I'm hoping someone knows about this. Can a map sensor affect the output of the liftpump? I think the stock liftpump has a hi and low setting. Will a map sensor giving bad reading defuel the vp? Or is it working in reverse and sensing boost and then adding fuel? 

None of the above. The ECM has two modes of operation. 100% duty cycle for running engine and 50% duty cycle (modulated) during cranking of the engine. MAP sensor only affect the timing and fuel of the VP44. Doesn't affect the performance of the lift pump.

 

What your seeing is high fuel demands and the fuel pressure falling. So what is happening is the demand volume is higher than the supply volume so the fuel pressure falls. Then is reverse when load is reduced the demand volume reduced and supply volume over whelms the VP44 so the fuel pressure rise and overflow valve opens more. 

Where is the overflow valve located on the vp44? 

Just forward of the fuel inlet. It connects the return line as well

Sean have you got any codes and what truck are you working on

Where is the overflow valve located on the vp44? 

 

 

The overflow valve...