Is this theory correct?
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Joined: Nov 2001
Posts: 4,199
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From: Branchville, Alabama
Originally posted by pcab4
Not necessarily. Pressure is the measure of restriction and not volume. If you put your finger over the end of the hose you create pressure and lower the volume and releasing your finger lowers pressure and increases volume. If you notice that your psi's slowly degrade over time and you've eliminated possible causes then you should be concerned with the pump giving out. But as long as you have 1 psi at WOT then you know that your pump is still producing more volume than the VP can use. Remember that the fuel return is about 70-90 percent of the fuel delivered to the VP depending on setup.
If you have 15 psi at idle and 0 psi at WOT what is your return line volume? Is your lift pump bad? If you have 15 psi at WOT is your lift pump good or do you have a restriction?
You can spend $600 on a FASS or replace your lift pump about 5 times. Is 30 minutes of labor worth $600? I still have my original pump at 33,000. Is this newest pump from Cummins a better one? They seem to think so. The older ones were horrible but the failure rates from Cummins on the latest released pump are down substantially.
Bill K did a big write up on this in the TDR and did it with a flow meter on the return line. Read this and see how people measure PSI and confuse this with a good or bad lift pump verses what the actual volume of the pump output is.
With this said if you used to get 15 psi at idle and now 6 months later you are seeing 8 psi at idle and you've eliminated possible causes, then replace the pump. But this is what the pressure gauge was really meant to do, was to monitor the normal pressures and not the extremes and to warn of a possible problem.
My .02..
Not necessarily. Pressure is the measure of restriction and not volume. If you put your finger over the end of the hose you create pressure and lower the volume and releasing your finger lowers pressure and increases volume. If you notice that your psi's slowly degrade over time and you've eliminated possible causes then you should be concerned with the pump giving out. But as long as you have 1 psi at WOT then you know that your pump is still producing more volume than the VP can use. Remember that the fuel return is about 70-90 percent of the fuel delivered to the VP depending on setup.
If you have 15 psi at idle and 0 psi at WOT what is your return line volume? Is your lift pump bad? If you have 15 psi at WOT is your lift pump good or do you have a restriction?
You can spend $600 on a FASS or replace your lift pump about 5 times. Is 30 minutes of labor worth $600? I still have my original pump at 33,000. Is this newest pump from Cummins a better one? They seem to think so. The older ones were horrible but the failure rates from Cummins on the latest released pump are down substantially.
Bill K did a big write up on this in the TDR and did it with a flow meter on the return line. Read this and see how people measure PSI and confuse this with a good or bad lift pump verses what the actual volume of the pump output is.
With this said if you used to get 15 psi at idle and now 6 months later you are seeing 8 psi at idle and you've eliminated possible causes, then replace the pump. But this is what the pressure gauge was really meant to do, was to monitor the normal pressures and not the extremes and to warn of a possible problem.
My .02..
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Joined: Nov 2001
Posts: 3,733
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From: Austria Europe
pcab: Since we can't measure the pressure where it really matters, inside the vp44, we will have to go on assuming.
If you read 1psi at WOT before the VP44 will there still be enough pressure after the restriction of hte inlet- after the channel to the rotary vane pump? Now we can just assume. If there was not enough pressure the rotary vane pump inside the VP44 would start to cavitate, therefore deliver significantly less fuel to the timing module and the high pressure pump. And since cavitation will lead to less fuel flow there and also to not properly filling the elements of the high pressue pump it will make the plungers run on partially dry until the plunger travel has reduced the volume of the element that much that it must hit fuel. Actual injection timing will be influenced by this. Shock loads due to high plunger speed while on the steep part of the ramp when hitting the fuel instead of relatively slow loading of the plnger on the flat part of the ramp.
Just my 2c
AlpineRAM
If you read 1psi at WOT before the VP44 will there still be enough pressure after the restriction of hte inlet- after the channel to the rotary vane pump? Now we can just assume. If there was not enough pressure the rotary vane pump inside the VP44 would start to cavitate, therefore deliver significantly less fuel to the timing module and the high pressure pump. And since cavitation will lead to less fuel flow there and also to not properly filling the elements of the high pressue pump it will make the plungers run on partially dry until the plunger travel has reduced the volume of the element that much that it must hit fuel. Actual injection timing will be influenced by this. Shock loads due to high plunger speed while on the steep part of the ramp when hitting the fuel instead of relatively slow loading of the plnger on the flat part of the ramp.
Just my 2c
AlpineRAM
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Joined: Apr 2003
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Here's a sample of what you can read if you search. Or just throw $600 at your problem. Restriction does change as you go from idle to WOT at the inlet to the VP.
(From TDR)
we installed a Sonnex digital flow meter ( inside ID is 5/16") in the return line back to the tank , cut in right at the trans crossmember . gph numbers are fuel returned to the tank .
DD2 , DD TTPM
FP volume
idle 11 psi 18gph
cruise 8 psi 24gph
WOT 2 psi 30gph
DD2 , TTPM , HOT PE ANY LEVEL
FP volume
idle 11 psi 18gph
cruise 8psi 24gph
WOT 0 psi 30gph
yes you read that correctly , fuel pressure went to 0 and the fuel volume back to the tank did not change from WOT with 2 psi . the truck did NOT stumble , buck , spit , fart , fall on it's face , nothing , ran like all heck at 0 psi , i will note , my truck ran the same at 0 psi with the DD3's , EZ and a HOT PE .
