Can anyone tell me what the little pump looking part is on the passenger side inner fender well, just in front of the blower motor is/what it does? Mine seems to continue running, when I try to start my truck. I seem to remember this shutting down before-after We We u it built up pressure? Would this cause a no start condition? Thanks for your input.

 

If you have a diesel, its the vacuum pump for the HVAC controls and the auto-lock hubs if your truck has them. Running constantly means there's a leak somewhere in the system that you need to repair before the pump burns up. Its not made to run constantly.