Ok Gang... You all have seen the Dodge FSM book minimum pressure of 10 PSI. Well I started to do some thinging and relized even that is too low. Here lets take a look at a pic here... http://forum.mopar1973man.com/attachment.php?attachmentid=878 So now let assume your fuel pressure average is about 12 PSI which is above the 10 PSI minimum pressure. Driving down the highway at 55-65 MPH your going to be flowing at least 2-3 GPH through the pump. But now let add a twist to it. New situation... Going up to Seven Devils Campground its a 17 mile drive up a 1 lane dirt road that is steep. I'm dragging a utility trailer with 2 ATV's. Ok for the sake of the post we'll say the fuel pressure is still 12 PSI. Climbing that grade I'm going to be flowing 7-10 GPH through that pump which is a good thing because the high flow keeps the pump lubed and cooled. But now the weekend is over time to drive home. So on the way down I set the exhaust brake and put it into 2nd gear and let it limp down the rough old trail it take about 45 minutes to 1 hour to return to the highway. So now the whole way down the mountain there is 12 PSI heading into the VP44 and but now your not throttling much if at all coming down the steep grade. So this means there is no flow to the injectors... And the Overflow valve is closed because you need 14 PSI to open it. So basically there is no fuel flow. So now the electronics and the mechanical part start to heat up the stale fuel and break it down. Remember diesel fuel is a very poor lubricant anyways. So now you see this is really easy to burn up or damage a injection pump. Now I'm starting to suggest 14-15 PSI as a minimum pressure and 20 PSI as a maximum pressure. This now keeps the overflow valve open and the fuel flowing through the pump while its spining and keeping the VP44 electronics cooled... Any thought on this???

From what I can find on that it delievers fuel to the rotor shaft and parts. It continues forward to the vane pump from there. But still the fuel pump in the rotor area got to return back...

Mike the fuel from the inlet enters the vane pump first. From there, the low pressure fuel goes to the timing device and pressure solenoid / distributor shaft. The passage that the wire goes through last in your picture is where excess fuel from the pressure regulator returns fuel to the vane pump to be recirculated. It is also the passage from the timing device that returns fuel to the vane pump. The only fuel that exits out the overflow (outlet) is excess fuel from the distributor shaft / pressure solenoid (which comes from the vane pump). There just is no way for an external fuel pump to force fuel through the VP44 and out the overflow.

So you say coming down hill stops the fuel flow!I made a test, went down our road made fuel pressure checks, then came down a hill and down shifted to second gear.I thought the fuel pressure would climb. Well it did but very short, then it dropped down a couple pounds like i was giving it throttle.shifted up and pressure went up gave it throttle and pressure drops back to normal driving.

So you say coming down hill stops the fuel flow! I made a test, went down our road made fuel pressure checks, then came down a hill and down shifted to second gear. I thought the fuel pressure would climb. Well it did but very short, then it dropped down a couple pounds like i was giving it throttle. shifted up and pressure went up gave it throttle and pressure drops back to normal driving.

To the injectors... As long as the ECM seeing Idle Validation switch being true the fuel rate is governor only if at idle speed above that there is ZERO fuel flow to the injectors (coast down hill). As for overflow flow if the pressure high enough to open the overflow valve then fuel will flow through there. But I still question if the vane pump internally is enough at times to open the valve when then pressure are weak?

Ok... Here is a video to show the fuel return rate for a VP44 at 17 PSI.

http-~~-//www.youtube.com/watch?v=YREpPrMxkHU

I love the funky green/blue color of my fuel...

I heard rumors about a major pump builder planing on building a lift pump timer to keep the lift pump running after the engine is shutdown to cool the VP44. Well after doing the video I hardly doubt the idea will hold up being there is very little volume of fuel leaking though the overflow valve.

OK Mike you have just proven wrong the people who say that (with the engine off) fuel cannot get past the VP44 internal vane pump regardless of the LP pressure.Now the question is:Would higher LP pressure push more fuel through and open the vp44 return relief valve for a higher flow rate?

Well... I could increase the fuel pressure temporarily but using a pair of vise grips on the return line of the AirDog. Now I don't mean crimp it off completely but I mean to limit its return to increase even more fuel pressure into the VP44 for a test run say go up to 20 PSI and 25 PSI...

