Good afternoon, I put a titanium fass lift pump along with a Fleece sureflo sending unit on my 2001 pickup about a year and a half ago. It’s been flawless for a few thousand miles. I had been noticing some slow starts when warm. Not bad just a few more revolutions than what was normal. I put some NAPA cross referenced fuel filters on because I didn’t have FASS replacements and didn’t have the time to wait . I started to take a trip recently and fuel pressure dropped significantly. I got the truck home and parked it for a month . I wasn’t real confident about the NAPA filters so i replaced with the direct replacement FASS filters. Test drove the truck for an hour or so and all seemed fine. Today it did the same thing again. I had gotten 17-19 psi at idle and 15-17 WOT. Today it dropped to 15 idle and drops significantly at WOT.

I talked to tech support at FASS and he mentioned something about a valve in the VP 44 might be a problem. Now when I turn the key on PSI jumps to 17 and drops to zero momentarily when I start the pick up. I’m in the process of putting a mechanical gauge in line to verify that the brand new auto meter pillar gauges are not the problem. Does this sound like a VP44 problem? Truck only has a 160000 miles, but I assume VP44 and everything beyond is original equipment .

Thanks, Kevin

Edited December 29, 2025Dec 29 by kbf98520

I believe you're experiencing a suction side air leak. Double-check the suction line side fittings, make sure they are snug and clean in the fittings. Let me know what type of hardware you're using.

The other part could be that you have a return line leak? Like, possibly a crossover o-ring issue or maybe injector o-ting. Sounds weird, but I've done a head gasket after the head came back, cleaned the baked injector o-rings would weep air in, allowing the system to drain, making a hard start. I've see nthis with crossover tube o-rings as well. Make sure to double-check the return tee in the rear of the engine; it might be weeping air into the system while parked.

Ok, the return line wouldn’t cause the low pressure while running would it

45 minutes ago, Mopar1973Man said:

I believe you're experiencing a suction side air leak. Double-check the suction line side fittings, make sure they are snug and clean in the fittings. Let me know what type of hardware you're using.

The other part could be that you have a return line leak? Like, possibly a crossover o-ring issue or maybe injector o-ting. Sounds weird, but I've done a head gasket after the head came back, cleaned the baked injector o-rings would weep air in, allowing the system to drain, making a hard start. I've see nthis with crossover tube o-rings as well. Make sure to double-check the return tee in the rear of the engine; it might be weeping air into the system while parked.

Everything is JIC and I beiieve everything downstream of VP44 is stock

1 hour ago, kbf98520 said:

I talked to tech support at FASS and he mentioned something about a valve in the VP 44 might be a problem.

The tech is probably referring to the 14 psi overflow valve in the VP44 injection pump. This pressure control valve is very reliable and I am almost certain it is not your problem. People have replaced this valve because of not understanding how the fuel system works, and then found after replacement, their symptoms didn't change.

Even if the overflow valve failed (as in opening at a lower pressure), it would have little or no effect on lift pump pressure. All fuel that enters the VP44 must pass through in internal fixed displacement vane pump. This pump is regulated at 100 - 300 psi depending on engine rpm and engine load. Because the internal vane pump is fixed displacement, additional fuel cannot be forced through it regardless of lift pump pressure.

Both Airdog and FASS lift pumps use a ball and spring check valve to regulate fuel pressure by recirculating any fuel that is not being used by the injectors and the VP44 cooling system. If the lift pumps are high volume pumps (which yours probably is), then a lot of fuel has to recirculate, in fact more fuel recirculates than gets used. The pressure control valves on these lift pumps are usually not of high quality and the ball and seat can take a beating, which can cause symptoms that you are experiencing. I am surprised that the Tech didn't mention this.

I would be looking at two possibilities for low / erratic lift pump pressure.

• Check the condition of the ball, spring, and seat in the lift pump.

• Check for any restriction / leaks in the low pressure fuel supply. Pay special attention to the suction fuel lines from inside the fuel tank to the lift pump.

Many people that have observed erratic lift pump pressure have stretched the spring in the lift pump's pressure regulating valve for a bit more tension. For some it gave desired results.

• John

Edited December 30, 2025Dec 30 by Tractorman

1 hour ago, Tractorman said:

The tech is probably referring to the 14 psi overflow valve in the VP44 injection pump. This pressure control valve is very reliable and I am almost certain it is not your problem. People have replaced this valve because of not understanding how the fuel system works, and then found after replacement, their symptoms didn't change.

Even if the overflow valve failed (as in opening at a lower pressure), it would have little or no effect on lift pump pressure. All fuel that enters the VP44 must pass through in internal fixed displacement vane pump. This pump is regulated at 100 - 300 psi depending on engine rpm and engine load. Because the internal vane pump is fixed displacement, additional fuel cannot be forced through it regardless of lift pump pressure.

