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???

Are you sure this diagram pertains specifically to the VP44 or is it a generic diagram for most solenoid-valve controlled radial-piston distributor injection pumps like it is labeled? Which part on the diagram is the infamous diaphragm? This is labeled as the high pressure stage, is this implying that the overflow valve is venting the high pressure side of the pump? I'm just a little confused by the drawing perhaps you could clear it up for us?. Jim

Although this diagram shows a four lobe HP pump, I am sure it's operation is the same as our beloved VP-44. It's a little difficult to see, but if you look closely, you'll see three different shades of blue. The dark blue is the high pressure Pump out to the injectors; the medium blue is downstream of the internal pump (about 14 PSI mostly controlled by the overflow valve); the light blue on the left is from the lift pump. As you see, there are a lot more solenoids, valves, etc it there which are controlled by electrical signals and medium pressure. There is also a relief valve bypassing the LP pump back to it's inlet if pressure gets too high. The overflow valve is shown on the far right. I'm not too sure where the famous (infamous) diaphragm lives; I'll have to do a little more looking. It's been a couple of years since I really dug into this. I'll see what else I can find that might be of help. humbly jimmy

This is a very interesting thread. It's good to see so many minds at work. Just to add another thought into this, I believe we are all overlooking some of the design of the VP44~~like the internal low pressure pump INSIDE the VP. By design, this pump should try to keep the pressure above the 14 psi set point of the overflow valve; thus, as long as there is adequate flow to the VP, the overflow valve will always be open somewhat to maintain the 14 PSI. I believe (there's that word again), the 10 PSI minimum suggested by the FSM is to ensure adequate flow, mostly through the overflow valve, to cool and lubricate the VP. Looking at the picture of the VP, it looks like the overflow valve is very near the inlet fuel inlet; in reality, as far as operation of the VP, it is actually in another chamber downstream of the internal low pressure pump.

Just my thoughts.

Jimmy

I also believe you are right Jimmy. Several years ago there was a rather heated argument on another forum about whether the lift pump can pump fuel through the VP44 and out the overflow valve or just to the chamber before the internal vane pump. The consensus among people who understand fuel flow through the VP44 was there is no route for the lift pump to push fuel through the VP44. It would require pushing fuel past the spinning vane pump which isn't possible. To confirm this I emailed an expert on the VP44 to ask if increasing fuel pressure from the lift pump would increase fuel flow through the injection pump and out the overflow. This was his response:

" Hi ED,

This is a really good topic. A lot of people assume that the inlet and outlet of a VP44 are directly connected, this is false. You are right the transfer pressure is fed to the injection pump enters a vane pump (supply pump) where pressure can be increased up to 250 psi, which is regulated by the pressure internal regulator.

Increasing the supply pressure over 15 psi will increase the internal supply pressure slightly due to the mechanical operation of the pressure regulator, but will not make a difference between fuel flowing out the overflow valve or not.

One thing to always remember is that not all failure of the VP44 pumps are caused by transfer pump pressure. People like to simplify things when it comes to vehicles and make assumptions with no proof.

Here is a diagram of the fuel flow through a VP44 pump."

Here's a link from TDR with some excellent pictures of a torn town VP44. http://www.turbodieselregister.com/user_gallery/displayalbum.php?&albumid=17275

Here's a link from TDR with some excellent pictures of a torn town VP44. http://www.turbodieselregister.com/user_gallery/displayalbum.php?&albumid=17275

WOW, great display pictures of all the parts of the VP44. You are right the only thing that higher pressure will do is provide a greater available volume if the supply line is larger than stock. It cannot open the overflow valve further due to the pump's internal design. I'm not going to reduce the pressure on my lift pump though, I'm comfortable with the 17-17.5 at idle. Thanks for the information, never too old to learn. Jim

No kidding this is worth taking the tidbits of information here and re-writing my fuel pressure page... Of course i gave proper thanks to these people! Even voted on the thread! (Excellent!)

This is THE topic have been looking for for over a year. If I understand what you are saying..."increasing the inlet fuel pressure ,will not help hold open the overflow valve, thus allowing more fuel to flow through and cool the pump.

Now...Here is my question, If the overflow valve is sticking open, will it cause a decrease in the measured pressure at the inlet banjo bolt. I ask this because I have been having problems with sudden drops in LP fuel pressure when there is little or no throttle applied.

Example: I'm driving along either towing something or climbing a small grade, fuel pressure generally hovers around 14 psi, as I come to a stop light or sign and decrease throttle, my LP climbs to about 17 psi for a second or 2, then suddenly drops to 3-5 psi, as I accelerate away, my LP pressure drops to almost 0, then suddenly jumps to 11 or higher depending on how hard I'm into the throttle. As I approach cruising speed and decrease throttle, my LP begins climbing to 17 psi again, then suddenly drops to 5 psi until I romp on the throttle for a few seconds.

