Engine/Transmission (1998.5 - 2002) Static Fuel Pressure Question

[chazj]

I notice that prior to starting my engine the fuel pressure drops to zero. It is my understanding that this is normal.



Am curious if there is any benefit to "cycle the key" (without starting the engine) after shutting off the motor to create a ZERO psi situation as Opposed to having 15 plus #'s of static pressure in the system when it's not running? I don't know "the mechanics" of what part or parts are involved in maintaining that pressure. My thought process is that if it's similar to a "check valve"... it maybe advantageous towards its longevity to relieve this static pressure when the engine is off



Appreciate the education... .

 

[JLandry]

Hi Charlie,



The fuel supply pressure will ultimately bleed off to zero after shutting down the engine. If you use a mechanical fuel pressure gauge you'll be able to observe this. So no, there's no benefit or need to activate the lift pump either before or after running the engine. The ECM controls the lift pump as needed.



Best regards,



John L.



You may have already read this, but just in case...



From the Dodge service manual:



FUEL TRANSFER PUMP



DESCRIPTION



The fuel transfer pump (fuel lift pump) is located on the left-rear side of the engine cylinder block above the starter motor. The 12±volt electric vane-type pump is operated and controlled by the Engine Control Module (ECM).



OPERATION



The purpose of the fuel transfer pump is to supply (transfer) a low-pressure fuel source: from the fuel tank, through the fuel filter/water separator and to the fuel injection pump. Here, the low-pressure is raised to a high-pressure by the fuel injection pump for operation of the high-pressure fuel injectors. Check valves within the pump, control direction of fuel flow and prevent fuel bleed-back during engine shut down.



Normal current flow to the pump is 12 amperes.



With the engine running, the pump has 2 modes of operation: Mode 1: 100 percent duty-cycle with a minimum pressure of 10 psi except when the engine is cranking. Mode 2: 25 percent duty-cycle with minimum pressure of 7 psi with the engine cranking.



The 25 percent duty-cycle is used to limit injection pump inlet pressure until the engine is running.



The transfer pump is self-priming: When the key is first turned on (without cranking engine), the pump will operate for approximately 2 seconds and then shut off. The pump will also operate for up to 25 seconds after the starter is engaged, and then disengaged and the engine is not running. The pump shuts off immediately if the key is on and the engine stops running.



The fuel volume of the transfer pump will always provide more fuel than the fuel injection pump requires. Excess fuel is returned from the injection pump through an overflow valve. The valve is located on the side of the injection pump. It is also used to connect the fuel return line to the side of the injection pump. This valve opens at approximately 97 kPa (14 psi) and returns fuel to the fuel tank through the fuel return line.

 

[chazj]

Thanks John for your help here. I have an electric autometer fuel guage so have never seen a drop in fuel pressure as you describe.



After years of reading about fuel pressure and its relationship to the lonegevity of the VP44, I've perhaps grown a little over sensitive in trying to avoid any problems.

Case in point... I notice my fuel pressure guage may register as low as 8 lbs. (depending on driving conditions and engine RPM) and around 12 lbs. when engine being worked while climbing the highways that cross the Sierra Nevada. A normal visual of the autometer guage indicates around 13 to 15 lbs around town and 18lbs at idle.



Thus my new borrowed motto perhaps should be What me worry"... .

 

[JLandry]

I notice my fuel pressure guage may register as low as 8 lbs.

This is perfectly fine and your other pressures some normal.



As long as the VP44 is supplied with some level of positive pressure at all times, that's adequate. Negative pressure and excessive positive pressure can cause internal damage. Keeping the pressure above about 5 psi is a good limit because it gives a safety margin for any gauge error.



Relax and enjoy your truck.



John L.

 

[JR]

The check valve in the vp is set at 14 psi. So anything lower than 14 psi the fuel is not able to overcome the check to return to the tank via fuel return line. Its been common practice by many to set the lift pump so that it is above 14 psi at all times so that there is always a supply of fuel circulating through the vp, thus giving it maximum cooling. Now Im not saying that if your lp drops below 14 psi your going to prematurely wear out your vp, but the theory does make sense to me. The most common failure of the vps seem to be electronics related from heat soak. So in theory, if youve got fuel constantly circulating through your vp instead of just pumping fuel to it and deadheading there, your keeping your vp as cool as you possibly can. Ive kept mine above 14 psi since it was new, well see how long it lasts I figure it certainly cant hurt.
Chaz, has your pressure always dropped by as much as 10 psi? It seems like alot, just going by comparison to my Raptor which I have set at 20 to 21 psi at idle and drops to no more than to 14 to 15 psi at wot. One thing Ive noticed has helped alot was going to 1/2 inch line over the factory fuel line. The factory stuff is so small its hard for any lp to recover when your engine is under load. Going to 1/2 gives the pump more volume of fuel which helps it to keep up with the engines demands a little better.

 

[Bob4x4]

Yes the secret is more about the volume not pressure. remember also any system other than the original engine mounted pump will be charging system voltage not pwm

 

[chazj]

The check valve in the vp is set at 14 psi. So anything lower than 14 psi the fuel is not able to overcome the check to return to the tank via fuel return line. Its been common practice by many to set the lift pump so that it is above 14 psi at all times so that there is always a supply of fuel circulating through the vp, thus giving it maximum cooling. Now Im not saying that if your lp drops below 14 psi your going to prematurely wear out your vp, but the theory does make sense to me. The most common failure of the vps seem to be electronics related from heat soak. So in theory, if youve got fuel constantly circulating through your vp instead of just pumping fuel to it and deadheading there, your keeping your vp as cool as you possibly can. Ive kept mine above 14 psi since it was new, well see how long it lasts I figure it certainly cant hurt.

Chaz, has your pressure always dropped by as much as 10 psi? It seems like alot, just going by comparison to my Raptor which I have set at 20 to 21 psi at idle and drops to no more than to 14 to 15 psi at wot. One thing Ive noticed has helped alot was going to 1/2 inch line over the factory fuel line. The factory stuff is so small its hard for any lp to recover when your engine is under load. Going to 1/2 gives the pump more volume of fuel which helps it to keep up with the engines demands a little better.

Around town I typically see 12-15 lbs psi and a strong 12 lbs WOT. An occasional 8 lb reading I may see after "upshifting" and using very little throttle. The fuel pressure guage is an electric autometer and I still have the original fuel lines feeding the FASS 95 HPFP.



Only have 176000 miles on the truck so keeping my fingers crossed that I can add a couple hundred more thousand miles without undue expenses...

 

[SnoKing]

I would replace the banjo bolts with Geno's. I did that years ago and also drilled out the one at the VP-44 myself. That is between the FF and VP-44. Ahead of the FF is now have Vulcan fittings and AirTech pump on the frame. I never go below 11-13 lbs. I think I have extra HD banjo's in the shop!!! SNOKING