Both Airdog and FASS lift pumps use a ball and spring check valve to regulate fuel pressure by recirculating any fuel that is not being used by the injectors and the VP44 cooling system. If the lift pumps are high volume pumps (which yours probably is), then a lot of fuel has to recirculate, in fact more fuel recirculates than gets used. The pressure control valves on these lift pumps are usually not of high quality and the ball and seat can take a beating, which can cause symptoms that you are experiencing. I am surprised that the Tech didn't mention this.

I would be looking at two possibilities for low / erratic lift pump pressure.

• Check the condition of the ball, spring, and seat in the lift pump.

• Check for any restriction / leaks in the low pressure fuel supply. Pay special attention to the suction fuel lines from inside the fuel tank to the lift pump.

Many people that have observed erratic lift pump pressure have stretched the spring in the lift pump's pressure regulating valve for a bit more tension. For some it gave desired results.

• John

Lift pump has a lifetime warranty. Hopefully I get it narrowed down tomorrow.

1 hour ago, kbf98520 said:

Make sure to double-check the return tee in the rear of the engine; it might be weeping air into the system while parked.

I have not even looked for the tee in the back of the head yet as I have not seen any evidence of fuel leaking from there. What’s the best way to access that tee?

1 hour ago, kbf98520 said:

I have not even looked for the tee in the back of the head yet as I have not seen any evidence of fuel leaking from there. What’s the best way to access that tee?

On my truck the tee at the back of the head started to leak fuel about 15,000 miles ago (at 390,000 miles). At the time I had the transmission out while doing a clutch job and I noticed the leak. The connections were loose, so I just tightened the them from below. They have never leaked since.

A leaking tee fitting (even if it's only leaking air) can cause long cranking times, but it will not affect fuel pressure after the engine is running - so, not likely to be your problem.

I haven't tried accessing the tee fitting from the top of the engine. I would probably remove the valve cover and use a mirror to inspect the connections and then blindly reach around the back of the head to tighten them.

• John

@Tractorman Those o-rings are square cut and you would need to order from Cummins for replacement. Might be a good idea.

Edited December 30, 2025Dec 30 by Mopar1973Man

On 12/30/2025 at 8:07 AM, Mopar1973Man said:

@Tractorman Those o-rings are square cut and you would need to order from Cummins for replacement. Might be a good idea.

These are the o rings for the return on back of head?

Fuel pump is back on line. 20+ psi at idle, no less than 17psi WOT. Turns out the problem was the relay. Junk I suppose. This was the relay that came with pump. Eklund brand, I think. Funny, FASS gave me a Bosch part # to replace it with. No one carries Bosch where I live , and rather than waiting on an internet order I replaced with another brand that cross referenced from AutoZone.

Still have the slow start when warm though

22 minutes ago, kbf98520 said:

Still have the slow start when warm though

Does this mean NO slow start when the engine is cold? If so, your lift pump pressure may be too high. The OEM fuel pump setup uses PWM (pulse width modulation) to reduce supply voltage to the lift pump, thus reducing fuel pressure, while cranking. The reasoning is that high lift pump pressure during cranking can affect the position of the timing piston, thus making the engine hard to start when warm. Not everyone has experienced this, but some have.

Is your relay wired directly from the battery?

You could try reducing lift pump pressure to see if this helps starting a warm engine. Don't get caught up in the, "you gotta have at least 14 psi lift pump pressure", as it is not true. Take it down to 12 psi or so at idle and see if that helps.

Good to hear you found the problem with the lift pump.

• John

Edited Thursday at 01:27 AM4 days by Tractorman

On 12/31/2025 at 5:26 PM, Tractorman said:

Does this mean NO slow start when the engine is cold? If so, your lift pump pressure may be too high. The OEM fuel pump setup uses PWM (pulse width modulation) to reduce supply voltage to the lift pump, thus reducing fuel pressure, while cranking. The reasoning is that high lift pump pressure during cranking can affect the position of the timing piston, thus making the engine hard to start when warm. Not everyone has experienced this, but some have.

Is your relay wired directly from the battery?

You could try reducing lift pump pressure to see if this helps starting a warm engine. Don't get caught up in the, "you gotta have at least 14 psi lift pump pressure", as it is not true. Take it down to 12 psi or so at idle and see if that helps.

Good to hear you found the problem with the lift pump.

• John

Starts right up when cold. Slow start is only when warm. Relay is wired to battery and fuse block.

There are no psi adjustments on this pump. If I was to attempt by opening up, thus breaking the seal, it would void warranty.

On 12/30/2025 at 8:07 AM, Mopar1973Man said:

@Tractorman Those o-rings are square cut and you would need to order from Cummins for replacement. Might be a good idea.

Edited December 30, 2025Dec 30 by Mopar1973Man

Are these square cut o rings the ones for the return on back of head? What type of thread comes out of back of head? Any way to maybe convert this to a jic type fitting?

