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Is it supposed to open when driving or is it totally dependent on the outside temperature?
Yes it is supposed to maintain some back pressure while you are driving, I have and excursion and the EBPV remains on on it for about 8 - 10 miles going down the road, you almost don't notice it though except for a slight hiss and louder exhaust sound, the only way I can tell when it is done doing its thing is my EGT drops 150-200 deg. When the sensor or sensor tube gets plugged, that is when the EBPV remains on more than it should.
One thing to realize is that cold air is harder to push threw.Every winter truckers wonder why their milage drops even though their idle time is consistant.The difference is the cold air and longer warmup times for all your oils.Giving up 1-2mpg in winter months is normal.Also even though alot of fueling stations don't post blended fuel 70/30 blends are very common.Adding 30% no.1fuel is killer on mpg.This winter my truck has been running 12-14mpg locallly and 14-16mpghi-way.The best I have done is 16.5 this fall w/35" tires and 4.10s.
Another thing is speed any vehicle running over 70 will lose considerable mpg.Reason is you start pushing more are which requires more horsepower.I have seen this in my Honda Civic and big trucks and anything in between.It's just like racing the faster you want to go the more horsepower you need to get there,and certain increments of advance speed require surprisingly more power. Like the saying goes SPEED KILLS---MPG.