Hi to everyone
Have my self a 99 cummins with auto with 267000 on the clock
My question is bad mpg's 11.3 driving steady and a lot less with a heavy foot
Got 10 psi at pump drops to 8psi at max rpm. Was told vp44 causing trouble
Put truck in nutral and bring revs up slowly to full 3200 rpm engine didn't miss a beat
Also no smoke . The put truck in 4x4 slip into first foot on brake then bring revs up to max. All I got was 2200 rpm and 20 psi of boost no smoke truck didn't move . Has anyone got thoughts on this thanks

 

your lift pump is bad...

replace it with an aftermarket pump ASAP!

you should NEVER see less than 10PSI under any conditions. and no lower than 14psi would be optimal...

My truck runs 20psi at idle and 16psi with maximum fueling (over 500hp worth of fuel draw) anything under 10psi is murder for your injection pump.

as for your economy sounds like you got something dragging or super oversized tires or something...11.3mpg is like average MPG for a truck that's towing 10k lbs+

 

What Jigabop said. Stop driving it until you replace your lift pump. 18-20psi at idle and 10-14 psi WOT with a load is where you should be. 10 psi should be your low pressure alarm pressure.

I got mid 11 to high 12 mpg in both my 2008 (programmer, deletes) and my 98 12-valve (springs, fuel plate, 4" exhaust) towing a ~12000-15000ish pound enclosed trailer from Seattle to Tampa and back.

Fix your fuel pressure issue first, then worry about economy. The lift pump needs to supply more fuel than the injection pump requires because the extra fuel is dumped back into the fuel tank. It lubricates and cools the injection pump, and if you don't have enough flow you will overheat and burn up your injection pump.

 

am running 35,s but mpg,s havent changed any from running 30,s. also have 4.5'' down pipe into 5'' through pipe.it has a cone filter as well, changed the factory lift pump as the pressure was really low and for how long it has been like this i dont know. i put on a mr gasket 105 gph at 15 psi but only got 10 psi out , would moving it nearer the tank help or is it just junk? its on the engine at the moment . checked the check valve as well seems fine but will get another,
thanks,

 

its best to locate lift pumps on the frame so the engine heat does not help them fail...as for pressure, try stretching the spring inside the pump to bump up the pressure.