Engine/Transmission (1998.5 - 2002) whats average fuel pressure?

[continentaldrifter]

Hello,while humming down the expressway unloaded at 65 mph,my fuel pressure is reading 3. 5lbs,while idling it reads 6. 5lbs. I recently changed the filter and the gauge is set up to read pressure post-filter. I just read a few posts back it should read around 20lbs at highway speeds. Did i misunderstand or do i have a potential problem that is unseen to my virgin diesel eyes. Thank you for any advice,be well!!

 

[Thomas]

Most ISB's with a new transfer pump see around 15 psi at idle and 12 or so cruising empty. If your pressures are accurate, they're low.

 

[1dslram]

I have always idled at 17#-20# and cruises at 22#-25# with a max of 30# cursing at 80mph or so, or while pulling my trailer.

Bruce

 

[Thomas]

Bruce, Since you posted in a 24 valve forum, are your pressures with a 24 valve engine transfer pump?

 

[continentaldrifter]

Holy crap,im nowhere NEAR 17 lbs,what is the worst thing that can happen as a result of low pressure????

 

[Thomas]

Your injection pump will be burnt toast. The VP44 uses fuel from the transfer pump for lubrication.

 

[continentaldrifter]

I ordered a pusher pump from Piers,will this help?What are the symptoms of a transfer pump thats on its way out?Throttle response seems to be okay... top and bottom end. I'll be towing a trailer to Bike week,will the demand for fuel and low pressure leave me stranded on the other side of the long white line?... ... ... ... . Dont make me sweat the 2200 mile trip if i dont have to.

 

[CstlEng]

There is lots of info on this exact topic in LOTS of other threads. With my . 02 I will agree with thomas' numbers and the ascessment that yours sound low if correct. I will add that the relief valve on the fuel pump to the return line (the big funny looking banjo bolt on the VP44) is supposed to open at 16 psi (so you should never see much more than this unless you are throwing lots of fuel at the vp44 and the stock return line/ bypass setup can't keep up with the surplus flow to tank).



I will also add that I have personally replaced VP44s attached to lift pumps that gave less than 9 PSI at WOT, and have not replaced VP44s attached to lift pumps that have not dipped below 10PSI. Others experiences may vary.



Brian

 

[Thomas]

Use the search feature and find hours of lift pump and injection pump discussions. I replaced my lift pump when it idled at 9 psi and cruised at 5. If the life of the VP44 was decreased, I have no way of knowing.



The lift pump is about $140 from Cummins.

 

[CstlEng]

The pusher should help, but it sounds like you still have a marginal lift pump. Change the filter one more time to be sure it is not the cheapest thing causing the problem, and have the lift pump with you when you do. That way if you can't pull prime after the filter change with your potentially bad lifter, you can put on the new one. If it was the filter keep the pump as a spare or return it if, as they say, "you feel lucky. " Not a big job, maybe an hour and only that long because it is a bit of an akward reach.

 

[OTRPU]

Don't know about anybody elses, but can tell you what it was like when mine went out. LP was pumping 0. . . nadda. Then the poor old injection pump and to do all the sucking. Didn't notice it unless loaded and going uphill. Dam thing sorta. . . well. . . just ran outta fuel. Would stall, coast awhile, then start running again. Made for interesting trips delivering trailers all across the country. Called my DOC in Angola. . . explained the symptoms. . . when I mentioned boost went to 0 he said, crappo, you're dead. Get in here and bring lots of money. Got two new pumps now. . . plus other goodies. . . runs like a scalded a$$ ape. Ran with the big boys last couplea days, actually only gotta hour and a quarter left for the morrow. All those hills. . . Kentucky. . . Tennessee. . . Georgia. Talk about a BIG GRIN Nothin with a trailer could touch me. Fella witha Duramin tried for miles, finally caught me after I shut down to take exit ramp 160 off I-85S to land at this Flying J. I don't think passing somebody that's jumping off really counts much. Wonder what I'm going to have to fix when I get back.



Cheers,

Steve J.

 

[rweis]

There is a thread that implies 5psi post filter is actually a suction in the VP feeding its fuel pump. Don't know if correct or not.



10 psi min WOT loaded is my rule.



Each to their own.



Bob Weis



ps I'm in the process of adding a Stanadyne Fuel Manager with lift pump option. I only have 20k on my OEM relocated to the tank, but min loaded is 12 psi. Unloaded min (75 mph) is 14 psi.

