Lift pump shuts off after engine starts

[piñon&juniper]

I've got my 2001 2500 5-speed 4x4 down with a lift pump problem.

 

Currently the newly replaced (in-line style, original location on engine) pump is primed and working during starting and maintains over 15 psi (that's the my of my gage). If the truck has sat for a few minutes, it will turn on briefly when the ignition is turned on, and it responds to blipping the starter by turning on for 25 seconds. I can hear good fuel flow returning to the tank (which I just cleaned up when replacing the fuel sender). Once I got the fuel circuit bled of air, it started just fine, but as soon as the engine starts, the lift pump shuts off. The injection pump is apparently able to pull enough fuel to keep idling, but can't rev up much- obviously it's starving for fuel. Gage slowly drops to zero.

 

I've swapped relays around for the fuel system and the ASD, with no improvement. I replaced the earlier lift pump with a fresh one, though the earlier one wasn't very old. There are no codes showing.

 

My search of the forums suggests that the ECM might have a burnt out circuit that runs the lift pump while the engine is running as opposed to starting. If that's so, I don't know why it would be, and I'd like to positively diagnose it before embarking on that repair. I have had strange lift pump behavior, including drops in pressure that vary with load but also come and go intermittently. I did replace the lift pump that came with the truck from the previous owner after I had is pressure drop to zero at times, and at this moment I have just replaced the one I put in, because I thought it was causing the problem... not so, apparently, so I guess I have a working spare.

 

Can anyone point me to any discussions that address this, or provide any further troubleshooting guidance?

 

If my ECM is the problem, I'm more tempted to set up a new relay off some other signal source to turn the pump on, in hopes that I can retain the shut-down function that would trigger in case of a wreck. I'm guessing a new or repaired ecm isn't in my budget.

 

Thanks for any ideas.

 

[Tractorman]

You are performing some good troubleshooting here - you are definitely on the right track.  It sounds as if you still have the OEM lift pump circuit.  If that is the case, the ECM provides power to the lift pump with two circuits - one for engine start and one for engine run.  The ECM is carrying the full electrical current load for the lift pump - not the best design.  If this is your problem, relaying the lift pump using the ECM to provide power to the relay coil would be a good solution as long as the ECM is still performing properly.  This modification would reduce the electrical load on ECM lift pump circuit considerably.  

 

In the worst case scenario (ECM no longer supplying power to lift pump) you could use a switched circuit in series with an engine oil pressure switch that would supply power to the coil of a relay for the lift pump.  This would accommodate the "in case of an accident" feature.

 

3 hours ago, piñon&juniper said:

I've swapped relays around for the fuel system and the ASD, with no improvement.

 

If you are referring to the relay in the PDC, this relay is for the fuel control solenoid inside the VP44 injection pump.  It has no affect on the lift pump circuit.

 

- John

 

Edited July 8, 2022 by Tractorman

[IBMobile]

40 minutes ago, Tractorman said:

If this is your problem, relaying the lift pump using the ECM to provide power to the relay coil would be a good solution as long as the ECM is still performing properly. 

How to install a lift pump relay.

https://mopar1973man.com/cummins/articles.html/24-valve-2nd-generation_50/51_engine/59_fuel/installing-protective-lift-pump-relay-r501/?tab=comments#comment-55

 

This is another way to control the lift pump.

https://mopar1973man.com/cummins/articles.html/24-valve-2nd-generation_50/51_engine/59_fuel/asd-controlled-lift-pump-r649/

 

Before doing any of the above, test ECM lift pump voltage output with a voltmeter or test light, at pin #15 and #35 of the connector at the ECM with the engine running.   If no voltage the that part of the ECM is bad.

[piñon&juniper]

Thanks to both of you. I did mess with relays in the PDC before further research told me I was in the wrong track. I have tested the leads at the harness to the lift pump. I presume those are directly connected to the pins you mention on the ECM connector? If they are, then there is certainly a failure in the run circuit on the ECM but full voltage on prime/start. And I understand how that happened. I do indeed have a stock setup. 
 

I feel good about the relay approach and I appreciate the guidance. Just about to tip over 400K on this workhorse. 

[Royal Squire]

Does your WTS light come on immediately when you turn key on?  In my experience this is a good indicator of ECM condition. 

[piñon&juniper]

It comes on as it should, and all other engine and sensor functions seem fine. The history of erratic lift pump pressure and function seem to indicate just the run circuit for the lift pump is bad. 

Edited July 10, 2022 by piñon&juniper

[Mopar1973Man]

Stock lift pump lead from the ECM has no relay and is power directly through the ECM. If there is issues with the lift pump circuit the ECM needs to be sent to @Auto Computer Specialist for inspection and repair.

