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The following illustration is a diagrammatic representation of a generic "LS" cooling system.

Starting at the water pump and following the numbers in sequence, here's the flow of the system:

1. The water pump pumps coolant out of the lower two ports on its back side and into the block.

2. Coolant circulates through the engine block...

3. ... and through the cylinder heads.

4. Special LS-specific "steam tubes" or "engine vent lines" are installed on top of the cylinder heads. As vapour or steam will always seek the highest point, any steam pockets created by local hot spots, particularly in the cylinder head exhaust valve area, will migrate up and into the steam tubes which will carry them and a small amount of coolant away and either into a port located at the top of the rad (as shown here) and from there to the surge tank, or directly to the surge tank (depending on application) where the steam is separated from the coolant.

5.Coolant returning from the cylinder heads enters the two upper round ports on the left and right back sides of the pump

6. Some coolant circulates through the water pump's bypass circuit and is again pumped back through the engine. This keeps the circuit flowing when the thermostat is closed.

7. Hot coolant exits the smaller, rear "heater out" port of the thermostat housing on the water pump. This takes it to the heater core. The port is 5/8" ".

8. After exiting the heater core, coolant returning from the heater passes through the surge tank to keep coolant circulating through that tank.

9. After passing through the heater core and then the surge tank, coolant re-enters the engine via the larger, front 3/4" "heater in" port of the thermostat housing on the water pump.

10. Hot coolant exiting from the engine that doesn't follow the pump's internal bypass circuit exits the water pump via the top 1-1/4" port and enters the top of the radiator. Hot coolant flows across and down through the radiator, cooling as it goes. Cooled coolant exits the radiator via the lower 1-1/2" outlet and returns to the inlet side of the water pump. Returning coolant is blocked from re-entering the water pump inlet if the thermostat is closed. When the coolant on the inside (engine side) of the thermostat reaches the temperature of the thermostat (e.g. 190° F) the thermostat opens and the cool coolant enters the water pump inlet to be circulated through the engine again, starting over at #1.

11. A radiator bleed or vent port located at the top of the radiator connects to the surge tank. Any air or steam in the system, especially that coming from the engine's steam tubes, will naturally seek the highest point and will therefore exit the radiator via this port and travel to the surge tank. At the surge tank, coolant and steam enter and the steam or air is separated from the coolant. The lighter steam / air collects and remains in the surge tank at the highest point, just below the rad cap, to be eliminated first in the event the rad cap purges.

12. The cooler, denser coolant goes to the bottom of the surge tank where it is collected by the flow returning from the heater core and circulated back through the system.

13. In the event of a system over-pressure condition, the rad cap opens and burps excess coolant and steam out of the system to be collected by the overflow tank.

By Bill "BillaVista" Ansell