Please interpret these fuel pressure readings

[DOE-SST]

I have my opinions, I’d like to hear yours...

1994 E350, 7.3IDI, E4OD. 276,000 miles. Van was used for transporting nuclear weapons convoy assault teams, and had the very best maintenance possible. Van is empty, not pulling any load.


From the fuel tank, the fuel line runs 12ft forward to lift pump, 10ft aft to factory fuel filter, then 7ft forward to the IP. Fuel system is stock, except for an inline filter placed 22" before the IP.


All rubber fuel lines, return lines & caps, and fuel filter replaced with new ones. Fuel tank removed and cleaned out. Fuel pickup cleaned out. Pensacola rebuilt IP & injectors recently installed. Three tanks of diesel, with plenty of DieselKleen, have been run through the fuel system.


I installed the fuel pressure sender 18" before the injection pump.


At idle, the gauge constantly jumps from 1psi to 4psi.

In city driving, lightly accelerating from a stoplight, gauge drops to 0psi during acceleration.

At constant 50mph, gauge reads 1psi.


What say ye,

o’ wizards of the IDI...

 

[gatorman21218]

What kind of gauge are you running? How does the van run? Either you have a lift pump failure or a bad gauge. Thats my take on the matter

 

[93f250idi]

I'm with gatorman on This one. Bad lift pump or bad gauge.

 

[gatorman21218]

I think an Injection pump can pull fuel without the help of a lift pump. However I think it is very ******* them and you dont want to do it

 

[MR.T]

At idle, the gauge constantly jumps from 1psi to 4psi. In city driving, lightly accelerating from a stoplight, gauge drops to 0psi during acceleration. At constant 50mph, gauge reads 1psi.

Looked this up in an older 7.3 manual: Min 1 PSI at rated speed, no load (essentially 3300 RPM in neutral) at the fuel filter outlet. This really isn't much fuel flow, probably less then at 50 MPH in your example. Min 1 PSI sounds low, but that's what it says -- 2 to 3 PSI would sound better to me. side note: If it was 1 PSI at the small throttle it takes for 3300 RPM no load, it's probably a vacuum at full throttle. Fuel pumps are cheap, and they don't last forever.

At low RPM, a fast electronic pressure sender may be catching low pressure fluctuations in the mechanical fuel pump when the cam lobe presses on the lever for the suction stroke of the pump. Do the fluctuations go away as RPM increases? Are there any drivability problems? What's the pressure at 3300 in neutral?

 

[Exekiel69]

Looks like You have more than one filter on that fuel system, have You replaced them recently? All that fuel treatment could pick up stuff from the tank.
I like to keep the IDI always with 4psi feed to the IP.

 

[MR.T]

Exekiel69 might have hit it -- I didn't notice he had 2 filters between the lift pump and the pressure sender. That's twice the filter pressure drop, even more if it's a finer filter then the OEM.

 

[DOE-SST]

To answer a few questions...



Do the fluctuations go away as RPM increases? Are there any drivability problems? What's the pressure at 3300 in neutral?

------------ the fluctuations disappear, AND, the psi drops to a steady 1 or 2 psi, as the rpm increases and I reach cruising speeds. No drivability problems. It smokes a little on acceleration, but that is probably due to less than optimal IP timing.
I don’t have tach on the vehicle (yet). I do have a handheld optical tach. Could someone suggest a good spot on the engine to measure rpms? It IS a van, and not a lot of working room.




Looks like You have more than one filter on that fuel system, have You replaced them recently?

------------Both filters have less than 10 hrs of run time on them. This van had an algae problem. Virtually every component in the fuel system was either cleaned by hand, or replaced with new/rebuilt parts. There is no algae left in the system. I’ve visually inspected the system for leaks - none found. Fresh diesel, with DieselKleen is the only thing run through the system. The second filter is a small see-thru inline style, installed as a last-chance filter to protect the IP.




I didn't notice he had 2 filters between the lift pump and the pressure sender. That's twice the filter pressure drop, even more if it's a finer filter then the OEM.

-----------I will remove the 2nd filter today and see if the psi readings change. It is a 10 micron filter.



I will compare the gauge to a known NIST traceable gauge this week, just to be sure it is accurate.


Is there any other possible cause for the pulsing pressure besides the lift pump?

 

[Exekiel69]

I removed the mechanical pump on most the IDI I use now since I like the benefits of the electric pumps. You can use a closed loop on the mechanical pump and run a cheap electric pump temporarily see if it gets better fuel pressure this way and if You like it enough to keep it as a main pump You can use a chevy big block off plate and remove the mech pump.

 

[Dieselcrawler]

i have noticed this on mine, i have the holley red pump 6 inches from my tank outlet, into the gauge then up to the filter head. most of the time it reads 5-6 psi. but then others it will say 3-4 psi or lower at idle. thought that was weird. no filter before pump either.