I have been suspecting the Overflow Valve for some time, but have had a hard time getting answers.

If I may interject one observation to your scenario MoparMan, as to the pump getting no cooloing/Lubricating fuel flow during long coast/decents. Just tonight I read a post somewhere else that claimed that the overflow valve had a small bypass port (always open) to make sure there is always some fuel flowing through the pump and back to the tank thus insuring a constant supply of cool fuel flowing through the pump.

Other posts I've found on this topic tonight:

Read Post #5 - http://www.cumminsforum.com/forum/98-5-02-powertrain/234058-funky-fuel-pressure-overflow-valve.html

http://www.blacksmokin.com/98-5-02-cummins-isb

OOps fire call...gotta run...

---------- Post added at 06:12 AM ---------- Previous post was at 04:02 AM ----------

Just found another possability as to why my fuel pressures keep dropping...Nothing's simple!!!

I stole this from http://www.dieseltruckresource.com/dev/fuel-pressures-and-flows-lift-pump-etc-t138789p2.html

Post # 30

I revived this thread because my friend's newly acquired '02 HO truck's lift pump died the other day in a bad part of town. I went to rescue him with a new NAPA pump I had purchased as a spare days after I bought my own truck. This left me with no spare pump, and also motivated me to get my Walbro system installed that has been sitting in a box.

I disassembled the failed Carter from the '02 to determine why it failed. I was rather astonished to see that it looked brand new inside (unknown mileage) and the vanes looked perfect. The plastic drive slot was also intact and the motor was working (we could obviously hear it running) but got no fuel through an open fuel filter top. Swapped pumps after I installed a new filter element and he was good to go.

What happened was the pump's internal bypass check valve died and caused the pump to suck from the internal cavity vs the inlet because is was the path of least resistance.

It appears like the OEM Carter installation in the CTDs is bypassing (or recirculating) A LOT of fuel within the pump when the engine is at idle. When the fuel pressure drops below the bypass threshold (15 psi?), the bypass shuts off and delivers enough volume to the VP44 to keep the LP from recirculating it's own fuel--which heats and aerates it. This makes me think that simply idling the engine does the most damage to the LP and possibly the VP44 by shoving all that aerated fuel into the fuel filter. I am not sure if the filter can remove all the aeration, but I doubt it.

Relocating the Carter down next to the fuel tank helps a bunch with cavitation, but the pump will still bypass internally when the engine is not consuming much fuel. The Dodge fix of the intank pumps help, but they are still problematic from what I have read here and elsewhere.

It appears like the best set-ups (LP) are the ones that aerate or heat the fuel the least amount possible. Walbro or other high flow pumps (insert favorite LP here) with an external pressure regulator located at the VP44 inlet, set-up to dump excess fuel back to the tank (not into the inlet of the pump!) seems like the easiest route to gaining a constant 13.5 psi of fuel supplied to the VP44 under all conditions, but they do recirculate a bunch of fuel.

The RASP pump or other mechanical pump (I'm not picking on RASP here--just an example) is closer to providing variable fueling based on engine RPM, but as HOHN pointed out, a Cummins will use FAR LESS fuel at 2,000 RPM while in neutral vs 2,000 RPM under a heavy load, but the RASP will provide exactly the same volume in either case because it is RPM dependent.

The system Bent_Valves has looks like the ultimate set-up. Run the Walbro (or other LP) with a PWM regulator and feed the engine exactly what it needs when it needs it. The bypass would be there incase of PWM failure. It would not use it otherwise as most systems (internal or external bypass) need to do.

Now if there was only a ready made PWM kit with a harness to connect the LP and a pressure transducer, we would be set!

---------- Post added at 06:31 AM ---------- Previous post was at 06:12 AM ----------

Actually it may not be so hard to test both the Overflow Valve and the Lift Pump's internal bypass. At present, If I bump the starter, my Lift Pump Pressure (LPP) goes up to 17 psi, then drops to 3-5 psi.

THE TEST...

If I clamp off the return line, and the LP is operating correctly and the problem is in the Overflow Valve the LPP should stay at about 17 psi.

If I clamp off the return line, and the Overflow Valve is operating correctly and the problem is in Lift Pump's internal Bypass then the LPP should do as it's doing now, climb to 17, then drop to 3-5 when the Internal bypass opens and sticks.

I'll have to try this later as its 0230 here...

Good luck with clamping off that METAL return line.Just replace the return valve in the VP44 with a new one and you will have the answer.Forget about the OEM lift pumps everyone knows they are not worth the time to mess around with anyway.Put on a better lift pump and be done with it.

