General

COOLING SYSTEM - K SERIES KV6

26-3-8

DESCRIPTION AND OPERATION

Operation

General
When the engine is running, the coolant pump draws coolant through the pipe connected to the thermostat housing 
and pumps it through the outlet at the right front corner of the cylinder block and through the cooling jackets in the 
cylinder block, cylinder heads and inlet manifolds. From the outlet at the right front corner of the cylinder block, the 
coolant flows through the hoses and the coolant rail to the IRD cooler, the engine oil cooler and the transmission fluid 
cooler. From the top of the cylinder block, coolant flows through the outlet elbow and the top hose to the heater matrix. 
Coolant returning to the thermostat housing, from the cylinder block, the coolers and the heater matrix, bypasses the 
thermostat and flows back to the coolant pump inlet to begin the cycle again.

Coolant also flows through the expansion pipes to the expansion tank, from where it is drawn back into the system 
through the expansion tank outlet hose connected to the coolant rail. The increase in coolant volume, created by heat 
expansion, reduces the air space in the expansion tank, which pressurises the system and raises the boiling point of 
the coolant.

Coolant flows through the radiator, from the top of the RH end tank to the bottom of the LH end tank, and is cooled 
by air passing through the radiator. When required, the two cooling fans provide additional air flow through the 
radiator, particularly when the vehicle is stationary. Operation of the cooling fans is controlled by the ECM via the 
cooling fan ECU. The temperature of the cooling system is monitored by the ECM using inputs from the thermostat 
monitoring sensor and the Engine Coolant Temperature (ECT) sensor located in the cylinder block, to the left of the 
thermostat housing. 
 

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 ENGINE MANAGEMENT SYSTEM - SIEMENS, DESCRIPTION AND OPERATION, Description.

Pressure Relief Thermostat (PRT)
The thermostat is exposed to 85% hot coolant from the engine on one side and 15% cold coolant returning from the 
radiator bottom hose on the other side. This allows the thermostat to react to the ambient conditions and provide 
coolant control for both winter and summer use. Hot coolant from the engine passes via holes in the by-pass flow 
valve into a tube which surrounds 85% of the thermostat sensitive area. Cold coolant from the radiator conducts 
through the remaining 15% of the sensitive area. In cold ambient conditions, the engine temperature is raised by 
approximately 10

°

C (50

°

F) to compensate for the heat loss of 15% exposure to the cold coolant returning from the 

bottom hose. This improves heater performance and engine warm-up.

The by-pass flow valve is held closed by a light spring and operates to further assists engine and heater warm-up. 
When the main valve is closed and the engine speed is at idle, the coolant pump does not produce sufficient flow and 
pressure to overcome the spring and open the valve. In this condition the valve prevents coolant circulating through 
the by-pass circuit and directs coolant through the heater matrix only. This provides a higher flow of coolant through 
the heater matrix improving passenger comfort in cold conditions.

When the engine speed increases above idle, the coolant pump produces a greater flow and pressure than the heater 
circuit can accommodate. The build up of pressure acts on the flow valve, overcoming the spring pressure, opening 
the valve and relieving the pressure in the heater circuit. The valve then modulates to provide maximum coolant flow 
through the heater matrix and allowing excess coolant to flow into the by-pass circuit to provide the engine's cooling 
requirements at higher engine speeds. The thermostat then regulates the flow through the radiator to maintain the 
engine at the optimum temperature. Maximum opening of the thermostat, and therefore maximum flow through the 
radiator, occurs if the coolant temperature reaches 102

°

C (216

°

F).

Operation of the thermostat is monitored by the Engine Control Module (ECM) using an input from the thermostat 
monitoring sensor in the bottom hose.