 

This is one reason I like the AirDog fuel system because it uses a protection relay and ECM is only used to trigger the relay. Any fuel pump that directly powers from the ECM is bad idea. When the pump has issues like locked rotor the amperage load will spike and damage the ECM. 

 

Oh little secret I'm going to release a AirDog install video soon. I just finish the video work for the draw straw. It's on my laptop and needs a bit of editing.

Edited July 9, 2022 by Mopar1973Man

[piñon&juniper]

Still having problems...

 

I did the ECU lift pump control bypass following IBMobile's second method (using the ASD relay output terminal 87 to trigger a new relay for the lift pump). I now have the lift pump powered during starting and running, which is an improvement over what I had before (it ran during starting and intermittently or not at all when running). But I still have the same fuel pressure issue: fuel pressure goes up briefly when the key is turned on, and will go up and stay up if I run the pump with a jumper, but as soon as the engine starts, the pressure drops steadily to zero. The truck can still idle but will starve under any load or higher rpm. I took the trouble to break out a junction in my new lift pump wiring so I can attach a trouble light and see that the pump is powered when it should be (though of course I can't hear whether it's running when the engine is idling).

 

More potentially pertinent information: 

 

This round of troubleshooting began when the truck ran low in fuel and would not re-prime. I took the opportunity to drop the tank, replace the broken fuel sender, and check things down there. The short flexible fuel lines between the hard lines at the frame and the fuel tank fittings were kinked during the tank removal (I don't see how that could be avoided) so I replaced them with good quality 3/8" and 5/16" fuel lines and hose clamps with a generous loop to ease fuel tank servicing. While replacing the sender, I tried to clean the screen elements at the bottom of the housing, and in opening it, I tore the screen, of course. Since those are very expensive as far as I can tell, I replaced the part with a different style strainer that had the right size fitting (7/16 ID) and a lug that I could use to fasten it in place. That seemed to work well enough.

 

Getting the lift pump to prime and bleed the air was very difficult, as it had been when I installed it last October. I assumed it was another lift pump failure, but that would be strange with an 8 month old pump. Still, because of the bad reputation on the (inline style) basic lift pump, I had a spare handy. But it also made me wonder whether there could be some kind of issue with the tank-to-lift pump plumbing. In fact that possibility has been in the back of my mind for a long time, and that's one reason I went ahead and replaced the stock flex lines at the tank with new rubber.

 

Is it possible that there is a leak in the suction line into the lift pump that would allow the pump to develop full pressure without the engine running, and then let in enough air to break the pump vacuum once the injection pump starts to pull? I don't have any wet leaks that I can see. Are there any good ways to check this that I can do without a lot of special tools? I'm pretty confident in my recently installed rubber lines and their connections, but could there be such a leak inside the tank's pickup tubing, or at the engine end?

 

One more detail- this truck has had slightly hard starting when hot and when the weather is hot (only under those conditions) for as long as I have had it). Someone somewhere suggested that could have to do with air in the fuel. Does that make sense to any of you? 

 

Thanks for any ideas.

 

 

 

Edited July 10, 2022 by piñon&juniper

[IBMobile]

30 minutes ago, piñon&juniper said:

Is it possible that there is a leak in the suction line into the lift pump that would allow the pump to develop full pressure without the engine running, and then let in enough air to break the pump vacuum once the injection pump starts to pull

If there was an air leak in the supply side the fuel would run back to the tank when the engine was shut off and there would be a hard starting problem due to a priming problem. 

 

You stated that the lift pump is "in-line style, original location on engine".  Is it a Carter pump that looks something like this?

These pumps are not reliable.  I went through 3 of them in 2 years, that is why I installed a Fuel Boss mechanical lift pump.   

[piñon&juniper]

Mine is what seems to be called 'airtek' style. it mounts in the same location but the body is horizontal, with output toward the front, and inlet toward the back with an elbow on the inlet to accommodate the factory flex-line/quick connect.

[piñon&juniper]

Here are two videos.

 

First, I jumper the lift pump so it is simply always running. Fuel pressure on my gage is pegged, so above 15 or 16 psi. Once I start the truck, fuel pressure drops slowly, though I know the pump is running.

 

Second, without the jumper. In this scenario, the new relay I installed actuated by the ASD relay 87, is starting the pump briefly on ignition, and continuously on running, as expected. I keyd over to ignition 3 times, each time raising the pressure a little, but after start, fuel pressure drops slowly again.

 

I have no hard start issues (except as described earlier, on hot days with a hot engine, end even then only 2-5 seconds of cranking and then starts fine).

