Just installed the low pressure piston lift pump (3936320) on my truck this morning. At idle it puts out 14psi which is much better than the 4psi from my old pump. with the piston pump no load psi tops out at 16 psi around 2k rpms then drops to 12 at WOT no load. Sounds good right? Well i took it on a test drive and i can suck the fuel pressure down to 0 in third gear at 2700 rpms. I could only get the pressure down to 0 in 5th under full load at 2500 rpms with the old pump, in 5th with the new pump the fuel psi gauge drops to 0 very quickly when i get on it. What's the deal???

 

Have you checked to see that the Fuel Filter and suction side of the pump are up to par (, Fuel Supply Plumbing, Sucking Air, Ect)?

 

Yeah, better have a looksie at your Fuel filter, and fuel heater screen.

 

^^^ i agree. My piston pump feeds my truck just fine. Mine draws down to about 8psi at WOT.

 

yeah it feeds mine fine 10 psi at wot for me stock non ic injectors maxed pump and denny t pin

 

the question is did you buy this pump new or used

 

Pump is brand new from cummins.

Fuel filter, heater etc. should be fine. Like I said, right before I changed the pump pressure was ok, just low like always. I only have 500 miles or so on this filter anyway.

You guys all running the spacer between the pump and block? I assume you have too, its tight enough with the spacer. I was just thinking, doesn't that spacer shorten the effective stroke of the pump therefore decreasing flow?

 

the spacer make the pump sit out farther allowing the cam to push it to full stroke without the spacer the pump would be compressed all the time

 

Without the spacer you will quickly bend or break the pump's rod. Upgrading your fuel pickup tube and line to the lift pump will help keep pressure up to the injection pump. It doesn't take much to suck the 5/16 hose dry, along with the even smaller pickup in the tank.

 

It should still do better than that. He's running stock injectors, from what I see in the sig, and still seems to be doing even worse under load than the stock pump that was on there before.

 

Exactly! I plan to run bigger hoses all around from the pickup to the injection pump eventually, but right now there is something else causing the problem.

 

Where'd you get it? If that was all you changed it must be a bad pump. Take it back and get another pump.

 

Got it new from cummins, but i don't know if they will take it back if its been run already, guess i'll give them a call. I really hate to condem a brand new part.

 

Are you sure you don't have an air leak? Pressurize your tank, maybe the fitting on the inlet side of the pump is leaking?

On my truck with stock injectors, making 20psi manifold the pump hasn't been under 10psi.

 

Yes, double check everything, of course. One thing I've always noticed about suddently appearing problems with the parts I just touched, is it's almost always because I just did something wrong.