The coolant expansion tank is used to compress the air at high pressure. In older cars that were not equipped with an expansion tank (unpressurized system), the coolant was drained along the filler cap at a higher pressure. With a closed system like the one in our waterboxer (WBX) and diesel engines, this discharge can still take place through the filler cap, but it will only occur in extreme cases, usually caused by engine problems.
The pressure that occurs in the system causes the boiling point of the coolant to increase, the coolant can thus be used over a larger temperature range without boiling.
When replacing, check the connected hoses immediately and replace any hoses that are convex or have fine cracks, not forgetting the coolant.
If the coolant warning light stays on, check that the coolant level is correct and, if necessary, replace the switch in the expansion tank (not supplied).
The cap is included with the tanks and can also be supplied separately.
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