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Topic: lift pump psi on an 8.3c (Read 8681 times) previous topic - next topic

Re: lift pump psi on an 8.3c

Reply #25 –
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

Re: lift pump psi on an 8.3c

Reply #26 –
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
1993 U280 SE 40' WTBI, Build: 4359
C8.3 300hp, 6-Speed, Exhaust Brake
960 watts on the roof (6 x 160)
Sorento (or BOLT) on a Kar Kaddy SS
"Nature abhors a vacuum"

Re: lift pump psi on an 8.3c

Reply #27 –
Quote from: Chuck & Jeannie –
"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

So it means full rpm and no load?  Thanks. 
"Riding and rejoicing"
Bob
1997 U320 40' Mid entry, build 5132,  wtbi ce27, 4th owner
2007 Solara convertible
2 prodeco tech outlaw ss electric bikes

1095 watts solar
08 Ls 460 and a sc430
2000 Ford F-250 superduty 4x4

Re: lift pump psi on an 8.3c

Reply #28 –
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



S/W Houston 95" U320C SE/40' 
Build #4778  Cummins M11
Repairs & Covered RV Parking (BAO)
PPL is close..

Re: lift pump psi on an 8.3c

Reply #29 –
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.

Re: lift pump psi on an 8.3c

Reply #30 –
Quote from: moss9994 –
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.
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!
1993 U280 SE 40' WTBI, Build: 4359
C8.3 300hp, 6-Speed, Exhaust Brake
960 watts on the roof (6 x 160)
Sorento (or BOLT) on a Kar Kaddy SS
"Nature abhors a vacuum"

Re: lift pump psi on an 8.3c

Reply #31 –
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?
1998 U270 34'

Re: lift pump psi on an 8.3c

Reply #32 –
Quote from: Caflashbob –
So it means full rpm and no load?  Thanks.
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
1993 U280 SE 40' WTBI, Build: 4359
C8.3 300hp, 6-Speed, Exhaust Brake
960 watts on the roof (6 x 160)
Sorento (or BOLT) on a Kar Kaddy SS
"Nature abhors a vacuum"

Re: lift pump psi on an 8.3c

Reply #33 –
out of respect for the gearbox, no thanks!  pc
S/W Houston 95" U320C SE/40' 
Build #4778  Cummins M11
Repairs & Covered RV Parking (BAO)
PPL is close..

Re: lift pump psi on an 8.3c

Reply #34 –
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.
1998 U270 34'

Re: lift pump psi on an 8.3c

Reply #35 –
 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.

Re: lift pump psi on an 8.3c

Reply #36 –
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.
Toby a 94 u280
Cummins 8.3
6 speed Allison
Exhaust brake


Adopted by Derek and Annabelle

Re: lift pump psi on an 8.3c

Reply #37 –
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.
Toby a 94 u280
Cummins 8.3
6 speed Allison
Exhaust brake


Adopted by Derek and Annabelle

Re: lift pump psi on an 8.3c

Reply #38 –
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

Re: lift pump psi on an 8.3c

Reply #39 –
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?

Re: lift pump psi on an 8.3c

Reply #40 –
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.
1993 U280 SE 40' WTBI, Build: 4359
C8.3 300hp, 6-Speed, Exhaust Brake
960 watts on the roof (6 x 160)
Sorento (or BOLT) on a Kar Kaddy SS
"Nature abhors a vacuum"

Re: lift pump psi on an 8.3c

Reply #41 –
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...
1993 U280 SE 40' WTBI, Build: 4359
C8.3 300hp, 6-Speed, Exhaust Brake
960 watts on the roof (6 x 160)
Sorento (or BOLT) on a Kar Kaddy SS
"Nature abhors a vacuum"

Re: lift pump psi on an 8.3c

Reply #42 –
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.



Re: lift pump psi on an 8.3c

Reply #43 –
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
S/W Houston 95" U320C SE/40' 
Build #4778  Cummins M11
Repairs & Covered RV Parking (BAO)
PPL is close..

Re: lift pump psi on an 8.3c

Reply #44 –
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.




Re: lift pump psi on an 8.3c

Reply #45 –
On my tractor, the injector pump is gravity fed.
1998 U270 34'

Re: lift pump psi on an 8.3c

Reply #47 –
Anybody ever get higher fuel pressure on their 8.3 cummins?
1998 U270 34'

Re: lift pump psi on an 8.3c

Reply #48 –
Quote from: krush –
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).
1993 U280 SE 40' WTBI, Build: 4359
C8.3 300hp, 6-Speed, Exhaust Brake
960 watts on the roof (6 x 160)
Sorento (or BOLT) on a Kar Kaddy SS
"Nature abhors a vacuum"

Re: lift pump psi on an 8.3c

Reply #49 –
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.
96 U270 BUILD 4810
85 380SL
Drummonds TN.