The valve stays closed until the engine is warm... unless you give it a lot of throttle, then it opens up. If yours is opening up when the engine is warm, but isn't opening up when cold but you give it a lot of throttle, then that is weird. Usually they just stick one way or the other. (What I find annoying about it is the range of speeds and loads where you give it a little throttle to maintain speed, but that makes it open up, which makes you speed up too much, so you let off a little bit, so that makes it closes, which makes you slow down too much, so you give it a little throttle... and so on...)

Diesels are also just sluggish when stone cold, EBPV or no EBPV. Is that maybe what's happening to you? You could always just lash the EBPV open temporarily to test that theory. When mine's dead cold and I'm driving it gently to warm it up, I always make sure I have a huge gap before pulling into traffic.

Duncan