Engine/Transmission (1998.5 - 2002) Fuel Pressure 2001 dodge ram

[MGrantham]

Installed air dog lift pump, what should the fuel pressure be under load, and at no load?

 

[Road Dog]

My FASS is 12 - 15 psi at idle and 7 - 10 under load. Too high (over 20) is bad for the diaphragm in the injector pump and too low (0) is bad for different reasons.

 

[KATOOM]

There is no "high pressure limit" on the VP and.....the diaphragm cannot be damaged. Both are a myth which simply wont go away.

The VP's low pressure chamber is supplied by the fuel transfer pump which feeds the high pressure chamber. That fuel pressure is then multiplied and any excessive fuel volume not used to run the engine is for cooling and lubrication and is purged out various ports via the distributor and overflow valve. And diaphragm used to be a thick plastic disk between the rotor and distributor but since the past decade has been changed to a metal disc.

For best reliability results the fuel pressure feeding the VP needs to be no less than around 14 psi at all times. So depending on the fuel pump and the fuel lines and fittings, and configuration, the idle pressure generally needs to be set at around 18-20 psi. This will have the cruising pressure in the 16-17 range with WOT in the 14-15 psi range. Perfect for ample flow/volume and pressure to achieve lubrication and cooling of the VP while running.

There is no problem running pressures over 25 psi but its just not necessary to do the job needed. That said, by virtue of how the fuel timing solenoid and timing ring function, a new aftermarket fuel pump will achieve more "cranking" pressure when trying to start the engine and can sometimes cause a hard "hot" start problem. To eliminate that problem if it arises is to install a delay relay on the fuel pump.

 

[MGrantham]

Thanks for the info!!

 

[klcammie]

On my 01 I have 20# at start 16# cruise and 11# wot. I've had 0 problems since this setting three years ago.

 

[cholmes]

Cummins will tell you that it is not about pressure, it is about volume which also makes sense to me. 5lbs pressure with xxx gallons a minute/hour or what ever is better than 20lbs at x gallons an hour. I have an in-tank pump pushing to the block mounted pump. Lots of volume, lots. Also have 18 lbs pressure and if either pump fails the truck will stop. Cummins said there are literally 10's of thousands of 5.9's in bigger trucks, buses, motor homes, heavy equipment with thousands of hours with a block mounted lift pump and no issues. They change out the pumps yearly as part of the PM program

 

[KATOOM]

Cummins will tell you that it is not about pressure, it is about volume which also makes sense to me. 5lbs pressure with xxx gallons a minute/hour or what ever is better than 20lbs at x gallons an hour. I have an in-tank pump pushing to the block mounted pump. Lots of volume, lots. Also have 18 lbs pressure and if either pump fails the truck will stop. Cummins said there are literally 10's of thousands of 5.9's in bigger trucks, buses, motor homes, heavy equipment with thousands of hours with a block mounted lift pump and no issues. They change out the pumps yearly as part of the PM program

I could respond as to why this isnt applicable anymore..... I've been part of so many in-depth threads about this topic over the past decade and I'm shocked it remains a topic of discussion. There are way too many variables for a simple black and white answer. If someone wants to run a 5 psi then so be it. Its your truck and to each is own.

 

[bboxall]

The problem with running pressure below 12-14 psi is that you don't have enough pressure to open the overflow valve which returns unused fuel to tank. The flow of unused fuel back to tank is what cools the VP44.

 

[KATOOM]

The problem with running pressure below 12-14 psi is that you don't have enough pressure to open the overflow valve which returns unused fuel to tank. The flow of unused fuel back to tank is what cools the VP44.

Actually thats believed to NOT be true.....

Even though the inlet and outlet are sitting side by side, its understood that the inlet fuel pump pressure is a "charging" pressure for the first two of three chambers within the VP. Therefore the outlet overflow valve maintains a uniform pressure value for VP's low/mid pressures.....and why its crucial to never mess with or adjust the outlet overflow valves regulated pressure. This is considered messing with parameters which no one yet fully understands.

So just pointing out that outlet fuel flow is a byproduct of fuel unused during the entire process of fuel passing through the VP, not directly from inlet to outlet, nor just pertaining to the fuel volume over and above what the engine uses. And yes, is what the VP uses to remain cool and lubricated.

The reason the general consensus of pressure is to be up around no less that 14-15 psi is because that not only happens to be whats generated by the rotor/distributors in the form of hydrodynamic fluid pulses counter acting pressures, but also because this inlet pressure assures that the chambers vane pump and timing piston remain in a constant charged state. There's more to it but I'm not sure who's bored with reading that much so far.....
And remember, pressure and volume are inversely proportional.

 

[Joe Mc]

Just my $0.02. Running a Glacier Diesel Products fuel system (Walbro pump, big lines, mechanical regulator) I have the regulator bushed to provide 20#. I see 20# at all conditions; idle, cruise and/or WOT. I've had this system in-place shortly after replacing my VP (see month & year in sig below). Not trying to brag about anything here, just saying "for years this has worked for me". Oh, still have same VP as mentioned in sig.