Overflow Valve

[claymike]

I know this subject has been brought up in other posts, but it still has not sunk in to my thick head. I have about 20k on my second lift pump and I have noticed in the morning once in a while the pressure starts going all over the place. Other then these morning events the pressure is always at 14 lbs and 11 lbs at WOT. I was wondering if this could be the overflow valve. My question is what exactly does the overflow valve do, and do you think this could be the problem?



Mike

 

[Evan A. Beck]

The overflow valve is basically just a fuel pressure regulator. Inside it, there is a spring and a small checkball. If this spring becomes worn after time, it can cause erratic and/or low pressure. This is much mor common on the 12V P7100 trucks, because of the pulsation of the mechanical lift pump.



You can test your overflow valve using a piece of 1/2" fuel hose and an air pressure regulator. It should begin to open at 10-12 psi, and be fully open by 13 psi.

 

[rubberneck]

grid heaters?

Mike next time it happens, pay attention to the volt meter. Because you said it only happens in the morning I think what you may find is that the pressure is dropping when the grid heaters cycle on and draw all the current from the system. When my grid heater comes on my pressure drops from 16-17 down to 11-12. check it out.

 

[claymike]

Thank Evan I will test it out.



Rubberneck it happens after the grid heaters have cycled. It is responsive to the throttle. While going through the gears it will go as low as 2 lbs and that is with only 7 lbs of boost so it is not taking much to loose the pressure. The strange part about this whole thing is that it never does it after the truck warms up.



Mike