with the type of fan u r running u have to have a shroud. it doesnt matter how many cfm it has it can pull shit through u radiator unless it has a shroud. thats why it fine when ur driving and got airflow through u radiator. so just get a shroud and u'll be fine
trust me on this we just went over all this crap at school nascar technical institue. for best flow get a shroud where the blades r half from the front and half from the rear for maxium flow

^^Work in progress pic
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