But I think what is occuring that the internal vane pump is limiting the flow through the pump and there isn't enough volume leaking through to build to 14 PSI to open the overflow valve to flow. So in theory incresing the inlet pressure should get to a point of opening. (Only in theory)

I know you ran a test when your LP was dieing and said it would hardly fill a bucket. How much was it supposed to fill in how much time? I would like to see how much flow it has idling at <14psi, when the overflow supposedly shuts.

From a long time ago when my lift pump was dieing (Stock carter) at 8 PSI it looked like the flow of the AirDog with the starter bump (the tiny trickle). I'll see if I can produce a lower pressure and do it again... As for the the test it was gallons per hour flow rating...

This much I know for sure Any pressures above 22 -25 psi you will see problems starting the truck when the Temperature outside rises. It may take as long as 30 sec's of cranking to get the truck to fire. It has something to due with holding the shutoff solenoid closed when High pressure hits the internals of the vp44. So If you have hard start condition in warmer 75* - 99* ambient air temps try to keep vp feed presure below 22 psi. Or install a cutoff switch to your secondary fuel pump start the truck and throw the switch back on.

Or install a cutoff switch to your secondary fuel pump start the truck and throw the switch back on.

Or a relay cutoff...

^^ That's how mine is set up.

I've been reading a lot of these threads about fuel pressure at the inlet of the VP44, min of 5psi or 10 psi, or 14psi, or 16psi, etc. I think the approach that has been taken that more inlet pressure translates into more flow through the pump is not linear. This is because the the VP44 has a displacement pump built into itself. This can easily be demonstrated by running a lift pump to the VP44 without the truck running and dumping the return to the pump into a bucket, practically no flow from the return of the VP44 without the engine running, doesn't matter if it's 5psi or 15 psi. Only when the VP is running will you see a good flow out of the return. The return has a pressure relief valve set at about 10 psi with a rather small area for the return fuel to flow through and only a certain volume of return fuel is capable flowing through this small port. Pressure and flow are NOT directly proportionate, meaning you can have all kinds of pressure and no flow, just as you can have a lot of flow with very little pressure. It would only stand to reason that any fuel (except for the small amount that leaks past the displacement pump in the VP44) that flows through the VP44 is controlled by rpm of the VP44. Even if you change the inlet pressure from 5 psi to 15 psi there should be very little flow change out of the VP44. Such as if you had 1 gpm return flow at 5 psi inlet pressure at 850 rpm, I would venture that in the same scenario at a 15 psi inlet pressure you may only see an increase of maybe 10 percent in the return line. End result being no matter what you try to push into a VP44 with stock lines or 1/2" lines at 5 psi or 15 psi, the built in displacement pump and the small (less than 1/8th inch) overflow valve is what really determines the flow through a VP44. Remember this style pump was used on the VW's with NO lift pump only a couple of parts being different in the VP housing and they run next to forever.

I think most of the problems that have been had with the VP44 can be attributed to the VP trying to pull fuel through a dead lift pump, matters not what kind, stock, intank, fass, or whatever, that causes insufficient flow and a vacuum state in the VP that causes, cavitation, which combined creates a serious lack of cooling and lubrication trashing the pump. This issue could have been solved by having a check valve that would allow the VP to pull fuel around a dead lift pump unrestricted. Their are other design issues such as the advance piston on the bottom with poor at best lubrication and no flow through to move particulates along an out of the piston housing that creates excessive wear and accumulation, but thats a story for a different time.

--- Update to the previous post...

Also you have to take into account the change in area of the lines from the inlet to the pressure relief valve. Shrink the area for a given flow = an increase in pressure. Meaning your taking the voulume of the inlet line, 7mm stock i think, on pushing it with the VP44 internal pump through the pressure regulator's 2mm opening. So in theory a 5psi inlet pressure could easily translate into 14 psi at the ball of the pressure regulator opening due to the decrease in area.

I agree with ddossdd. Simply put pressure does not equal volume. All the pressure in the world can't flow volume thru a 1/8" pipe. Read up on basic fluid dynamics. I'm not saying there isn't flaws in the VP44 But some people seem to put to much emphasis on pressure. Smoothing out the restrictions in the plumbing might be more productive. Lots of good stuff being said, just had to put my 2 cents in. MEH

