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The square tank showed is a custom/aftermarket replacement, Robert - hopefully functioning like the original.
here were basically three versions of the cooling system lay-out - your 74 has the header tank, top front of the engine. The cap on header tank is sealed and not used after the initial fill of drained system. In addition; you have the expansion tank as shown.
As coolant heats up, it expands - into the header- and expansion tank. Any air in the engine is funneled into the expansion tank by hose from the header tank to the expansion tank - originally at its bottom.
At initial fill; remove the header and expansion tank caps - and set cabin heat to ‘hot’ position. Pour coolant into the header tank until full, fit cap. Check that the expansion tank is also full - then fit that cap. Idle the engine for some minutes - stop engine and recheck levels. Refill expansion tank(!) if required.
As engine is run up to temp, the expansion tank cap opens as pressure rises. As Gary et al says; the expansion tank cap opens at 13 - 15 psi. It vents out excessive coolant through a vent - usually in the expansion tank filler neck. Either directly to open air or through an attached hose. This venting is very important - and must(!) function…
As the engine cools down after the full heat cycle, the coolant contracts. Checked cold, the level in the expansion tank is now at the ‘normal’ level - but it may require a couple of heat cycles to stabilize. Always check coolant level at the expansion tank with the engine cold - against the ‘normal’ level. Do not add coolant beyond this level - it will simply be expelled; giving the impression idea of a leak…
As an aside; the S3 system is different; there is no header tank and cap - just the thermostat housing. Two vent hoses, from radiator and thermostat housing goes to the expansion tank. Venting air that may have collected at high points - preventing air lock in coolant circulation…
In both cases; the expansion tank, like engine, is pressurized to cap pressure - raising the boiling point of the coolant to some 120C. In both systems a hose can be connected from the expansion tank vent to an atmospheric recovery tank - catching ejected coolant instead of loosing it to ground.
Note; checking coolant level in the expansion tank repeatedly with the engine cold - without adding coolant - is the first step in verifying that you have a leak…
Frank
xj6 85 Sov Europe (UK/NZ)
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