With my fuel pressure gauge, I noticed something I don't understand. When I am climing a grade in 5th gear and WOT, with 13 psi of boost, the fuel pressure gauage will go between 5 psi and then a few seconds later go to 2 psi, then go back to 5 psi, etc. During this time the boost remains constant. Can someone explain?

Brian

 

Where is the sending unit for the gauge? Sounds like air (I know, not again...) or a lousy lift pump or a crappy fuel filter. Aside from that, maybe the vane pump in the IP or the pressure control valve associated with it is doing some weird things...Dave, where are you?!

 

I use a Westach electrical fuel pressure gauge. I have a hollowed out banjo bolt for the line that goes to the injection pump. A 45 elbow is threaded into that, and the sending unit threads into the elbow. Connections are sealed with Permatex #2 no leaks there. The fuel filter is a Fleetguard FS1232 that I changed on Sunday.

Is there a chance these fuel pressure readings are consistent with a collapsing fuel line? Now that I have less air in the system, there should be more suction forces on the rubber line to the tank and the line might actually collapse. Before I think I may have been sucking air from the treads on the lift pump quick disconnect fitting. Two days ago I replaced it with a barbed fitting and sealed the threads with Permatex #2.

Brian

 

I think it would take alot more suction than any lift pump could generate to collapse even a rubber fuel line.

 

That's an odd one. I never saw the gauge fluctuate at a constant load. There's gotta be something up with the injection pump on this truck.

I'm with Ace, unless there is a blockage you are not gonna suck the line shut. If my Walbro isn't sucking 3/8" fuel hose shut a diaphragm lift pump sure isn't going to.

 

My gauge is the same one and it does similar things. I wonder if the sending units are too sensitive.