(From TDR)
we installed a Sonnex digital flow meter ( inside ID is 5/16") in the return line back to the tank , cut in right at the trans crossmember . gph numbers are fuel returned to the tank .
DD2 , DD TTPM
FP volume
idle 11 psi 18gph
cruise 8 psi 24gph
WOT 2 psi 30gph
DD2 , TTPM , HOT PE ANY LEVEL
FP volume
idle 11 psi 18gph
cruise 8psi 24gph
WOT 0 psi 30gph
yes you read that correctly , fuel pressure went to 0 and the fuel volume back to the tank did not change from WOT with 2 psi . the truck did NOT stumble , buck , spit , fart , fall on it's face , nothing , ran like all heck at 0 psi , i will note , my truck ran the same at 0 psi with the DD3's , EZ and a HOT PE .
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Joined: Apr 2003
Posts: 6,564
Likes: 6
From: Cummins Technical Center, IN
Originally posted by pcab4
Not necessarily. Pressure is the measure of restriction and not volume. If you put your finger over the end of the hose you create pressure and lower the volume and releasing your finger lowers pressure and increases volume. If you notice that your psi's slowly degrade over time and you've eliminated possible causes then you should be concerned with the pump giving out. But as long as you have 1 psi at WOT then you know that your pump is still producing more volume than the VP can use. Remember that the fuel return is about 70-90 percent of the fuel delivered to the VP depending on setup.
If you have 15 psi at idle and 0 psi at WOT what is your return line volume? Is your lift pump bad? If you have 15 psi at WOT is your lift pump good or do you have a restriction?
You can spend $600 on a FASS or replace your lift pump about 5 times. Is 30 minutes of labor worth $600? I still have my original pump at 33,000. Is this newest pump from Cummins a better one? They seem to think so. The older ones were horrible but the failure rates from Cummins on the latest released pump are down substantially.
Bill K did a big write up on this in the TDR and did it with a flow meter on the return line. Read this and see how people measure PSI and confuse this with a good or bad lift pump verses what the actual volume of the pump output is.
With this said if you used to get 15 psi at idle and now 6 months later you are seeing 8 psi at idle and you've eliminated possible causes, then replace the pump. But this is what the pressure gauge was really meant to do, was to monitor the normal pressures and not the extremes and to warn of a possible problem.
My .02..
Not necessarily. Pressure is the measure of restriction and not volume. If you put your finger over the end of the hose you create pressure and lower the volume and releasing your finger lowers pressure and increases volume. If you notice that your psi's slowly degrade over time and you've eliminated possible causes then you should be concerned with the pump giving out. But as long as you have 1 psi at WOT then you know that your pump is still producing more volume than the VP can use. Remember that the fuel return is about 70-90 percent of the fuel delivered to the VP depending on setup.
If you have 15 psi at idle and 0 psi at WOT what is your return line volume? Is your lift pump bad? If you have 15 psi at WOT is your lift pump good or do you have a restriction?
You can spend $600 on a FASS or replace your lift pump about 5 times. Is 30 minutes of labor worth $600? I still have my original pump at 33,000. Is this newest pump from Cummins a better one? They seem to think so. The older ones were horrible but the failure rates from Cummins on the latest released pump are down substantially.
Bill K did a big write up on this in the TDR and did it with a flow meter on the return line. Read this and see how people measure PSI and confuse this with a good or bad lift pump verses what the actual volume of the pump output is.
With this said if you used to get 15 psi at idle and now 6 months later you are seeing 8 psi at idle and you've eliminated possible causes, then replace the pump. But this is what the pressure gauge was really meant to do, was to monitor the normal pressures and not the extremes and to warn of a possible problem.
My .02..
PCAB, everything you posted is correct. But HAULIN is saying exactly what you are. He is saying that if you maintain pressure AT HIGH VOLUME, then you know you have the volume. This makes perfect sense since normally higher volume means less measurable pressure. Again, this is consistent because the higher fuel usage of the VP poses less of a restriction.
I have posted till I'm blue in the face trying to get people to understand the view of pressure as a restriction. But it's all relative to where you measure the pressure. For example, you see MORE pressure with less restriction in the supply side, while you see LESS pressure with less restriction on the VP side.
You have to keep your apples and oranges straight when talking about this stuff.
Good posts, guys.
Justin
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Joined: Apr 2003
Posts: 6,564
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From: Cummins Technical Center, IN
Yes, it should be less. With less restriction you have more flow (volume). Since the restriction posed BY THE VP is not changing, but the available volume of fuel is larger, then the ratio is different now, and you should see much less pressure drop.
This is why some guys with the FASS/Prep systems are seeing almost NO DROP AT ALL even at WOT. This is because the delivered volume of the system is so great that the variable restriction the VP poses is a drop in the bucket.
Justin
This is why some guys with the FASS/Prep systems are seeing almost NO DROP AT ALL even at WOT. This is because the delivered volume of the system is so great that the variable restriction the VP poses is a drop in the bucket.
Justin
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