The engine cooling system maintains the engine temperature during operation. Correct coolant flow through the engine, radiator and remainder of cooling system passages and components is essential to maintaining a correct engine temperature.

Engine coolant flows primarily from the engine to the radiator circuit and back to the coolant pump. Coolant is sent from the coolant pump through the engine block and cylinder heads. A separate circuit from the engine also feeds the heater core with coolant. The coolant pump circulates the coolant. The coolant thermostat is a control valve actuated by coolant temperature. When the thermostat is closed, coolant flow bypasses the radiator circuit and returns to the coolant pump. When the thermostat is opened, coolant flows through the radiator circuit to transfer engine-generated heat to the outside air.

The engine cooling system incorporates a coolant bypass solenoid valve and a coolant shutoff solenoid valve. Using these two valves, the coolant flow through the engine block can be restricted or completely stagnated. The restriction or stagnation of the coolant flow makes it possible for the engine components to warm up faster during the warm-up phase. The coolant shutoff solenoid valve is normally open and closes when energized to stagnate coolant flow throughout the cooling system if the coolant bypass solenoid valve is also closed. The coolant shutoff solenoid valve blocks or allows coolant flow through the thermostat housing, coolant pump, engine block, heater core, turbocharger, transmission oil cooler, and engine oil cooler. The coolant bypass solenoid valve provides another coolant circuit for the engine block and is normally closed and opens when energized to provide coolant flow between the outlet of the engine block to the thermostat housing. The primary function of this valve is to increase coolant flow through the engine block reducing cooling system pressure and temperature fluctuations during high engine loads. If one or both valves do not open when commanded by the PCM an engine over temperature condition can result.

Engine overheating generally occurs when there is a disruption in the ability to control either coolant flow at the correct rate, the inability to transfer heat from the engine through the coolant (including low coolant) or an inability to transfer engine-generated heat to the outside air through the radiator.

Possible Sources 

• Low coolant level
• External engine coolant leak
• Airlock in system
• Pressure relief cap installation
• Restricted airflow through A/C condenser/radiator
• Internal engine coolant leak
• Turbocharger
• Coolant condition/concentration
• Non-OEM engine enhancement components
• Electric cooling fan
• ECT sensor
• Radiator
• Thermostat
• Temperature gauge
• Coolant pump
• Coolant flow restriction
• Coolant shutoff solenoid valve
• Coolant bypass solenoid valve

Diagnostic Trouble Code (DTC) Fault Trigger Conditions