Lift pump shuts off after engine starts

[piñon&juniper]

Truck running again.

 

The modification of the pickup in the tank worked. In the process, I found that there is no check valve in the tank. The part of the fuel canister that I though must be a check valve was in fact another fuel strainer, coarser than the one fuel had to pass through first. Go figure.  I have no explanation for why I couldn't pull fuel through, but now I can.

 

The 8 month old Airtex style lift pump was indeed bad. Maybe it went bad sooner due to the blockage? I don't know. Im ready to shop for a better lift pump set up now... seems cheaper than a week of lost labor and mysteries. Nothing like three problems at once to make for good troubleshooting.

 

Thanks for all the help-

[LorenS]

1 hour ago, piñon&juniper said:

Truck running again.

I'm glad you got it ironed out.

Relocating my lift pump to the frame helped my fuel pressure a lot. I put a spin-on strainer ahead of it since I gutted (on accident) the strainer/sock in the tank.

 

Here's the link to an amateur photo of my install.

https://mopar1973man.com/topic/18093-donaldson-upstream-spin-on-screen/

Edited July 14, 2022 by LorenS
Add link

[piñon&juniper]

11 hours ago, LorenS said:

Relocating my lift pump to the frame

What pump setup did you use?

[LorenS]

FASS DRP 100. Relocating the stock pump to the frame was probably sufficient as the pressure rose and was consistent, but I had already bought the FASS so eventually installed it just to see if it gave any noticeable improvement. It may have been better at maintaining a higher pressure in a high load situation.

[piñon&juniper]

Ok, if anyone is still paying attention...

 

I got it all back together, and it runs, but something ain't right.

 

Summary of current state of affairs:

 

Problem 1 (Solved): ECU control of fuel pump is bypassed using @IBMobile's ASD relay technique. Works great.

 

Problem 2 (soon to be solved): The ASD is controlling an Airtex style inline lift pump in the stock location, not Airtex brand. Seems to work, but only pumps about half the expected volume of fuel when tested. This pump is brand new (Standard Ignition STD FTP5 from O'Reilly). With all my recent experience with the unreliability of these pumps, I have ordered an AirDog FP 100 upgrade to mount on the frame rail. I't not here yet.

 

Problem 3 (sort of solved): In messing with the fuel tank basket to replace the fuel sender and resolve a blockage of some kind, I replaced the original damaged fuel screen and another original fuel screen with my own made up draw tube in which I included a check valve. That valve now seems unnecessary, but it made sense at the time. The blockage issue is fixed, but the check valve probably adds a little unnecessary restriction to the line.

 

Problem 4 (not yet addressed): I have a pretty janky looking fuel pressure sender and gauge setup, and I'm not too confident in its accuracy. There have been wild variations in its readings. It's range is 0-15 psi, and to be useful, it should be more like 0-30. Historically it mostly stayed pegged and only dropped a little under heavy load. More recently it's at 12 or so at idle, 5-9 on light load driving, and plummets under heavier loads down to 1 or 2. Observing this behavior for the first time is what started me off on my lift pump quest. Even since getting things kind of working again, the gauge has behaved in both ways (reading mostly pegged and now, consistently, the more alarming low pressure readings).

 

Putting in a more reputable pump system will eliminate one of my variables. Getting a reliable fuel pressure gauge will eliminate another.

 

My Question for now is: Is there any known possibility of an issue in an otherwise functional VP44 that would cause intermittent fluctuations in the return fuel volume and/or the pressure set point for that return? As I assembled things the most recent time, I observed the return flow, which was steady at idle but which increased dramatically with RPM. Perhaps that's normal with the VP44's internal vane pump?

 

I'll update again after the AirDog installation.

 

Thanks

[Mopar1973Man]

I'll be releasing a install video soon.

[LorenS]

1 hour ago, piñon&juniper said:

As I assembled things the most recent time, I observed the return flow, which was steady at idle but which increased dramatically with RPM. Perhaps that's normal with the VP44's internal vane pump?

This is correct.  @Tractorman I believe did a bunch of testing around this topic.

[piñon&juniper]

I just found the article on that valve, so I'll pursue all that good info.

 

[Tractorman]

1 hour ago, piñon&juniper said:

My Question for now is: Is there any known possibility of an issue in an otherwise functional VP44 that would cause intermittent fluctuations in the return fuel volume and/or the pressure set point for that return? As I assembled things the most recent time, I observed the return flow, which was steady at idle but which increased dramatically with RPM. Perhaps that's normal with the VP44's internal vane pump?

