I read on here all the posts i could find on the subject. I couldn't find one where the resolution was actually obtaining 30 psi or more at higher rpm. I thought I would ask if anyone was able to get the 30 psi+ that is mentioned as ideal when the engine is in the upper rpm range?
What I have done so far.
I bought a pressure gauge, snubber valve and line to get my pressure readings.
With the gauge hooked up I was reading 15 psi or so at idle, maybe 8 psi at 2000 rpm.
So I ordered the tork tech adjustable over flow valve. Installed it idle pressure was around 23 psi that was great. The psi would drop to 14 psi at 2000 rpm.
Called and talked to the guy at tork tech, he said I have air entering the fuel line somewhere.
I read all the posts about changing fuel line. I decided to replace the fuel line between the larger filter and the lift pump
on the engine first and it was easiest.
Then I made a rookie mistake and did several steps at once. I replaced the lift pump thinking what the heck I am going to be down there anyway why not. Also after some reading I decided a larger fuel line might also help.. The factory line was 5/16" between the lift pump and large filter. So I went and got half inch fuel line. Hooked it all up and got 15 psi at idle and 2000 rpm. I decided maybe the fuel line was 5/16" for a reason and put 5/16' fuel line back in place. No change at idle still 15psi and and the psi drop at 2000 RPM.
Side note apparently all lift pumps are not built the same.
I replaced the original lift pump and went back up to 23 psi at idle and the drop of psi at 2000 rpm.
Then I decided to try the 1/2' fuel line again and that helped 23psi at idle and 23psi at 2000 rpm.
I don't know if the larger fuel line will hurt anything but it sure stopped the psi drop between idle and 2000 rpm.
At this point I am thinking that might be as good as I can do for lift pump pressure, unless someone has had success at obtaining an increase of fuel line pressure as the rpm increase.
Any thoughts or comments are appreciated.
You are doing some interesting experiments. I did some of these same things a couple years ago, although I never got into changing out fuel lines. You might have already read the long thread we had going on this subject, but if not:
Fuel System Science Project (http://www.foreforums.com/index.php?topic=21795.msg163432#msg163432)
To answer your question, I have never determined what my fuel pressure is at "full throttle". I couldn't come up with any easy way to do it. I have a fuel pressure gauge in the engine compartment, but the DW was not excited about my suggestion that we could raise the bed and she could watch the gauge while going down the highway at full throttle. I don't see what the problem is...
I get about 25psi fuel pressure right after a cold start. It seems to decrease slowly as the engine warms up. Pulling off the road after a hard run, I'll see around 18psi at idle. In spite of having the snubber on the line, my (liquid filled) gauge jumps around a LOT - I need to work on that (when Bill gives me another round TUIT). 8)
I feel like if you are maintaining around 20psi or so at normal cruise RPM's then you are doing OK. You are obviously providing the high pressure fuel pump with all the fuel it can use, plus some.
Keep posting reports on your project. Photos are also good!
Was there an issue with the engine running correctly, or just curiosity? As long as the lift pump delivers enough volume to the high pressure injector pump that's all that matters in the end?
I had just gone through all this with an older 24 v cummins in my dodge truck. Enough fuel pressure on the dodges is critical to the fuel injection pump with the vp44 pump.
This may be a completely different animal with the p7100 pump. But since I was now armed with enough knowledge to be dangerous I thought I would apply it to the motorhome.
I was reading the fuel pressure while the rv was stationary, all I did was use the cruise to get up to 2000 rpm and hold it.
Chuck and Jeannie you gave me the idea of moving the gauge to where it can be read going down the road during driving driving conditions, I don't know if that makes a difference but I will try and find out. It maybe a few days before I can accomplish that. I did read the thread you mentioned several times. I don't remember anywhere that someone was able to increase the fuel pressure as rpm increased. I missed it if they did.
bbeane I don't know the answer to that question, the only way I have to measure what's going on is the psi gauge. Maybe this is as good as it gets or needs to be, thats what I was hoping to find out.
