mine stays partially closed till it gets about a needle and a half up off the line. when i bump the throttle it opens quickly then will partially close at a cruise. mash it, it opens. just on the colder mornings. I dont think anything is wrong with it, it is just trying to warm it up quicker, do its job. You can actually turn it into a exhaust brake if you want. your fine.

 

It is quite normal for the valve to close as you are driving if the motor is not up to temp... Under hard accel it should open.

 

I was meaning if it wouldn't open when accelerating. They can start to have problems where they will stay shut and the truck will not have the power it should. I don't know what causes them to get 'sticky' like that. They are operated by an electric solenoid that controls a valve. The valve uses engine oil pressure to move the butterfly. I don't know if the solenoid valve doesn't operate correctly or if carbon build up makes the butterfly sticky. When it acts up you are almost left with only idle. I'm trying to get more info on it to know what inputs their are to the computer. I would think there is a over ride switch or something in the throttle pedal. Maybe that is going bad and not forcing the valve open when more power is required.
Don't get me wrong. When they work correctly, they are great. It really aides in engine warm up. If you wire it up, it will also work as a good engine brake.