I have a problem I'm sure you guys might help me solve. Truck starts good most of the time, but occasionally the lift pump will not pump. If I let it sit for 5-10 minutes and try starting again it will usually decide to pump. It's been doing this for the past 6-8 months or more, I have changed the lift pump several times and it never has cured it. I have swapped relays and can't say that helped either. I have a b-d idiot light, and it has never given any problems once running, its just the starting.

As a side note, about once every 1-2 weeks I get a code for the crank or cam positioning sensor but was told its just the cold temp. swings, maybe a related problem? Thanks

 

For all the more a crank sensor is i would change it. It also helps control performance. I have never herad of temps interfereing with them. I asume you have a stock lift pump, even if you dont, try bumping the starter then listen for the lift pump. It should run till it gets to pressure. I would also try to get a fuel pressure gauge also, so you can see where your lift pump is at for pressure. I did however have an issue with the ecm not powering the lift pump. Turned out to be the computer. There was an intermitten problem also. But with the cost of the computer it was cheaper to install a FASS on it. The problems ended. Good luck1

 

But then I have also had relays work sometimes and not other times. Just try swapping one from the horn or something to see if that changes it.

 

i had code P0336 "Crankshaft Position Sensor Performance" twice this week, never affected my fuel pump running. first time i just cleared the code, second time i got a new one.

i put it on today, genuine Cummins $54. i hate walking and wreckers.

the pump will not run for long when you turn on the key, just a 1/4 second or so, once it knows it's running the ECM turns the lift pump back on which is just about instantaneous once it starts,

turn key on, bump the starter and the lift will run for 15 seconds or so.

you need a fuel pressure gauge to diagnose this, to see when the pump is coming on, if it is coming on.............................first

 

Is an aftermarket lift pump wired in differently or does it use the stock relay? I know for sure the lift pump doesn't run at all when it does decide to act up, I do need to upgrade to a gauge someday soon, but I'm used to listening for it and the light has been correct so far... Swapping the relays hasn't seemed to help. I've swapped one, then another and another, but seems to not be the answer. Seems like it just has to make up its own mind which kind of makes me think ecm. Doesn't happen on a cold start, but after its warmed up a bit. like when you just want to run in the store or turn the truck off to talk to a neighbor in the middle of the road. I have gotten good about just wandering around or sitting innocently in the truck to not draw everyone's attention in a parking lot.

 

after market fuel system still uses the wire off the stock lift pump to fire up any fuel system, FASS and Air Dog you put that wire on there relay, so your still using all the stock systems to fire up the pump what ever one you have. FASS and Air Dog uses more amps, so they run a wire from the battery to a relay, relay to there pump triggered by the ECM (lift pump wire)

if your not getting any power to the stock lift pump wire when running, yes i could be the ECM or simply the wire on the engine harness.

 

I had an issue with my truck that I first thought was the lift pump relay, then I traced it to a blown fuse that kept blowing so I figured it was the lift pump gone bad. Turned out to be a small piece of silicone had broken loose from the prefilter and jammed between the gears of the pump. It was a Fleetguard filter too.
$9 for a new filter, an hour of trouble shooting and a half hr to open the pump housing to remove the chunk fixed it.

 

Just bought an 98.5 and someone ran a pink wire to my fuse block in the cab and the pump ran no matter what position the key was in. I moved it to and ignition only fuse bit the pump runs none stop, any ideas on an easy fix?

 

Toggle switch, but you just gotta remember every time!

 

put a Holly or Barry Grant fuel pressure NOS safety switch on the oil galley with one of there relays, pump will not run unless it has oil pressure.

that is how i had my pusher pump wired up when i ran one years ago.

usually the ECM give the lift pump a bump prior to starting, you could override it with another switch in the cab if it needs that bump to start.

 

I believe the relay that controls the lift pump is inside the pcm, so if it starts to go bad, you can't just replace it.

Very good reason to use the wire to control a second relay with a fused wire right off the battery. This way, the load is off the pcm relay. If the contacts are worn on the pcm relay it still may reliably kick on a relay to carry adequate current to the lift pump.

Hobbs switches (oil pressure) work nicely too if you want to bypass the system completely. Only problem is if the system needs prime, it won't turn on the fuel at all. This leaves you stranded. You can wire in a relay from the starter impulse (small) wire to kick it on when you turn the engine over, but then you might get "hard starting" from high fuel pressure at startup. Especially if you use dual LP's, or have an aftermarket lift pump.

 

Well, my problem went away for a week or two, then decided not to start again. After getting a volt meter out and checking deeper, I was getting the initial 1/4 second bump at key on from the lift pump, then nothing. It finally started after about a 15 minute rest. Gave in and changed the crank positioning sensor and seems to be ok so far....

I'm thinking that the ecm must not have been getting any signal from the crank sensor so was not allowing the lift pump to have current for normal running? Thanks for the help!

 

Not sure the crank sensor is needed to run the LP... Its supposed to kick on for a period of time after you bump the starter. Might try bypassing the pcm and just power the lift pump off the battery to isolate it as the problem.

 

Did you find the problem?
I have a very similar case...but LP runs w/+12v up to 15psi. I'm checking grounds and then leading towards ECM($264) relay issue.