Here is what I took off. It was 5/16" air brake line with two 90 deg elbows.
I Replaced it with 1/2" fuel line with no 90 deg elbows. Everything I read said to avoid 90 deg elbows if possible.
If this is on your 96 then you have a 12 valve engine,not sure what your symtoms were before you started fooling with the
engine.What are you trying to achieve?How many miles were on the engine when you got the coach?
I have a 8.3c in the 95 u280.
Fuel pressure from the lift pump to the secondary filter / injector Pump stays at about 20psi at idle or 2000. Doesn't seem to change with rpm.
The P7100 should be a oil cooled / lubricated pump. Unlike the VP44 in my 99 dodge, and yes I've had to replace the vp44 because of a lift pump failure.
I now have a pressure switch on the fuel supply.
I've got theory on this as I've experienced a similar thing as you are reporting. I've installed an electronic fuel pressure gauge just prior to the injection pump so I get fuel pressure readings while driving.
The theory is this, our engines (325hp C8.3 for mine) have been turned up higher than the normal OTR trucks and I don't think the fuel system is quite able to supply the extra fuel necessary for the larger injectors and injection pump on our engines. When under load, my fuel pressure drops pretty quickly compared to idle as the injection pump sucks down a lot of fuel to send to the injectors.
The on engine fuel filters that the mechanical C8.3s use are really tiny filters (16 x1.5 threads and less than 30gph in most cases) that don't flow much fuel and I've noticed differences in the fuel pressure based on the filters actual ability to flow fuel. For example, the Fleetguard FS1251 (fuel/water separator) is only rated for 20GPH but the Fleetguard FF42000 (plain fuel filter) is rated in the 50GPH range. I've noticed superior fuel pressure when using the higher flowing filter. Also, even a slight restriction on the primary fuel filter can start to starve the lift pump of its pumping ability and decrease the fuel pressure.
After reading a few posts awhile back on various Dodge/Cummins sites, I'd also wonder if the line that runs from the output side of the lift pump all the way to the injection pump is holding back fuel flow. I seem to recall a poster on those sites who replaced all of it on his Dodge and reported much better numbers. This is also what the various aftermarket electric fuel pumps too. They call for 3/8" ID tubing directly to the injection pump from the electric lift pump.
Fuel pressure is not the same as flow. I fail to see the real dilemma with 8psi vs 18psi when both the lift pump and injection pump are positive displacement devices.
These 8.3 liter engines are in boats and cranked up to 450hp or more. I've never seen anybody measure or worry about fuel pressure in them.
As long as the injection pump is getting a good flow of fuel with no air bubbles in it, all should be well.
Cummins Diamond 450 (http://www.sbmar.com/articles/identifying-the-bosch-lift-pump-on-a-6cta-8-3-450-diamond-others/)
I think you'll see in this picture that the marine engines (at some point) use a completely different lift pump than our engines do. I seriously doubt our lift pumps flow enough fuel for that horsepower. Our engines use completely different CPLs than the non-marine engines do too. I'd guess that the high pressure fuel lines have a larger ID than ours to flow enough fuel to the much larger injectors on the marine engines.
The fuel pressure of our engines is tied directly to the fuel flow since the overflow valve is the last piece of equipment on our injection pumps and regulates the fuel pressure.
if your fuel line to the lift pump is the issue a quick test would be to run a short hose from the inlet of the pump into a five gallon container of fuel and see if your pressure readings increase. You can then move the hose around the fuel system to locate your restrictions and areas where air is getting in.
We found similar results after installing dash fuel pressure gauge using the same parts as Moss. And I have come to the conclusion that as long as I have fuel pressure, the injector pump may have enough fuel and higher pressures could mean that more fuel is available than needed. But more fuel flow then needed for injection could still cool pump.
