Lift pump cycling on and off

[MRiley]

I'm looking for a little help figuring out a problem. Tonight while driving home i notice that my lift pump was cycling on and off. At one point the fuel pressure dropped to 0, then the pump came back on and the pressure jumped right back up. I have a fass pump so it is pretty loud and I could hear it going off and on. When I came home it tried to check the power leads to see if it was an issue with he pump or something else but I could not get it to cycle.... Is this a lift pump failing or am i looking as something else? About 6 years ago i had the ECM fail and got a new cummins unit to replace it. The new ECM only has abut 20K on it. I shouldn't be the ecm again ???

 

[Gearclash]

I’m not familiar with FASS pumps, but fuel pressure dropping to zero and coming back up again was the symptom that my OEM pump was done for. Would work fine at lighter loads, but if I really got on the power the pressures would jump around some, then go to zero for a little while, then come back up again. I would never have known what was going on without a fuel pressure gauge.

 

[JR]

Are you positive the pump is actually shutting off intermittently or is the fuel pressure simply bouncing around?
Does the pump run if you bump the starter?

 

[DavidC]

Is the fass remote mounted from where the original was ?. If so the relay kit should have been installed and mounted in the engine bay somewhere, usually at the firewall drivers side as the original pump wires now energize the relay and power for the pump comes from the battery to a inline fuse to the relay then from the relay to the pump. Where the pump ground is landed is anyone's guess but I would start there looking for a corroded/loose connection and make sure the relay is tight in it's socket. All of this is assuming it is an electrical problem and the pump is doing the on/off routine.

 

[MRiley]

Are you positive the pump is actually shutting off intermittently or is the fuel pressure simply bouncing around?
Does the pump run if you bump the starter?

100% sure the pump is shutting down. The fass is kinda loud. i actually got under the truck to verify the pump is intermittently shutting down. If I bump the starter some times the pump will run as normal, other times it will give a little bump then stop...

 

[MRiley]

Is the fass remote mounted from where the original was ?. If so the relay kit should have been installed and mounted in the engine bay somewhere, usually at the firewall drivers side as the original pump wires now energize the relay and power for the pump comes from the battery to a inline fuse to the relay then from the relay to the pump. Where the pump ground is landed is anyone's guess but I would start there looking for a corroded/loose connection and make sure the relay is tight in it's socket. All of this is assuming it is an electrical problem and the pump is doing the on/off routine.

The Fass was mounted remote by the tank not on the block. it was relocated with a vulcan relocation kit. I will check the relay and the ground.

 

[MRiley]

I’m not familiar with FASS pumps, but fuel pressure dropping to zero and coming back up again was the symptom that my OEM pump was done for. Would work fine at lighter loads, but if I really got on the power the pressures would jump around some, then go to zero for a little while, then come back up again. I would never have known what was going on without a fuel pressure gauge.

I've had gauges on this truck for a long time for that exact reason. This was actually happening to me at idle. I was sitting at a traffic light and watched the pressure drop to "0" then it popped back up to normal...

 

[Gearclash]

I've had gauges on this truck for a long time for that exact reason. This was actually happening to me at idle. I was sitting at a traffic light and watched the pressure drop to "0" then it popped back up to normal...

This is why these trucks MUST have fuel pressure gauges . . . I put gauges on mine and found out very shortly thereafter that the lift pump was acting up. Most likely saved the VP for another 100K miles of life. Put gauges on shortly after 100,000 miles, caught the lift pump failing; just short of 200,000 the VP quit, almost certain the computer on it failed as it never threw a timing code.

 

[MRiley]

After working on this yesterday i think i have a fix. The pump seemed to run fine when i supplied a direct 12volt wire to it. So i started checking the wiring in the relocation kit and found a relay. I replied the rely and so far it seems to be running good. I've never seen a relay go out like that before. For me they usually just stop.... But things are acting normal again.

 

[JR]

I always carried a spare relay and fuse when I had my Air Dog. Glad it was an easy fix.