The problem is not lacking fluid, the expansion tank can hold more than needed, it's that the fluid doesn't flow down into the cavities sometime and no one can figure out why....

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2006 Liberty Limited CRD, Deep Beryl Green, Yeti tune, Arp studs, new cams, rockers, lifters, TB. "Green Monster"
2005 Liberty Sport CRD, Deep Beryl Green, GDE Hot Tune, ARP studs. "Rocket"
1982 Fiat 124 Spider Convertible. "Fiona"

Because sometimes the air pocket can be higher then the coolant level and won't bleed out and where that funnel comes in handy.XJ's with the 4slow in them are well known for that same issue since the upper radiator hose goes above the AC compressor and thus the highest point in the cooling system.