When I install my big line kit from FF to VP, and if I keep my pressure gauge tapped at the fuel filter test port, will the increased fuel line size result in more pressure at the VP inlet that I won't be able to see on my gauge because it's mounted at the fuel filter test port. Just curious. Thanks.

The test port is fine don't worry about it... The gauge will show proper pressure...

The test port is fine don't worry about it... The gauge will show proper pressure...

So, in other words the pressure through the big line from filter to vp will go'backward' to give me correct pressure... thanks.

The pressure from the filter to the VP44 through all the lines will be the same... If you mount the gauge on the VP44, or the output of the filter... All the pressure should be the same...

Agree with M73M, the line will be too short in length to have any pressure drop only a foot away. I have the OE line going from my OE filter housing, and I am getting 20+ PSI at the VP44 inlet, I am sure it would be a little higher at the filter housing, but at that short of a distance, only a very little bit higher. I might pick up a couple of banjo fittings and put the left over hose to use later?

If the test port is on the outlet side of the filter then you will not be able to measure the differenc between the FF and VP. If its on the inlet side thats a different story.

If the test port is on the outlet side of the filter then you will not be able to measure the differenc between the FF and VP. If its on the inlet side thats a different story.

If you had a electric fuel pressure gauge and 2 senders you could hook up a sender in both port (with needle valves to protect the senders). But then wire the senders to a toggle switch in the pod (or anywhere you wanted) then be able to toggle between the 2 sender and see how plugged up your filter was. If the fuel pressure drop was more than 5 PSI then its time for a filter. But remembering the minimum pressure... Just a crazy dumb idea... But goes along with AH64ID...

Good info - now for the next question: If my pressure valve on the return line doesn't open until 14 PSI, but I'm rolling down the road with 13 PSI, what happens to the fuel in the VP which isn't used by the injectors, but does not return back to the tank because the valve is not open? I'm still learning this system, so please bear with my questions. Thanks. In response to AH64, I thought the whole idea was to measure pressure on clean side to give you a clean pressure reading with a clean filter, and when you see a pressure drop (in my case, filter is clean - just changed it couple weeks ago), the first culprit to check would be the filter before suspecting the lift pump. All the recommendations I've read stated to tap the pressure on the clean side, maybe I didn't understand your response, so just seeking further clarification - thanks again.

Good info - now for the next question: If my pressure valve on the return line doesn't open until 14 PSI, but I'm rolling down the road with 13 PSI, what happens to the fuel in the VP which isn't used by the injectors, but does not return back to the tank because the valve is not open? I'm still learning this system, so please bear with my questions. Thanks. In response to AH64, I thought the whole idea was to measure pressure on clean side to give you a clean pressure reading with a clean filter, and when you see a pressure drop (in my case, filter is clean - just changed it couple weeks ago), the first culprit to check would be the filter before suspecting the lift pump. All the recommendations I've read stated to tap the pressure on the clean side, maybe I didn't understand your response, so just seeking further clarification - thanks again.

I would think you would be still getting fuel return through the injector return lines at 13 PSI? I would suspect you would still be getting good VP cooling at that pressure?

Good info - now for the next question: If my pressure valve on the return line doesn't open until 14 PSI, but I'm rolling down the road with 13 PSI, what happens to the fuel in the VP which isn't used by the injectors, but does not return back to the tank because the valve is not open? I'm still learning this system, so please bear with my questions.

Nothing... There would be technically no flow... The injectors are not firing so there is no return... And the Overflow valve is closed... So the only fuel that would be moving is what little fuel there is bleeding past the 0.016 size hole in the overflow valve. Maybe that why I opt for the 15-17 range of pressure most of the time... At least then I know the overlfow valve is open...

So, nothing returns from the injectors? When does fuel get returned from the injectors? I may be ignorant, but there is a return line from them right? At least I know there is a Tee in the return line. Refresh my memory please.

So, nothing returns from the injectors? When does fuel get returned from the injectors? I may be ignorant, but there is a return line from them right? At least I know there is a Tee in the return line. Refresh my memory please.

I think I mis understood the last post... I was assuming coasting/deceleration conditions with <14 PSI of fuel pressure... (re-posted to clarify). If you coasting down hill or decelerating... There is no fuel pumped to the injectors if there was you would be accelerating or maintaining road speed. The injection pump is literally turned off. So there would be zero return from the line at the back of the head. The only time fuel returns from the rail is when the injectors are fired (bleed off pressure) is drained to the return rail. This small bit fuel is the remain of fired injectors and bleeds off the line pressure and drain into the return line. So since coasting/deceleration tends to shutdown the VP44 from firing the injectors that means that return rail will be dry and if the fuel pressure is below 14 PSI so is the overflow return line. (except what will ooze out of the 0.16" inch bleed hole...)

