Fuel pressure issue, works but doesn't.

[Stanley]

Thought I'd make another little one for this. 

Doing some diag and upkeep and stuff from other thread this weekend and came across this...

 

So I can turn key on, visually see needle on gauge move 0-5 psi, next do a key bump and it shows nothing, no matter how any times I try key bump. I removed fuel line from vp44 and into a bucket. Performed key bump again, fuel flows out great fills up a gallon pail and then some. 

 

When truck is started gauge shows 15psi even while driving. 

I also have a spring kit from FASS to increase pressure just a tad. However I have never seen more than 16psi, only sits at 15. Idk why that is??? Stock is around 15, I was hoping for 17 ish...

 

New vp44 came with a new overflow valve, could that be failing or failed causing this? I'm not sure what else it would be. Fass has flow but can't build any pressure maybe another idea, should still have warranty on it.

 

Any thoughts?

[Mopar1973Man]

Washer shim... I got a few small washers that don't restrict flow but just tightens up that spring a bit more. I just the same on my AirDog since the fuel pressure was sliding off a bit so I tweaked the spring a bit and then reassembled and got 18-19 PSI no issues. 

[Stanley]

I'm gonna do some more key bumps in the morning and check again, but this worries me... I should have a constant pressure not 0...

I might tweak their "upgraded" spring (shouldn't have to do this @fass)  and add a washer maybe, if this little issue gets solved. 

[LorenS]

14 hours ago, Stanley said:

So I can turn key on, visually see needle on gauge move 0-5 psi, next do a key bump and it shows nothing, no matter how any times I try key bump. I removed fuel line from vp44 and into a bucket. Performed key bump again, fuel flows out great fills up a gallon pail and then some. 

This doesn't sound like a problem with the lift pump as it's clearly operating and does build 15 PSI when engine is running. I assume your lift pump is triggered by the ECM; if you hotwire the lift pump does it build pressure?

 

Mechanical or electronic gauge/sender? It may have more to do with something getting weak and really needs 14.4V to be happy, versus 12V with alternator not operating - so maybe it IS the pump motor.

Edited April 17, 2022 by LorenS

[Tractorman]

15 hours ago, Stanley said:

New vp44 came with a new overflow valve, could that be failing or failed causing this?

 

Overflow valve operation has little effect on lift pump operation.  In the bump start mode the lift pump output flow is basically deadheaded against a non-rotating vane pump in the VP44.  This is why lift pump pressure is usually the highest in this mode.  Also, overflow valves rarely fail - they get misdiagnosed many times - but rarely fail.

 

15 hours ago, Stanley said:

So I can turn key on, visually see needle on gauge move 0-5 psi, next do a key bump and it shows nothing, no matter how any times I try key bump.

 

Since it is not mentioned, I will asked the question.  Can you hear the lift pump running each time the bump start mode is used?  

 

While pumping into the bucket in the bump start mode, did you try restricting the flow to build pressure?  If not, that may be worth a try.

 

Is the lift pump being operated by the ECM directly, or has a relay been wired with the ECM operating the relay coil?

 

 

I agree with @LorenS, it does not sound like a lift pump problem.

 

- John

 

[Tractorman]

Another thought - is the lift pump fuel pressure gauge electric or mechanical?  If it is electric, does it have power when the bump start mode is used?

 

- John

[Stanley]

The FASS system plugs into the factory LP plug. Pump is running when these tests were done. Gauge is mechanical also. 

 

I did another couple of bump tests this morning and wouldn't you know the gauge showed 15psi... seems to be intermittent now... I'll have to monitor this week and see. It has shown the previous symptom all last week. 

[LorenS]

Since it's now an intermittent problem, that really makes me think electrical connection, or even the harness's relay. This may be hard to diagnose, but on the plus side, I bet the fix is dirt cheap!

[Tractorman]

2 hours ago, Stanley said:

The FASS system plugs into the factory LP plug. Pump is running when these tests were done. Gauge is mechanical also.

 

Okay.  Does the FASS lift pump use a relay that is operated by the ECM, or is the lift pump directly wired to the ECM?

 

- John

[Stanley]

Sorry @Tractorman it is from (ECM) main harness LP plug to relay, then relay to LP. 

[Mopar1973Man]

So the ECM is relay protected. Be aware the lift pump ground and fuel heater ground is behind the starter.

[Great work!]

Next time when the pump should be running but is not, slide under and wiggle wires where they enter or terminate at the pump see if it's intermittent there.

[Stanley]

It might just be the gauge going. This morning done some micro testing and had to key bump twice before it showed pressure at the gauge, somethings bleeding off? 

 

The pump runs at any point, there's no problem there. I just dont see pressure till I key bump a few times or the truck starts. Its something that just started happening maybe a week or so ago...

 

That silly {upgraded} spring has disappointed me. Still at that solid 15 psi which is good but I wanted 16-17psi for security. Ill have to do what @Mopar1973Mansuggested and find some washers that will fit inside there. Im assuming opposite side of check ball, on fitting side...

[Doubletrouble]

8 hours ago, Mopar1973Man said:

So the ECM is relay protected. Be aware the lift pump ground and fuel heater ground is behind the starter.

 That's good to know.

[Max Tune]

If I remember correctly, it's ball, spring, washer(s), fitting. I couldn't find an exact fit washer on hand, had washers that a pencil would fit through and ground down the outer edge to fit. Moparman tip: put pencil through fitting, put washer(s) on, then insert pencil into spring and start fitting. Tighten up and remove pencil. 

Pure genius. This insurse the washers are seated properly on spring and fitting.

 

Been awhile since I had to do this, so if I have something wrong I'm sure it'll get corrected.

[Stanley]

3 minutes ago, Max Tune said:

Moparman tip: put pencil through fitting, put washer(s) on, then insert pencil into spring and start fitting. Tighten up and remove pencil. 

That's one of those tips to put in the tip section we were talking about I think it was @LorenS that mentioned it.