Before installing the new fuel pressure gauge, our coach seemed to be running fine when Cindy & I proactively replaced our original 18 year old mechanical lift pump ourselves. To our surprise, after the new pump, we felt more uphill power, smoother takeoffs and generally better operation. Figured that before our new pump, we must have had some injector pump starving, but without a fuel gauge, we had no awareness.
So after the new pump, we installed Tork snubber, electronic fuel pressure sender parts & dash gauge. Wish we had it installed before our new lift pump to see how low the pressures were. We now saw pressures readings all over the place but mainly below 15 psi.
Then like Moss, we replaced original injector pump return valve with a new adjustable Tork valve and pressures increased, are still all over the gauge, now with some 30 psi peaks. The new return valve did not seem to increase our feeling of power, like the new lift pump did, but we like having the new fuel parts. Years ago we replaced fuel hoses from tank to primary fuel filter and from injector pump return to tank, but we did not make Moss changes from lift pump to injector pump.
Before the new mechanical lift pump, we examined installing one of the popular electric fuel lift pumps, but it started to get too confusing. And then a Forum member said why change something that is working, so we abandoned spec'ing out electric pump and went with a new Cummins mechanical lift pump.
Barry & Cindy, I think I will order the same lift pump mentioned in your other posts and try that. The off brand I purchased was not as strong as the original that is on it.
I am not sure why Foretravel put the small size hose between the filter and lift pump. You would think there was some purpose. It wasn't to be cheap, what I took off looked like it would be expensive to replace.
For whatever reason mine just won't get 32 psi at this time on my gauge at the banjo fitting that enters the fuel injection pump. What that really means I am not sure.
I did find this in the Tork Tek literature about the lift pump "During the pumping cycle, the pressure rises rapidly to 50 to 60 PSI. On the return stroke of the piston, the pressure drops to 10 to 15 PSI".
Maybe the gauge I have is just not sensitive enough to read what's really going on.
We pretty much have what you have relating to fuel pressure gauge.
We are running pretty well, not overheating and steady power on mountains pulling our 5,700 lb Jeep Grand Cherokee v8.
Our gauge max's at 30psi, but we very seldom that peak. I figure that it is good enough and who knows what a higher pressure would even do for us. We also have the pressure gauge tap at the injector banjo input.
I wonder if our gauge can react fast enough, especially with our dampening aids of the tiny banjo port, 2-foot braided hose, dampener at end of 2' hose, and then an electric sender into the 2nd dampener. Gauge needle does move very fast from one psi to another, so it does not look sluggish. Gauge also has a digital display that also quickly changes.
Just not in me to chase high pressures.
Thanks for sharing... (& directly to Foretraveler email address) Barry & Cindy
Barry, my electronic fuel pressure gauge runs up to 100 psi IIRC. The only time I will see more than 30 psi is when I quickly take my foot off the accelerator pedal while driving at higher RPMs. Fuel pressure will spike up to about 50 psi when that happens because it takes a moment for the overflow valve to deal with all of the excess fuel.
You could crank up the relief valve, and have very little flow, but nice high fuel pressure. You could also let off the relief valve and have lots of flow, but lower pressure. I do not believe pressure is a valid indicator of flow, in this case.
I think we are looking at "pressure" here, not as a indicator of flow rate, but rather a indicator of having a sufficient quantity of fuel available to the high pressure pump under all operating conditions. For example, back before I replaced my old overflow valve with the adjustable Tork Tek unit, I saw 3 psi on my fuel gauge at idle. It was not hard for me to envision a situation where this very low pressure might drop to near zero at some higher RPM, possibly resulting in cavitation of the high pressure pump. The lift pump was doing its job just fine, but my stock overflow valve was bypassing WAY too much fuel, resulting in low indicated fuel pressure.