Something without o rings

3 hours ago, kbf98520 said:

Starts right up when cold. Slow start is only when warm. Relay is wired to battery and fuse block.

There are no psi adjustments on this pump. If I was to attempt by opening up, thus breaking the seal, it would void warranty.

You could just disconnect the fuel pump relay to test a warm start condition. Rig up a jumper wire to hold in your hand while you start the truck - that way you can control when the lift pump runs (or doesn't run) for your test.

At least, it would let you know if too high lift pump pressure is causing your long crank on a warm start.

• John

Lost pressure again today on a short trip. Psi dropped to 15 at cruise, 12 at WOT. Got it home , tuned it off, re keyed it went to 18, started , it dropped to zero , but still ran. Shut it off quickly. Going to get on it again tomorrow

So, it sounds like the lift pump relay was not the issue? Symptoms remain the same?

• John

Edited yesterday at 05:46 PM1 day by Tractorman

Seems to have fixed it for 100 miles or so. Odd, going to run another bucket test and see what’s going on…… PIA!

21 hours ago, kbf98520 said:

Lost pressure again today on a short trip. Psi dropped to 15 at cruise, 12 at WOT. Got it home , tuned it off, re keyed it went to 18, started , it dropped to zero , but still ran. Shut it off quickly. Going to get on it again tomorrow

Is the check ball getting shoved back into the spring? I typically bend the tail of the coil over so there is a tail tip in the middle of the coil. This will prevent the check ball from wedging into the coil.

49 minutes ago, Mopar1973Man said:

Is the check ball getting shoved back into the spring?

I assume you’re talking about an internal check valve in the pump? I can’t open up pump without voiding warranty.

While I got ya , the square cut o rings you mentioned, are they for the return on back of head?

Edited 21 hours ago21 hr by kbf98520

24 minutes ago, kbf98520 said:

I assume you’re talking about an internal check valve in the pump?

Actually, you can open this up. The return line to the filler neck to the tank is the fitting you remove, and behind that is a spring and check ball. Just be aware the check ball is plastic and could wedge it way down in the coils of the spring. This is why the sudden change in fuel pressure and then later it pops back up when the vibration shakes it loose again.

Here is my new spring for Thor.

While I got ya , the square cut o rings you mentioned, are they for the return on back of head?

Yes, you are correct, those square cut o-rings are for the return tee in the rear of the engine on the driver side.

Edited 21 hours ago21 hr by Mopar1973Man

11 minutes ago, Mopar1973Man said:

Yes, you are correct, those square cut o-rings are for the return tee in the rear of the engine on the driver side.

I have a service manual but , have not opened it up for this yet. Why is there a return tee and not just a single discharge to tank?

14 minutes ago, Mopar1973Man said:

Actually, you can open this up. The return line to the filler neck to the tank is the fitting you remove, and behind that is a spring and check ball. Just be aware the check ball is plastic and could wedge it way down in the coils of the spring. This is why the sudden change in fuel pressure and then later it pops back up when the vibration shakes it loose again.

I’m pulling and returning from a new fleece sending unit. I’m not sure there is a check valve there . I did a bucket test the other day and confirmed the relay was cycling on and off. Replaced relay and it seemed to have fixed the problem…. Not

2 minutes ago, kbf98520 said:

I have a service manual but , have not opened it up for this yet. Why is there a return tee and not just a single discharge to tank?

So the pop off fuel from all 6 injectors will bleed off inside the rail in the head that exits the head at the rear of the engine. Then the second line is the return of the VP44 which is the second line to return to the tank. This is what the square cuts o-ring will seal at the rear of the head.

5 minutes ago, kbf98520 said:

I’m pulling and returning from a new fleece sending unit. I’m not sure there is a check valve there . I did a bucket test the other day and confirmed the relay was cycling on and off. Replaced relay and it seemed to have fixed the problem….

I'm talking the return of either of a FASS or AirDog fuel systems use a check ball and spring to control fuel pressure and flow back to the tank for seperating the air bubbles and fuel excess return is on this line. The fitting in the pump will have a black plastic check ball and a spring.

2 minutes ago, Mopar1973Man said:

I'm talking the return of either of a FASS or AirDog fuel systems use a check ball and spring to control fuel pressure and flow back to the tank for seperating the air bubbles and fuel excess return is on this line. The fitting in the pump will have a black plastic check ball and a spring.

I’ll definitely take a look as soon as I can get back on it. I just realized I have a P1693 code. I also have not yet done the W-T ground wire mod. I’m starting to wonder if this is what’s giving me relay problems

3 hours ago, kbf98520 said:

have a P1693 code

Just a flag or marker there is another error. Use a actual OBDII code reader.

Edited 17 hours ago17 hr by Mopar1973Man