I just finished my little test. BTW, the return line is rubber down at the front of the frame rail. It allows for flexibility between the engine and rail. The bad news is, it does appear to be the pump bypass sticking open. Clamped off the return, bumped the starter, 17 psi for about 5 sec, then sudden drop to 7 psi with an increase in pump speed. I wonder how hard it'd be to disassemble the pump and try cleaning that bypass.

Sealed unit... sorry...

Twas afraid of that...

Twas afraid of that...

Dan, as has already been noted above your lift pump is shot. They will behave in the manner you described and eventually will produce no pressure at all. The BEST and all application fix is to replace the lift pump with an Air Dog or FASS. For many, the $500 plus dollars and installation is just a bridge too far. A low cost fix for your application that would bolt into the OEM stock pump location is the Airtex 7153 fuel pump which is now being sold in place of the original Carter lift pumps at many auto parts store. They cost about $160 and are a straight bolt in replacement for the OEM Carter lift pump. They work well for STOCK applications. Since you have the Diablosport Power Puck, Boost Elbow, and Quadzilla Boost fooler installed; I strongly recommend going with the Air Dog or FASS. NO OEM style lift pump is going to work well or in some cases at all with a non stock engine application. The Airtex pump on my wife's truck works well but the truck is stock and the Airtex 7153 lift pump has been relocated to the frame under the fuel tank and a 1/2 inch big line kit has been installed all the way from the pump to the injector pump. Relocating the lift pump provided a gain of about 2-3 psi fuel pressure. This particular setup MAY work on your truck but you will have to monitor fuel pressure VERY closely. Again, the Air Dog or FASS is the best fix in your case.

I second LIVEOAK's assessment, Another option to look at is a Raptor pump less expensive than a FASS or AirDog and without filters & air/water seterator. But it is a good pump.

Ive got a DDRP and hate it due to its pressure drop on my STOCK 99. Im waitin in line for DTT to release their mechanical lift pump, which is rumored to be cheaper than the electric pumps.

I second LIVEOAK's assessment, Another option to look at is a Raptor pump less expensive than a FASS or AirDog and without filters & air/water seterator. But it is a good pump.

Dan, as has already been noted above your lift pump is shot. They will behave in the manner you described and eventually will produce no pressure at all. The BEST and all application fix is to replace the lift pump with an Air Dog or FASS. For many, the $500 plus dollars and installation is just a bridge too far. A low cost fix for your application that would bolt into the OEM stock pump location is the Airtex 7153 fuel pump which is now being sold in place of the original Carter lift pumps at many auto parts store. They cost about $160 and are a straight bolt in replacement for the OEM Carter lift pump. They work well for STOCK applications. Since you have the Diablosport Power Puck, Boost Elbow, and Quadzilla Boost fooler installed; I strongly recommend going with the Air Dog or FASS. NO OEM style lift pump is going to work well or in some cases at all with a non stock engine application. The Airtex pump on my wife's truck works well but the truck is stock and the Airtex 7153 lift pump has been relocated to the frame under the fuel tank and a 1/2 inch big line kit has been installed all the way from the pump to the injector pump. Relocating the lift pump provided a gain of about 2-3 psi fuel pressure. This particular setup MAY work on your truck but you will have to monitor fuel pressure VERY closely. Again, the Air Dog or FASS is the best fix in your case.

Raptor and Big Line Kit installed...'Nuff said.

I was getting ready to add to the HTML pages and found that yes both input and overflow areas under the VP44 electronics are hooked together! Check out this picture closely... Notice you can see the black wire in the other hole on the overflow valve side. So yes both the inlet and overflow valve are connected to each other...

So...get me oriented here. The elbow is the inlet line, the boss to the right of it is where the overflow valve connects? What bolts on over the cavity? The Computer?

This is a great thread. The thought processes that you guys are putting to text are awesome. I'm actually working on a fuel pressure (well, lack thereof) issue on my '01. This one should help me work through it, or at least, make me look at it from different perspectives.

If your looking at it from a new perspective at least is a good thing... Maybe you'll see something we are all missing...

I was getting ready to add to the HTML pages and found that yes both input and overflow areas under the VP44 electronics are hooked together! Check out this picture closely... [ATTACH=CONFIG]1436[/ATTACH] Notice you can see the black wire in the other hole on the overflow valve side. So yes both the inlet and overflow valve are connected to each other...

Mike, if I understand what you're saying correctly, the second hole to the right that you can see the wire going to is the one that delivers fuel to the vane pump. Fuel enters threw the hole to the left (where the wire has a spade connector) and exits out the second hole into the input area for the vane pump. The hole to the right isn't connected to the fuel return port (overflow).

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...