Installing the adjustable overflow valve allowed me to raise my fuel pressure (at idle) to around 18-20 psi. At this pressure, I am more confident that there is always a sufficient quantity of fuel available at the injection pump inlet, and it will not starve. The lift pump is not pumping any more fuel (ie the flow rate has not increased), but more fuel IS available for use by the injection pump, because less fuel is being returned to the tank.
IMHO
The increase in pressure is created by restricting the discharge (aka return line) after the injection pump. The entire time, fuel is flowing through the loop and the injection pump sucks what it needs.
Restricting the return doesn't provide any more fuel to the injection pump--it really just created more work for the lift pump.
I understand there is anecdotal evidence and tork tek has pages of words explaining why it is so great. But there are lots of cummins engines on the road and nobody touches this stuff...and there are even more bosch inline pumps out there.
You make a good point, and I can't refute it because I don't really know anything about this subject except what I read. I have no concrete proof that "raising" the fuel pressure at the injection pump inlet has
any positive effect. But what, exactly, constitutes "raising the pressure"?
Here's a question: the original factory overflow valve fitted to our engines was designed to maintain some desired pressure in the fuel manifold.
What was/is that desired pressure?
I went looking for this number...
Cummins QuickServe Online has all the info on my engine. (If you have a Cummins, and haven't registered at QuickServe, you are missing out on a GREAT information resource) Cummins QuickServe Online (https://quickserve.cummins.com/info/index.html)
Under the "Service" tab, you will find "C Series Troubleshooting and Repair Manual". Opening that manual, going to Section 5, "Fuel System", then under "Fuel Pump", we find the procedures for checking, removing, testing, and installing the lift pump. The link below is a portion of the "Test" section. It instructs you to fit a test pressure gauge between the lift pump and the inlet to the injection pump. (Exactly where my fuel gauge is installed) Here is the desired test result:
"
Minimum pressure at
high idle is 138 kPa [
20 psi]"
High idle for my engine (listed under Parts/Fuel System/Delivery System/Base Fuel Pump/Remarks) is
2760 RPM.
SO, the factory wants to see a
minimum fuel pressure of
20 psi at 2760 RPM. Most of us who care about this subject have been measuring the pressure at idle. When we raise the RPM's, we have all witnessed the fact that the fuel pressure seems to either decline, or at best remain constant.
Therefore, I think it is reasonable to conclude that fuel pressure at idle should be no less than 20 psi.Yes, there are lots of Cummins engines on the road, and nobody touches this stuff. However, if you desire optimum engine performance from your coach (and who doesn't?), you should at the very least ensure the minimum standards of fuel pressure are being met. The only way to do that is to put a gauge on your engine and read the pressure. Or, you can ignore the whole subject and hope for the best.
Do what makes you happy!
2760 rpm at high idle? Seems a lot. My m11 varies from 900 to 1300 on the raised idle settings
An ISC runs more RPMs then the ISM at a given speed.
"High Idle", in diesel terminology, does
not mean the same thing as "raised idle like to help warm up the engine on a cold day".
I found this definition online in several diesel "textbook" references:
"Maximum no-load speed or
high idle is used to describe the highest engine rpm obtainable when the throttle linkage is moved to its maximum position with no load applied to the engine."
It is one of the functions of the engine governor to prevent the engine from exceeding this RPM limit. Here is the full reference document if anyone is curious, or wants to learn more about diesel engine technology:
http://www.free-ed.net/free-ed/Courses/15%20Transportation/HeavyVehicles/HeavySystemsShow.asp?iNum=0401
Chuck, can you post the entire test procedure? It also has a volume test. I'm curious to read what it says.
Krush, Here you go!
Seems like the thread is running slightly rough.
I have made a little more progress. I have witnessed for the first time an increase barely of psi at 2000 rpm.
I went and bought the cummins lift pump installed it. Also after noticing when I removed the fuel line between the large filter and the lift pump whoever installed that had a hard time actually getting any sealent on the threads. I decided to remove the two plugs on the filter housing and reapply sealant on the threads. Not sure what it did but easy enough to do.