 

[RustyJC]

With stock injectors towing our 36' long, 13'2" tall, 13500 lb 5th wheel RV (original lift pump @ 16000 miles):



16+ PSIG @ idle

15 PSIG @ 70 MPH cruise

12-13 PSIG @ WOT



I just installed the DD2's and will be towing this coming weekend with them, so these numbers may be subject to revision.



Rusty

 

[CstlEng]

5 PSI post filter is NOT suction from the VP. Suction is by definition a negative pressure: the only way to have positive pressure on the intake side of any pump is to put it there with another pump in series (like lift pump or lift + pusher), or by pumping and storing (a gravity feed tank or the like). 5 PSI instead of 12PSI or 16PSI post filter (on our trucks) can be from a few things: a dirty filter, a weak lift pump, an intake restriction (dirty intake screen in the tank or kinked line between tank and lift), or a small vacuum leak on the intake side of the lift pump (loose tank top connection, loose lead in to lift from the steel fuel line, chafed through steel fuel line), or a relief/ bypass valve on the VP44 that opens at too low a pressure. By far the most common causes are filter and lift pump. I have had to clean out my tank and intake screen once (I do have 185K though). The others are not real common but can occur.



The VP may allow sufficient quantity of fuel to pass through it that all the residual pressure post filter is used up (the part of the flow returning to tank, keeping the relief valve open) and the relief valve closes. This would happen during high fuel load (accelerating or trailering or up hilling). Then all fuel flowing past the filter is going into the VP 44 (none is bypassed to tank return) and making horse power (You may be familiar with the closely related smiles effect). If acceleration continues pressure has no choice but to drop: all the energy put into the fuel stream is going into moving the fuel and that energy is consumed in friction losses (the faster a fluid moves, the more friction it makes, the more energy is needed just to move it) . At this point there would be 0 pressure post filter, and if the VP44 were a pump capable of suction operation (it is... slightly... although VERY BAD for life of same) then some of its energy would need to go into producing suction. That is negative pressure to try to pull more fuel to it.



I guess all I am trying to say is that if you have any pressure at all post filter (or anywhere else post lift pump), the lifter is doing at least something, and that unless there is negative pressure (vacuum) measurable before the VP, the VP is not producing suction.

 

[Mtngoat]

Something happened with my pusher pump, by itself it increased its pressure, I can actually hear it running faster. My WOT is 15, cruise is 24 and idle is 29. I don't know if that is ok or if it can hurt the VP. Not taking any chances, I am getting a new pusher pump and installing a filter between it and the tank. Besides, if its running faster than its supposed to, it will prob burn itself up.



Robin

 

[gonehuntingagain]

Originally posted by Burnt Toast

Hello,while humming down the expressway unloaded at 65 mph,my fuel pressure is reading 3. 5lbs,while idling it reads 6. 5lbs.

I bought my truck used and the first thing I did is install a FP gauge - my pressures read the same as yours. I took it in and they replaced the fuel pump. The new pump had 15 psi at idle, 12 psi cruising on the freeway, and 10 psi at WOT.

 

[emarsh]

6 psi at cruise, 9 to 10 at idle, 12k miles changed pump, 6 psi at cruise, 9 to 10 at idle; saved old pump for spare. I do not worry anymore. I think far too much is made of this. I concur with others here that have said as long as you have some positive pressure, 2 to 3 psi you cannot damage the vp44. I now have 21k miles.

 

[mcoleman]

I have around 13 at cruise and 8 to 9 at WOT. But that is also with stock injectors. I plan on upgrading in the near future. Personally know of a fella who's truck was huffing on the big end of the 1/4 when his fuel pressure dropped to 5psi with bigger injectors. I'm not willing to take a chance on wiping out my VP44.

 

[Rebel_Horseman]

Recently bought my truck as well and installed the gauges about 2 weeks ago. Pressure, measured from the banjo bolt location at the bottom of the fuel filter housing, was 5-6 idle and about nil at WOT. Replaced the pump now I have 10. 5-12 at idle, about the same at cruise (sometimes down to 9) and about 6ish at WOT. Fuel filter change due this week, so I'm hoping it will be up afterwards.



Reb [><]

 

[Walleye mike]

13. 5 idle

10-12 cruse

6 WOT