If it has air trapped, the air needs to be bled out of the system. In order to do this, fill the expansion tank completely with coolant and let it run for a while. The air should bleed itself out of the system.

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Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record