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Fuel pressure rating

Old 12-26-2018, 06:05 PM
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Fuel pressure rating

Well, it's been a little while since I've found myself sitting on the side, or middle, of the Garden State Cash cow, broken down again. No, not today, but I did take that silly road for a nice ride, to see the wife, and get some miles on the new fuel pump from Cummins I just installed.

So, according to the FSM, the factory rating for the lift pump is supposed to be 3psi. From the installation, this thing has read 7 to 8 psi, pretty much at idle, not fluctuating much, which is what I expected, and hoped for (not fluctuating). Now that I seem to have a fully functioning lift pump, I decided to take it for the ride today, and closely watch the fuel pressure on the gauge in the truck, while cruising, accelerating, passing, and whomping on it.



For the most part while driving around it stayed in the 6 to 7 psi, and didn't move about much. When I approached the on-ramp, I decided to get about 1/2 to 3/4 pedal into it, and I watched the fuel pressure decline, the faster I went. At one point it read " 0 ". Freaked me out, as I thought I'd be stranded again, but once I took my foot off the pedal, it resumed back up to the 6 to 7 PSI all the way home. I decided to try the rapid accelerating again on the highway, and again, the gauge read lower and lower as the faster and more rpm, and boost I achieved. Returned back to normal, once I stopped peddling it.

So, do you guys have similar numbers? I really couldn't get it to do " 0 " again, but it did go down to 2psi a few times during speed increasing.

Man, I really don't want to have to think about another fuel pump again, but that " 0 ' psi, irked me.



Thoughts ? experiences ? I'll post a video as soon as I get a few moments to upload it to youbloob.



Thanks !



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Old 12-26-2018, 06:48 PM
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Thoughts...?
A HVLP lift pump is in your future.
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Old 12-26-2018, 07:34 PM
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Originally Posted by oliver foster View Post
Thoughts...?
A HVLP lift pump is in your future.
The option of having a pump that won’t dump fuel into the crankcase was a no brainer for me.
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Old 12-26-2018, 08:10 PM
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Or the supply line is sucking air, yet another possibility.
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Old 12-26-2018, 08:22 PM
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Originally Posted by NJTman View Post
Or the supply line is sucking air, yet another possibility.
That has been a problem you have been having for a while. It has not been ruled out yet.

If the condition gets worse as the tank get less full, that would point to an air leak.

I have built trucks that were tuned up to the point where they could drag the fuel PSI down to 0 at the drop of the skinny pedal.
In that case you would be looking at a bigger and better LP.

More testing is needed, but I could still see a HVLP LP as an future upgrade for you.
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Old 12-26-2018, 09:50 PM
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Originally Posted by oliver foster View Post
That has been a problem you have been having for a while. It has not been ruled out yet.

If the condition gets worse as the tank get less full, that would point to an air leak.

I have built trucks that were tuned up to the point where they could drag the fuel PSI down to 0 at the drop of the skinny pedal.
In that case you would be looking at a bigger and better LP.

More testing is needed, but I could still see a HVLP LP as an future upgrade for you.
Agreed. I'm going to detune the pump, and retry the beyond 1/2 pedal routine. See if the numbers change, easy peasy test.

Upgrading the hvlp in the spring ultimately will be required
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Old 12-26-2018, 09:54 PM
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I personally would continue to drive it as is to sort out the air in fuel issue, rather than detune the pump and add yet another variable in the mix...
Detuning the pump if you have an air sucking issue will not solve the real issue.

I would drive it as is, with a spare full Jerry can of diesel and the fuel filter wrench handy in the bed and keep a close eye on the fuel pressure gauge.
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Old 12-26-2018, 10:08 PM
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If I detune the pump, and the fuel pressure drop no longer occurs, then your theory about the stock, diaphragm lift pump failing to compensate for the increased fuel required by the ve., would be correct,

For now, I'll follow your. Advice, and start pricing the hvlp parts from,Cummins
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Old 12-26-2018, 10:34 PM
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Back to the question. What fuel pressures do you regularly see, and how do they vary, based upon driving actions.?
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Old 12-27-2018, 01:03 PM
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Edwin, what is your fuel pressure on the new pump you purchased, and the chinese unit you bought.

This guy bought a "cummins" part from an online dealer for part # 4988747 and he was running from 20 to 30 psi. Last thing I want to do is upgrade, and blow out the front seal in the VE.








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Old 12-27-2018, 01:27 PM
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From the sticky, I found these numbers. Called cummins and quoted pricing for me.
Cummins OEM priced all the parts to swap over to the HVLP pump.

Here's what they said:

(2) 3918191 seals (new number 3963983) $2.45 each
(1) 3914753 Fuel tube $ 42.56
(1) 3936320 Fuel pump (new number 4988751) $116.53
(2) 3931348 gaskets $4.47 each
(1) 3914284 spacer $16.62


Total for all parts $ 189.55 plus tax
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Old 12-27-2018, 03:07 PM
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Originally Posted by NJTman View Post
Edwin, what is your fuel pressure on the new pump you purchased, and the chinese unit you bought.

This guy bought a "cummins" part from an online dealer for part # 4988747 and he was running from 20 to 30 psi. Last thing I want to do is upgrade, and blow out the front seal in the VE.



I got my pump from Nascar Mark. I for get the part number but it is available on his site. IIRC he rated it at 15 psi but that is probably the static rating. It runs about 11 psi when the engine is running and pretty much stays there is I rev the engine. I don't have an in-cab fuel pressure meter, just a cheap one that goes up to 35 psi. The no-name pump I got off Amazon would peg the meter after a few pumps by hand which I would call a static test by connecting the pump to the meter and dipping the intake in a jar of fuel while pumping the piston. I didn't do a static test on the pump I got from Mark.

I hope this helps.

Edwin
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Old 12-28-2018, 02:18 PM
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Kinda sounds like a restriction somewhere. The pump has no problem making decent pressure, but can't supply enough volume to meet higher demand, so the pressure falls off. It would be interesting to tee a vacuum gauge into the suction side of the pump to monitor while driving, to see if a high vacuum develops at the same time the pressure drops under load.
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Old 12-29-2018, 09:38 AM
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I have a NAPA stock lift pump and my fuel pressure stays at 8-9psi with my mods. I've seen it drop to around 6 pulling a load up a hill or accelerating rapidly but that's about it. It's never even come close to 0 but I've never floored it either...
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Old 12-29-2018, 10:58 AM
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Originally Posted by KRB View Post
I have a NAPA stock lift pump and my fuel pressure stays at 8-9psi with my mods. I've seen it drop to around 6 pulling a load up a hill or accelerating rapidly but that's about it. It's never even come close to 0 but I've never floored it either...
thank you, fellas

Oliver mentioned he's seen the same "pulled to zero" scenario before, and a lot of it has to do with the degree the pump is turned up, type of fuel pin, and settings . it hasn't happened again, but I haven't been on a highway either.

Yesterday, my brass tipped FSS failed. I have since backed off the fuel screw a bit, and reinstalled the Bosch FSS Back in. Hopefully now it's turned down a bit, I don't have any failure issues. I cannot believe the brass tipped FSS failed. It wouldn't shut the truck off, and then it wouldn't activate at all with 12 v applied. Sent it Back. No reason it shouldn't function.

Need to go up on a highway to see if the reduction in power affected the numbers on the fuel pressure gauge.

On another footnote, I'm down to 1/3 rd full on my tank. Living vicariously through the dodge.
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