Just like I updated the article page with basic flow calculations...[TABLE][TR][TD=width: 33%]Pipe Size[/TD][TD] Gallon Per Hour Flow Rate[/TD][TD] Gallon Per Minute Flow Rate[/TD][/TR][TR][TD] 6mm ID pipe[/TD][TD=width: 33%] 75 GPH @ 15 PSI[/TD][TD=width: 33%] 1.2 GPM @ 15 PSI[/TD][/TR][TR][TD] 1/2" ID pipe[/TD][TD] 570 GPH @ 15 PSI[/TD][TD] 9.5 GPM @ 15 PSI[/TD][/TR][/TABLE]

I saw a few refrences to not being able to pump fuel thru the VP. I have also seen this stated in other forums. I believe MM showed you could pump it thru the return in his video. When I installed my AD i was using the AD to reprime the fuel system and had 3 of my injector lines open. the AD pump fuel right thru the injector lines. I spent 45 minutes trying to find a leak because i forgot they were open. Is this unusual or the norm?

Not typically no... Usually you have to crank the engine to pump fuel from the high pressure side.

I have had a couple people call me on that, but i did get a fair amount of fuel thru it with just the AD. The VP is one from blue chip and had less than 10k on it wen i installed the AD. I have put 25k on it since with no issues with the fuel system. I guess i will just live with it.

I was referred to visit this site by dripely and I was reading all the inputs on the vp44, I by no means claim to be a diesel mechanic but I am intriguied by all the theories being brainstormed here , I just installed a AIRDOG ll on my 98 24v and am experiencing longer cranks than I am used to, prior to the install I experienced the longer cranks only a few times when engine is warm after a run to work stop and get back in , Now it cranks long 2 to 4 seconds or longer wont start then try again then it will start ,I had cummins check fuel pressures and the found it had 25 psi going to the vp44 they called airdog and told them what was going on and they said it was ok the pump is only spec for I think 18psi so they cummins cut the spring down per airdog tch says its common and adjusted airdog ll to reach 15 to 16 psi they say all is good but I am still having long cranks I do not have my gauge yet its on the way .Airdog says when starting dont wait till pump stops just get in and start it ,that works some of the time . any feed back :banghead: would be appreciated thanks

I was referred to visit this site by dripely and I was reading all the inputs on the vp44, I by no means claim to be a diesel mechanic but I am intriguied by all the theories being brainstormed here , I just installed a AIRDOG ll on my 98 24v and am experiencing longer cranks than I am used to, prior to the install I experienced the longer cranks only a few times when engine is warm after a run to work stop and get back in , Now it cranks long 2 to 4 seconds or longer wont start then try again then it will start ,I had cummins check fuel pressures and the found it had 25 psi going to the vp44 they called airdog and told them what was going on and they said it was ok the pump is only spec for I think 18psi so they cummins cut the spring down per airdog tch says its common and adjusted airdog ll to reach 15 to 16 psi they say all is good but I am still having long cranks I do not have my gauge yet its on the way . Airdog says when starting dont wait till pump stops just get in and start it ,that works some of the time . any feed back :banghead: would be appreciated thanks

The stock programming in the ECM turns on the LP for a second or so when the key is initially turned on. this builds the pressure that causes hard starting with an aftermarket LP running pressures over 10 PSI. To resolve this you can put on the switch to disable the LP until the engine starts (I believe Mike has the diagram for it) or you can find someone who has a SMARTY and have them put a SMARTY tune on your ECM then Put the stock program back on the ECM. What this will do is disable the initial one second pump run and also turn on the cold weather high idle and the 3 cyl high idle features. It should solve the hard start problem for you. Or as an alternative go to a dealer and pay $100 for them to do the same thing if you can convince them it can be done (they are usually in denial about the initial pump run and high idle features). I have a SMARTY and have done this on my 98 and a few others. I'm in Roundup MT so if you wanted to stop by I would do it for you, but it seems like a long drive from where you are for just that.

Yes , its a long trip indeed but I here it a nice place to visit ,even though I had a issue with longer cranking before the airdog install when engine is hot ? And it got worse when the airdog was installed ! Now I am hereing by the folks that installed the airdog ,thats the vp44 that needs replacing and now that the cummins folks seen it it started to be some other issue and not the VP and now that they charged 5 hundies in looking at it they they are stumped, one of the techs says it can only be the VP, is there any other way that a guy like myself with very little diesel knowledge do without bringing it in to a dealer its a $2100.00 fix at the cummins dealer using there reman injection pump ,do they go through the pumps or do they have them rebuilt by a a bosch certified tec and bench test it and replace all the componets includeing the computer,wow thats alot ,sorry and only give a 6 month warrenty .