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Chevrolet Sonic Repair Manual: Cooling System Description and Operation

Chevrolet Sonic Repair Manual / Powertrain / Engine / Engine Cooling / Information / Cooling System Description and Operation

Cooling System

The cooling system maintains the engine operating temperature at the efficient level under any speed and operating condition. When the engine is cold, the cooling system cools the engine slowly or not at all. The slow cooling of the engine allows the engine to warm up quickly. The cooling system includes a radiator and recovery subsystem, cooling fans, a thermostat and housing, an oil cooler, a water pump and a water pump drive belt. The accessory belt drives the water pump. All components must function properly in order for the cooling system to operate. Before the coolant reaches operating temperature of the thermostat, the coolant circulates through water jackets in the engine block, the cylinder head, the heater core and the oil cooler. The coolant pump draws the coolant from the bypass pipe which the pipe receives returning floe form the engine and the heater core. When the coolant reaches the operating temperature of the thermostat, the thermostat opens. The coolant then goes to the radiator where it cools. This system directs some coolant through the hoses or pipes to the heater core and oil cooler. This provides for heating, defrosting and engine oil cooling. The surge tank is connected to the radiator to recover the coolant displaced by expansion from the high temperatures. The surge tank maintains the correct coolant level. The cooling system for this vehicle has no radiator cap or filler neck. The coolant is added to the cooling system through the surge tank.

Water Pump

The belt-driven centrifugal water pump consists of an impeller, a drive shaft, and a belt pulley. The water pump is mounted on the front of the transverse-mounted engine and is driven by the accessory belt.

The impeller is supported by a completely sealed bearing. The water pump is serviced as an assembly and cannot be disassembled.


A wax pellet-type thermostat controls the flow of the engine coolant through the engine cooling system. The thermostat is mounted in the thermostat housing to the rear of the cylinder head. The thermostat stops the flow of the engine coolant from the engine to the radiator to provide faster warm-up, and to regulate the coolant temperature. The thermostat remains closed while the engine coolant is cold, preventing circulation of the engine coolant through the radiator.

As the engine warms, the thermostat opens. This allows the engine coolant to flow through the radiator where the heat is dissipated through the radiator. This opening and closing of the thermostat permits enough engine coolant to enter the radiator to keep the engine within proper engine temperature operating limits.

The wax pellet in the thermostat is hermetically sealed in a metal case. The wax element of the thermostat expands when it is heated and contracts when it is cooled.

As the vehicle is driven and the engine warms, the engine coolant temperature increases. When the engine coolant reaches a specified temperature, the wax pellet element in the thermostat expands and exerts pressure against the metal case, forcing the valve open. This allows the engine coolant to flow through the engine cooling system and cool the engine. As the wax pellet cools, the contraction allows a spring to close the valve. To provide fuel economy and low emission benefit during part load driving condition, the electric controlled thermostat enables changing the thermostat opening temperature. Basically, the thermostat begins to open at 105° C (221° F) and is fully open at 120° C (248° F) when the electric energy not assisted.

If a certain driving environment encountered that requiring better cooling performance, the ECM orders to put electric energy to the thermostat. The energized electric heater in the wax pellet gives thermal energy to the wax for assisting the wax expansion. Then the thermostat valve opens earlier than the specified coolant temperature. For this vehicle, the programmed temperature set value by the ECM is 105° C (221° F) for cold and part load driving mode and 90° C (194° F) for hot and full load driving mode.


The radiator is a heat exchanger. It consists of a core and 2 tanks. The aluminum core is a tube and fin cross-flow design that extends from the inlet tank to the outlet tank. Fins are placed around the outside of the tubes to improve heat transfer to the atmosphere.

The inlet and outlet tanks are a molded, high temperature, nylon reinforced plastic material. A high temperature rubber gasket seals the tank flange edge to the aluminum core. The tanks are clamped to the core with clinch tabs. The tabs are part of the aluminum header at each end of the core.

The radiator removes heat from the coolant passing through it. The fins on the core transfer heat from the coolant passing through the tubes. As air passes between the fins, it absorbs heat and cools the coolant.

Surge Tank

The surge tank is a plastic tank with a threaded pressure cap. The tank is mounted at a point higher than all other coolant passages. The surge tank provides an air space in the cooling system that allows the coolant to expand and contract. The surge tank provides a coolant fill point and a central air bleed location. During vehicle use, the coolant heats and expands. The increased coolant volume flows into the surge tank. As the coolant circulates, any air is allowed to bubble out. Coolant without air bubbles absorbs heat much better than coolant with bubbles.

Coolant System Flushing
Note: Do not use a chemical flush. Store used coolant in the proper manner, such as in a used engine coolant holding tank. Do not pour used coolant down a drain. Ethylene glycol a ...

Cooling System Draining and Filling
Draining Procedure Warning: To avoid being burned, do not remove the radiator cap or surge tank cap while the engine is hot. The cooling system will release scalding fluid ...

Other materials:

Separating Parts
Note: Disassembly of the piston, press fit design piston pin, and connecting rod may create scoring or damage to the piston pin and piston pin bore. If the piston, pin, and connecting rod have been disassembled, replace the components as an assembly. Many internal engine co ...

Park - Engine Running (Gen 2)
When the gear selector lever is in the Park (P) position, fluid is drawn into the pump through the transmission fluid filter assembly. Line pressure is then directed to the following valves: Fluid Pressure Directed in Preparation for a Shift Manual Valve Mechanically controlled ...

Vehicle Speed Sensor Replacement
Vehicle Speed Sensor Replacement Callout Component Name 1 Engine Wiring Harness Bracket Nut Caution: Refer to Fastener Caution. Tighten 9 Y (80 lb in) 2 ...

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