Now with the pressure actually going up I noticed now the fuel manifold has several leaks in it. That's what they call it, it is the line that runs from the injectors back to the secondary fuel filter and joins the fuel flow to the injector pump. I am not sure yet but I think a large part of that line is plastic. I have high hopes for that being more of the problem.
I am off to get one of those and all the fittings from the Cummins house.
To a person at all the large after market suppliers of cummins specific parts said there is a problem if the fuel pressure won't increase with rpm.
Also there has been some question as to what does it matter as long as it's running. I found this in the cummins quick serve web site on the 8.3c
Just a friendly discussion amongst those having interest in "fuel-ish" subjects. 8)
I don't think the condition (good or bad) of the fuel drain line manifold is going to have any effect on your fuel pressure. We had a discussion about that manifold back when I did my fuel project. I get the impression (I may be totally wrong) that the drain line simply carries excess fuel from the injectors, "running downhill" to the secondary filter. The flimsy sections of plastic tubing are a press-on fit, and don't look (to me) like they can hold any appreciable pressure. I'm sure you will want to correct any leaks, but I doubt it will further your quest for higher pressure.
Here's the link back to my old post (for what it's worth...):
Fuel System Science Project (http://www.foreforums.com/index.php?topic=21795.25)
So it means full rpm and no load? Thanks.
Often missed are fuel lines.
They are known to internally break down (collapse) on the suction side of the pump after a number of years.
My 24V 01' 1 Ton has gone through (many) modification's after 400K.
Line size, smooth elbows, and a great many nights spent wondering what's going on with this fuel system. Most all of the step by step mods made a positive difference and wasted lots of time & money.
BTW, changing return line (increasing) pressure isn't positive.
The large FASS pump/filter assembly will end your problems.
The fuel system on these engines are not well designed (sad to say.)
The lift pump is not up to the task and craps out which then starves (kills) the main pump.
Notice how far the lift pump has to pull fuel from the tank unless you have the updated design (pump in tank). Unless you redesign the fuel system, they will never be anything but a PITA.
Dodge and Co. have not designed any room for error into these fuel deliver/filter/etc., systems.
12V & 24V 5.9"s and the 8.3's are (basically) the same design and have the same fuel delivery issues.
pc
My thought on the fuel drain line leaking it sounds like a great place to suck air in the system. It goes directly back in at the fuel filter from the injectors. It's a process for me since trying to fix this, I am not an expert.
Everything I read or hear is the low fuel pressure as rpm increases means air in the system.
Anyway that line is bad and it's definitely made with plastic lines. I will have it in soon and see if it helps or not.
I guess this is possible, but the way I understand the fuel flow diagram, the drain line would never be on suction. However, I am also a novice at this stuff, so I welcome any other opinions on this idea.
I consider this whole thread a GREAT learning opportunity! Thanks for starting it!
Reviewing the entire test procedure shows that unhooking the discharge of the lift pump and measuring volume of fuel pumped for 30seconds crank provides tangible evidence of proper pump operation.
I would agree more with the "fuel pressure" reading, if there was a procedure to dead head the pump and measure pressure. However, at the end of the day, the main concern is that the injection pump is being supplied enough fuel.
Because there is not a fuel pressure regulator, but instead an orifice on the discharge, the PSI reading (IMHO) is not a truly effective measurement of proper operation.
For example: let's say I installed a 2 inch hose for fuel supply and installed a high volume low pressure pump. Let's say I put a 4 inch return pipe. Say this pump was putting out 10GPM and the PSI reading at the injection pump was 5psi. Is the injection pump getting enough fuel?
One place I looked explained it this way: "You get the engine up to full operating temp, put the transmission in neutral, set the parking brake, and give the engine full pedal-to-the-metal throttle. Then you wait for the RPM to stop climbing and stabilize - that is High Idle."