[IBMobile]

 

Ok, I have an Airtex fuel pump plumbed in my fuel system with the fuse for it pulled.  It is used as a backup if the belt breaks on the mechanical pump and is also used to prime the fuel system when changing injectors, vp44, or fuel filter.

 

Try this, get a fuel container with a couple of gallons of fuel in it and enough fuel line to go from the suction side of the fuel pump to the contain, disconnect the fuel tank hose from the suction side of the fuel pump and install the new hose from the container.  Prime the system and start the engine.  If the fuel pressure stays stable, then the problem is in the tank or the fuel line to the pump.  If it drops then it's in the pump, fuel filter or VP44.

 

[piñon&juniper]

I was getting toward that conclusion. I’ll try it ASAP and let you know. Thanks. 

[Tractorman]

1 hour ago, piñon&juniper said:

I was getting toward that conclusion. I’ll try it ASAP and let you know. Thanks. 

 

@IBMobile's test is a good one.  

 

One reason you are seeing the pressure fall off after starting the engine is because the fixed displacement internal vane pump inside the VP44 takes over.  With the engine idling this vane pump returns at least 18 gph of fuel to the tank via the 14 psi overflow valve.   The lift pump is not keeping up with that flow rate, consequently, the pressure falls after engine is running.  Only two reason come to mind - the lift pump is faulty, or there is a blockage somewhere in the fuel line.  It is not air.  The lift pump can pump air mixed with fuel.  If there was too much air, the engine would probably die and would not restart. but a gauge would still show fuel pressure.

 

- John

Edited July 11, 2022 by Tractorman

[piñon&juniper]

18 minutes ago, Tractorman said:

the pump is faulty or there is a blockage somewhere in the fuel line. 

Yes, that makes sense. I'll try a free-flow test of the lift pump as well. I've read that it should pump about 45 oz of fuel during its normal 25 second priming period. Does that sound right? 

 

The weird thing is that this is happening with the eight month old pump and with the brand new pump exactly the same, and I can hear pretty healthy fuel return stream in the tank. 

 

I'll do my tests tomorrow morning- thanks to all of you-

Edited July 11, 2022 by piñon&juniper

[Tractorman]

22 minutes ago, piñon&juniper said:

I'll try a free-flow test of the lift pump as well. I've read that it should pump about 45 oz of fuel during its normal 25 second priming period. Does that sound right? 

 

Yes.  That is about 50 gph.  

 

23 minutes ago, piñon&juniper said:

The weird thing is that this is happening with the eight month old pump and with the brand new pump exactly the same, and I can hear pretty healthy fuel return stream in the tank. 

 

Are you hearing this when the engine is running?

 

- John

[piñon&juniper]

Here's the update:

 

Truck runs basically right with a dip tube from the pump inlet directly into. diesel can. Transfer pump (I'm using my other Airtex pump that is 8 months old connected to the line in the engine bay) cannot pull fuel through the stock fuel lines to empty the tank. I dropped the tank and disconnected the supply line there, and it blows clear. There is some kind of obstruction in the pickup cannister- I couldn't pull any fuel through at the tank. Once I pull the canister out and it drains, I can both pull and push air through. 

 

Am I correct that part of the canister is a check valve to prevent supply line fuel flowing back to the tank and help maintain prime? I think I see where that would be (a D-Cell sized inner part on the bottom half of the canister that the fuel pickup line connects to the top of. If that is malfunctioning, it could explain some of the difficult priming and intermittent fuel pressure behavior.

 

I'm attaching a picture of the alternative pickup I installed a couple of days ago, in case it illuminates anything, but my next move is going to be bypassing that inner check valve deal altogether, dropping a new line to the bottom with a stainless mesh strainer and an aftermarket check valve that I just ordered from NAPA, which I should get today.

 

More soon!

 

 

392C049F-4379-4A3E-9D7C-399236995ED8_1_201_a.heic

On 7/10/2022 at 9:08 PM, Tractorman said:

Are you hearing this when the engine is running?

I tried to listen for it but way too much engine and exhaust noise, even with a listening tube stuck down through the tank filler. Safe to say that stream was dried up on running.

[Silver_O1_Dually]

Sounds like it’s a good time for a aftermarket lift pump. 

[piñon&juniper]

1 hour ago, Silver_O1_Dually said:

Sounds like it’s a good time for a aftermarket lift pump. 

Yeah, but that’s still several paychecks away. And this in-tank stuff would not have been solved by that anyway. Still, I’m going to do it some time soon. 

[Mopar1973Man]

7 hours ago, Silver_O1_Dually said:

Sounds like it’s a good time for a aftermarket lift pump. 

Being I'm in the process of installing an AirDog 150. The Draw Straw video is done. I should be working on the pump by tomorrow. Only if it not 108°F like today. Ugh.