 

The VP44 fixed displacement internal vane pump is the only pump that controls flow through the VP44 and over the 14 psi overflow valve returning fuel to the fuel tank.  Because the vane pump is fixed displacement, it will always flow more fuel as engine rpms are increased, therefore more return fuel.  None of the vane pump's operation has anything to do with the lift pump.  The lift pump's only purpose is to supply more fuel to the VP44 inlet than the internal vane pump can use.  That is all.  Nothing else.  

 

Because the lift pump supplies more fuel than the internal vane pump can use and the internal vane pump is rpm dependent, there will always be a positive pressure at the VP44 fuel inlet.  The fuel pressure at the VP44 inlet will be the highest when the engine is at idle and the lowest when engine is at full rpm under full throttle.

 

I have logged similar miles on my truck (367,000) and over 270,000 miles on a replaced VP44.  My frame-mounted FASS 65 gph lift pump pressure is the highest at idle - 12 psi.  It is 6 psi at under near full power.  The 6 psi is of no concern.  It is a positive pressure at the VP44 inlet.  I operated a OEM relayed in-tank lift pump (modified under warranty by Dodge) for over 150,000 miles on the replaced VP44 - that lift pump produced 6 psi at idle and 3 psi under near full power.  Again, 3 psi is a positive pressure - no concerns.

 

I think you are on the right track with getting a reliable gauge setup and installing a frame-mounted lift pump.  The frame-mount pump will allow the lift pump to behave like a pusher pump instead of a puller.  Most of the time the fuel level in the tank will be at or higher than the lift pump inlet.  

 

Lift pump pressure should be steady at any pressure reading.  You mention you still have concerns with blockage.  I think it would be worth installing a temporary vacuum gauge at the lift pump inlet (on your frame-mounted lift pump) to measure the pressure drop.  If there is no blockage, I would expect to see a minimal vacuum reading (less than 5" hg) while driving the truck.

 

I just noticed that you are reading the 14 psi overflow article.  A good read, but in my opinion this valve is not your problem.  The operation of this valve is largely misunderstood.  It very rarely fails - if ever.

 

- John

 

 

Edited July 18, 2022 by Tractorman

[piñon&juniper]

4 hours ago, Tractorman said:

I think it would be worth installing a temporary vacuum gauge at the lift pump inlet

Thanks for the thoughtful reply. Can you recommend a good vacuum gauge setup, and for that matter, a reputable and not too costly fuel pressure gauge I could consider?

 

More soon-

[Tractorman]

Here is just one of many vacuum gauges that would be suitable.  Just make a temporary tee fitting to work for your test.  You don't necessarily have to drive the truck.  Just bring the engine rpm up to 2,000 and hold.  This will bring the tank return flow to about 28 gph via the internal vane pump.  Observe the vacuum gage reading.

 

As far as a permanent fuel pressure gauge setup, I have no recommendation.  I use a warning light mounted in the dash.  The warning light is triggered by a low fuel pressure switch under the hood.  I have never felt a need for a fuel pressure gauge inside the cab.  

 

I have also added a fuel line with a check valve that loops around the lift pump.  This means that fuel flow to the VP44 inlet will not be disrupted in the event of a lift pump failure.  The VP44 internal vane pump will continue to draw fuel from the tank and around the disabled lift pump.  There will be no disruption of engine performance or fuel return flow to the tank.  I have proven this with my own truck on a 28 mile roundtrip to town with the electrical supply disconnected to the lift pump.   During the trip I applied full throttle several times, set cruise control for a few miles, and stopped / started the engine several times.  No change in performance.  Of course, the low fuel pressure warning lamp stayed on the for the whole trip (no lift pump pressure), but there was no cause for concern.  

 

- John

 

 

 

 

[wil440]

As ever John great write up with facts

I use a mechanical lift pump and would NEVER go back to electric EVER

 

my fuel pressure is 7psi ish at idle and 22psi at anything over 2k which my truck never sees, I have tested fuel return to tank and at 7psi (mechanical gauge) it is a serious amount of fuel, my fuel temps are never high, I've just done 200mile today and highest was 105f

My fuel return goes back to the basket ....still.... I have a return made for the filler neck but haven't got around to it

 

I don't buy into the 15psi minimum deal, I buy into flow return not pressure as my truck proves

 

And my mechanical pump is a DTT which everyone slated as rubbish..... not so  and my lines right now are 3/8th just not got around to 1/2" and I don't see the need

 

I did change the return valve on my truck a while ago...... while chasing PSI  actually  made jack difference

What I am not saying is that 5psi WOT on a rubbish electric chinese pump is enough,  5psi and 10 GPM return is fine

 

I am saying check your own return as I have and I believe @Tractorman has 

 

And go mechanical lift pump and forget about all this PSI rubbish

 

I swallowed a huge shipping, duty and Vat bill to get mine into the UK..... best thing I did