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The front lower one goes to the water inlet, so yes, coolant will pour out. Ditto for the lower side ones which are coolant bypass lines to the throttle body and EGR, as I recall. The upper one over to the air intake shouldn't have any liquid in it.
Well I replaced the IACV on my SC400 and I only removed one hose...lol well whatever works!
You're probably referring to just the valve itself, not the entire housing? But even then, I'm not sure what one hose you would have removed because mine doesn't have any hoses connected to the valve. And looking at one of the all-in-one units with the valve inseparable from the housing, you would still have to remove all four hoses to get it off, so that can't be it... Did you mean the one [wiring harness] plug, rather than one hose?