I was thinking 13 PSI under load, what happens to the return rail under those conditions? But I still dont know why I am getting such a large fuel pressure drop coasting down a hill? Remember I did post that a few days ago.

I'm not sure... But there has been talk in the past that the VP44 has a small vane pump in it but how much pressure, suction, volume it capable of producing is totally unknown to me. But from what I've learned so far is if the truck is coasting then the injectors shutdown (VP44 doesn't produce line pressure). So this means that the overflow valve must be venting... But theat hard to measure return volume while cruising down the road... So maybe in your case the overflow valve is a bit weak opening at 13 PSI and venting back to the tank... Or that little vane pump is giving it that extra 1 PSI to open the overflow valve... (speculation)

I replaced the over flow valve less then a year ago, but that is no guarantee that it is not working properly.

Well' Give this a thought, If the overflow valve on the return side is stuck open or partially stuck open then the fuel pressure would only build up & hold as long as there was pressure comming in...When you are idling or deaccelerating very little pressure is flowing thru the VP Pump and with the stuck overflow valve there can't be any pressure because it is being released as quick as it comes in I mean i could be wrong so take for what its worth ...sound logical though

Good info - now for the next question: If my pressure valve on the return line doesn't open until 14 PSI, but I'm rolling down the road with 13 PSI, what happens to the fuel in the VP which isn't used by the injectors, but does not return back to the tank because the valve is not open? I'm still learning this system, so please bear with my questions. In response to AH64, I thought the whole idea was to measure pressure on clean side to give you a clean pressure reading with a clean filter, and when you see a pressure drop (in my case, filter is clean - just changed it couple weeks ago), the first culprit to check would be the filter before suspecting the lift pump. All the recommendations I've read stated to tap the pressure on the clean side, maybe I didn't understand your response, so just seeking further clarification - thanks again.

"I would think Nothing,.. it sets there until it builds 14psi and then opens & closes in a nano second, will be very hard to actually see on anolog gauge when only falling back 1-2psi.." with a lift pump thats shot it may not open at all which means 0 return... = wiped-out VP pump

Here is a link to the ISB OM. https://quickserve.cummins.com/qs2/portal/manualviewer.html?path=/qs2/pubsys2/xml/en/outlines/3666193.html See: Section F - Familiarization, (53-200-001-tr) Flow Diagram, Fuel System, and you will see at the back of the engine, a fuel return line from the injectors. So yeah it does have a return from the injectors.

Yeah it does... But remember there is only going to be flow from this return only if the injectors are firing... If the injectors don't fire the no return... The 2 pics show the return line from the head and the tee connector...

Yeah it does... But remember there is only going to be flow from this return only if the injectors are firing... If the injectors don't fire the no return...

That sounds logical.

Well, I'm on my way now, got the bug. Couldn't wait any longer and installed my 3/8 vulcan line from FF to VP this afternoon. Moved my FP gauge down to the tapped fitting for the VP inlet. Re-primed the entire system (running isolator with little to no loss tested against a test gauge). Still running around 14-14.5 at idle, so thinking that lift pump not quite where it should be, but primes (key-bump) at a higher PSI than before I changed out the line, so thinking that opening that line up a bit helped out (positive thinking). Stuck now on next move, been reading past couple hours on airdog v. fass, etc (again, don't want to stir up a debate, as I understand it all will boil down to personal preference), or raptor v. fass hpfp, we'll see. Stuck my head up under the truck today and got a decent look at the fuel tank top, and think I can do this no problem without dropping or moving anything once I decide on which system to go with. I appreciate all the insight and suggestions here, and look forward to keeping you all posted on my next move. I did drop another 150 or so today with Geno's on some nice to have things like drain plugs, etc. Question, can I expect a bigger PSI difference if I just moved the OEM pump back to the frame rail, or will it just be a matter of it being easier on the pump because it's pushing not sucking? Also, I was reading elsewhere here of running air dog with factory filter still in place for fuel heating, will that head loss of fuel moving through 3 filters v. the airdog's 2 before the VP affect the overall pressure too much, or is the regulated pressure of the air dog sufficient to overcome this head loss? The good thing with all this is I'm no longer a virgin in the ways of messing with my fuel system and getting air out of it. I remember when truck was new, and original VP went out due to incorrect fuel gauge and fuel starvation, that first time I changed out the filter how long it took to get started again made me really worry that I was hurting something. Now I can get it back up and running with a few bumps and turns of the wrench in no time. By the way, pulled 23.4 out of last tank of fuel, hand calculated compared against the SGII, and includes 2500 feet of elevation drop/gain for my round trip to work. Been really grandmothering the thing though to achieve that, i.e. 55-65 depending on the gradient. I calculated last night a savings of around 150 bucks a month v. 15-16 mpg at 82 mph which is where I was 6 weeks ago which equals more goodies for the truck.