ME, I don't have balls big enough to try that with our coach, but supposedly the engine is designed to live through it. I'll take their word for it! :o
out of respect for the gearbox, no thanks! pc
High idle on my mechanical 8.3 is 2400 or 2600...i cant' remeber...it's on the name plate. And it matches when I stomp the pedal. On boats they crank the engines up toe 2800 or more.
No luck with the return manifold. :'(
The only thing left for me to try is putting the fuel intake line from the lift pump in a 5 gal diesel can and see if it changes anything.
The lines from the lift pump forward, are all Cummins designed. They run all over the place and look quite involved as to what they accomplish.You would expect they ran the calculations to have the correct flow at a given psi or psi at a given flow and chose the lines to accomplish that. I don't have a flow gauge or a flow rate from Cummins, so I have used a psi gauge to get to try and get to their number.
If you have pressure in the system is that good enough, I don't know. A good question for a Cummins expert.
I will say again I talked to several place like TST, Tork Tek, BD diesel performance and they were all surprised the fuel pressure went down as rpm went up. Everyone of them said something is wrong pressure should go up with rpm.
If it doesn't change testing the fuel psi reading out of a fuel can and bypassing the rv fuel line, I guess that is as good as it going to get for now.
If it doesn't change I suggest moving the fuel container to the line where it hooks on the tank. I don't know what setup is used to suck inside the tank but on transports I have seen the metal tube suck air.
Another thought is if you want to see if you are sucking air is to put a clear section of hose in just before the injection pump. You will see even the smallest air bubbles in it.
While Cindy was behind the wheel on our San Diego to Yuma drive yesterday, I took a few fuel pressure readings. Our gauge max is 30 psi and has an analog needle and a digital display. PSI noted below is +/- 3 PSI, like 15 quickly ranged from about 13- 17, etc.
PSI
12 1600 rpm, 6th gear, 50 mph
15 1500 rpm, 6th gear, 50 mph
15 1700 rpm, 4th gear, 40 mph
20 1700 rpm, 5th gear
20 idle
27 1500 rpm, 45 mph driving downhill with retarder
30 some very quick peaks in all situations
Well I ran the fuel line direct from the lift pump to a 5 gallon diesel can. No change.
All the lines from the lift pump to the injection pump seem fine. If it is sucking in air from somewhere it's a mystery to me. It would have to be from somewhere on the engine and I see nothing obvious to cause that.
Since the same results seem to be posted by everyone about fuel pressure, it sure makes me wonder what is going on.
What would they use to measure the fuel pressure with at a Cummins shop? Maybe a snubber valve with a gauge hooked into the fuel injection pump inlet is not an accurate way to measure it?
That link I posted in Reply #24 (above) tells you exactly how they would test fuel pressure, if they follow the Cummins repair manuals. They have a fuel pressure test kit with a flex hose that replaces the lines from lift pump to injection pump. Pressure gauge is mounted inline on the hose.
You have already indicated that you have access to QuickServe Online. The Cummins repair manuals available there have all the test procedures spelled out in detail.
Moss9994,
It seems like your main concern is the fact that our fuel pressure does not increase when going from idle to higher RPMs. I agree this is troubling, from a theoretical standpoint. You would think, as RPMs increase and the lift pump cycles more rapidly, that pressure would increase. Perhaps PC, in Reply #28 above, has hit the nail on the head. The fuel delivery system on our coaches may be on the ragged edge of adequacy. Perhaps the increase in fuel consumption at higher RPM is greater than the increase in lift pump output, so fuel pressure actually drops. The lift pump can ALMOST keep up, but not quite. A bigger, stronger pump with more reserve capacity would seem to be the obvious solution (FASS?).
However, from a practical standpoint, it doesn't seem to matter much. My engine runs fine with 18-20 psi fuel pressure at idle, and "whatever" fuel pressure at 2000 RPM (normal cruise). How much benefit would I see from some incremental increase in high RPM pressure? Would it be worth the hassle and expense of adding a aftermarket pump? I don't know...
