kawi600

I installed a new lift pump back in december of last year. Did an overflow valve shortly after that in february and the pressure jumped to 30psi at idle. After while the pressure gradually creeped down to 22psi idle / 25psi at 2000rpm no load so I threw in another overflow valve from cummins. This is the same part I got last time but the pressure remained at 22psi. Squeeze the return line the pressure shoots up towards 40. Watch the pressure and it varies slowly all over the place. 22 to 28psi. Confirmed this with a real fuel gauge.
I also have an aux fuel setup plumbed in and the pressure problem pre-dates it. Switching to the aux fuel doesnt do much either. Aux fuel system bypasses everything and is pulled by lift pump right into the IP so no chance of air leaks on the heater doing it. I run a separate heat and filtering system behind it all.
The only things left are the IP itself and the needle valve / hose and gauge that might be leaking air in. Assuming leaking air in would cause pressure to drop, especially under heavy load where I see 12psi.
Any of you folks seen this before? Got any ideas? Im about ready to replace the needle valve, banjo bolt and hose I run to the gauge next.

TxDiesel007

I have had a bunch of fuel pressure issues in the past...

here is my question, how quickly does your gauge respond? and in conjunction, how open is your needle valve? My gauge takes at least a good five minutes to get to the 20 psi at idle, the truck must be warmed up for one and two the oil pressure must be at the level it needs to be at midrange as well (least mine is)

My needle valve is just barely barely cracked open, which is why it takes so long to respond. I have already changed the fuel filter, the prefilter, the rubber lines, the LP, the Needle valve, the OFV twice and still with the gauge cracked open a little more i idle at 10 psi, yes u heard right, i hear the gauge ticking, and the gauge responds almost instantly and drops quickly.. when i depress that little bugger with some smooth vice grips i actually get the 60 psi pretty quickly, and i do have alot of smoke, so i know im not starvin my pump for fuel, i do have an old ofv i need to run to see if tehre was any difference tho....

I have opened it all the way and i do see the gauge pulsate pretty badly, but i see the pressure get as high as sixty psi... i pretty much discovered by accident that i either have a bad gauge, or that i have my settings correct right now, i do perfer a super slow responding gauge, due to no pulsations, no cab noise and hey, it gives me the readings i wanna see

average of 20 psi at idle and 16-40 under load.. it usually climbs up quicker when i let off of a heavy throttle run...

Another thing you can try is running a setup similar to mine, i HAD an isolator, if you have that thing trash it, its worthless, and just run fuel to the gauge. I have a steel braided hose connected with a double male connector to the oil pressure tubing that runs into my cab, if i want (i found this out by accident) i can just undo the OPT and run another gauge to test my system, which for me would be sweet, if i can just get around to it...

Pressure dropping to 12 psi? mine drops to 16 at WOT (slowly i might add) but hey, its only cause the IP is sucking alot of fuel...

Keep in mind tho, Fuel pressure is just an average reading, what you got was normal, averages fluctuate, and they can be affected by temperature, fuel viscosity, and even if you add additives to it, so i wouldnt say that your pressure is off too badly, heck if it was me i wouldnt even worry about it..

If you want higher pressure tho, u can PM Cumminz4x4power, i think its what he goes by here, he was able to get 60 psi at idle with manipulating an overflow valve, i dont know how he did it but im sure that he can better explain it to you tho, Even tho i have read and understood more pressure might not always be better, id like to know how he did it as well...

These are my thoughts on ur situation, and experience, sorry for the book, and i hope that they help...

Rick

Mcmopar

I thought ANYTHING lower than 20 PSI was really bad for the P7100??

TxDiesel007

doubt it

the pump is oil lubricated, unlike the VP44 which is fuel lubed... i have no issues with my P7100

Rick

kawi600

Ive got the needle open just enough to get quick response. I wouldnt mind so much if it idled low and then shot up nicely... but its idling around normal and it dips as low as 10psi when you have your foot in it. Something really odd going on. I bet this is whats killing my fuel mileage too!
I just re-did all the plumbing for the fuel pressure gauge and packed all the threads with thread sealant, so now Ive eliminated everything but the IP. Can a worn IP cause weird pressure problems like this?
Ive replaced the overflow yesterday. I also tried taking one and stretching the spring to 12mm. I idle at 28psi with that and still dip down around 10 when under heavy load.
I guess the next thing im going to try is putting the feed and return hoses directly into a 5gal can of diesel and see if that changes the idle and 2k no load readings

TxDiesel007

Hmm

i neglected to mention... did you drop your fuel tank yet? the little mesh might be really really plugged up..

Rick

kawi600

I cleaned the strainer when I fixed the gauge sender over the summer. I may do it again when I get a chance so I can install some aluminum tubing to replace the steel fuel lines. I ran form that aux fuel system too, which would have ruled out a clogged feed.
I bought a pump from cummins last night, gonna install that and see what I get.