I have a 2000 1500 that is loosing vaccum when climbing hills causing the heater vents to open and close. It also has a cold start problem that goes away if you turn the HVAC off, it allows the truck to run without dying. The dealer gave me a little $8 check valve that goes on the right side intake manafold. They said that would cure it but didn't make any change at all. One other item that may be related is when decelerating from hi speeds there is a popping noise that sounds just like the pop off vavle on my cummins. It goes away at lower speeds. My thought is that both issues are related and that it is a bad check vavle or something like that. Any thoughts would be greatly appreciated.

Thanks in advance,

Scott