They are measuring with no restrictions between the lift pump and injection pump as the standard for the 30+- psi mark.
I have been doing it wrong. My gauge reads the the entire system, filter and all. My gauge setup is not what Cummings calls for in their test.
I am going to have to look at this again with that thought.
If the lift pump you have comes via CARTER and has the floating phanolic (sp) veins, you may have an issue.
I pull(ed) 36,000 lbs of oil field gear for over a decade with my 01' 24V HO 5.9L Ram until the patch hit the skids again. 390,000 miles makes me an EXPERT when it comes to the problems this series of diesel develops. Beside replacing the 6 spd OD transmission with an Eaton FS6406A assembly, the ONLY major issue was dealing with the Fuel System. I ended up with the Large FASS Kit after discovering all of the other mods were a water haul. Believe me, breaking down on the side of the road with a time critical load is not a good way to stay in the Hot Shot business.
If you decide to come up with a fuel system that will live... I suggest the LARGE FASS System.
No, you do not need to install their largest system but, I do like the idea of having a fuel system that will support over 1,000 HP and after 300,000 miles has not missed a lick.
T & T are on the market if interested.. PC
Thinking about this when working outside earlier. Seems the Cummins test is mainly to confirm the lift pump and return valve are working properly. Capable of putting out 30 psi and venting the excess back to the tank. And if that fails I am guessing the volume test to see if it is lift pump or return valve related.
It didn't say hook everything back up and check for 30 psi again at the inj pump.
The guy at tst diesel said there were many versions of the p7100 lift pump for the 8.3c
Thinking about all the fuel lines, filter and so on between the lift pump and injection pump, maybe Cummins puts everything where it needs to be for psi and yes flow to be set by that.
The lift pump is the same for 5.9 and 8.3 12v.
Maybe the fuel system design between the lift pump and the inj pump is what makes that possible?
If that is true maybe it's designed to be around the same psi at the inj pump at idle and higher rpm after going thru the entire fuel system after the lift pump. Because it is all mechanical it will seem off a little as rpm change when watching a gauge at the inj pump inlet.
Just thinking out loud trying to figure it out.
On my tractor, the injector pump is gravity fed.
Anybody ever get higher fuel pressure on their 8.3 cummins?
Higher than what? Mine runs at 18-20 psi at the injector pump inlet (at idle).
Research the Tork Tek overflo valves and/or Larry B's brand,if you have the Bosch fuel pump they are a good upgrade,the stock old one gets weak and lets more fuel to the overflo.
The lift pump has to keep the interior of the injection pump full of diesel at full load plus a bit extra. Checking the pressure at idle may not show any restriction in the filters, etc. The little ball on the return valve develops microscopic canals as it ages so replacing the valve at high mileage may be a good idea. The injection pump is a positive displacement pump and nothing will be gained by exceeding the manufacturer's recommended pressure.
Bosch P pumps are rugged but CUPS pumps don't tolerate low diesel pressure well.
Cummins lift pumps do fail so keeping an eye on fuel pressures via boost pressure is a good idea. A failing lift pump can also leak diesel into the crankcase diluting the oil. Check oil frequently and give it the sniff test at the same time. Never good to "make" oil.
Pierce
Where is the lift pump on an 8.3?
On the passenger side just below the engine mounted fuel filter.
Mechanical lift pump mounted on the engine block curbside mid engine.
See link below for photos:
new lift pump made a BIG difference (https://www.foreforums.com/index.php?topic=23353.msg181248#msg181248)
The Forum link just posted show our surprise how much better our coach ran after replacing C8.3 lift pump and return valve. We must have been light starving injector pump.
Should have installed our fuel pressure gauge before making any fuel changes to have a baseline to which we could compare.
Barry & Cindy