Allison 5sp: 2007 LLY prime issues

[ikeG]

Got an 07 lly van here that will only start and run for couple seconds. It will only start after letting the factory lift pump run for a second. It then starts and rail pressure never reaches desired and it then stalls. Will do this repeatedly.
I pulled the supply line off at the engine and ran the pump and it pumps fuel. Not crazy amounts but pumps it steady. Any ideas of how to further diagnose? I would lean towards cp3 after seeing the fuel its pumping. I might run a can of fuel to the engine fitting to eliminate the lines and filters and see if the cp3 will pull a prime by itself. Of course they bring it with the batteries drained down all the way....

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[ikeG]

I'll add that the van has only 89k on odo. And this problem come up after sitting and not running all winter, stored inside.

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[DAVe3283]

If you pop the return line off, how much total return fuel after cranking/starting/dying? Either the CP3 isn't able to move enough fuel (bad CP3), or it can move enough fuel but it is getting returned too fast (injectors, CP3, FPRV). Or there is a restriction somewhere, but that seems unlikely if it ran fine when parked.

 

[2004LB7]

Have you tried it with the regulator unplugged?

 

[ikeG]

Have you tried it with the regulator unplugged?

Not yet.

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[ikeG]

It will not start at all with the return line removed. But the lift pump pushes fuel out both the 1/2" supply and the 3/8" return lines. The returning fuel from the pump & injectors combined while cranking is just a dribble.
Could of swore I smelled gasoline in the first bottle of fuel so I took another sample and now I cant smell gas...

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[ikeG]

And if I feather the throttle I can get it to run roughly at 1600ish frp. Just for a few seconds longer. Has a p0087 set. I guess I'd start with a cp3?

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[ikeG]

Just ruled out the prv too. no fuel out its hose at all.

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[DAVe3283]

It will not start at all with the return line removed.

That's odd. Not 100% sure what to make of that.

It is sounding more and more like the CP3. Though like 2004LB7 said, you should try it without the regulator plugged in to see what it does.

 

[ikeG]

I know it's pretty straightforward diagnosis on pickups but this factory lift pump has me guessing. Why would it pump fuel out both return and supply. I am able to block off the return side and stop the flow, so maybe it just keeps slight pressure on the return circuit. Kinda like piezo injectors.
And there's the ecm control of the lift pump. It runs at first key on, but when it is commanded to quit, it will not start running again for almost a minute of key off.

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[2004LB7]

Can you jump out the relay to the LP to keep it running?

 

[DAVe3283]

I am not familiar with the factory lift pump, but if it is anything like my AirDog or any other return style regulated lift pump, it will send excess fuel out the return. The factory return line is very small, so it will try to push out everywhere, not just back to the tank. My truck does the same thing, if I turn on the AirDog, it pushes a few PSI into the return with no demand on the main line.

 

[ikeG]

I am not familiar with the factory lift pump, but if it is anything like my AirDog or any other return style regulated lift pump, it will send excess fuel out the return. The factory return line is very small, so it will try to push out everywhere, not just back to the tank. My truck does the same thing, if I turn on the AirDog, it pushes a few PSI into the return with no demand on the main line.

That is what I suspect

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[Tothemax]

i had the same problem with my brother 08 van. if you jump the lift pump to stay running it will probably keep the engine running. mine need a cp3. almost as if the suction side of the cp3 went bad

 

[THEFERMANATOR]

The van does not use a lift pump, it is a prime pump from what I have gathered. It only runs before cranking to purge the air out for filter changes. It does not run when the engine is running. Check it for air leaks. My bet is your sucking air. The purge pump pushes the air out, it starts, then shuts off once it sucks air again. Jump the pump to stay on and see if it will stay running, if it does, then you know you're sucking air, or have a massive fuel restriction that the cp3's pump can't pull through. And the factory pump can move some fuel. I know somebody that used one as a lift pump, and he said it pumped alot of fuel(was just too noisy).

 

[ikeG]

Ok, thanks ferm. So it's not a lift pump but a prime pump. I just jumpered the relay and it will run all day.

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[ikeG]

With the prime pump running full time, it will meet desired rail only at idle. As soon as it needs to build rail to move, it will stay around 6k and as you increase throttle it will decrease until it stalls.


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[ikeG]

If I do a rail pressure test with scan tool, actual will match desired until you reach 15k. Then it will not make anymore

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[THEFERMANATOR]

Do you have a way to check fuel pressure/vacuum to the cp3? This sounds more like fuel restriction than failed cp3 to me. Or can you hook a hose up to the prime pump and feed it from a separate fuel supply to eliminate restriction. When my cp3 went, it didn't have problems until you ramped pressure up, but under 15k it was fine. It started up fine and everything, but it would pour fuel out the cp3 return line when ypu ramped it up.

 

[ikeG]

Do you have a way to check fuel pressure/vacuum to the cp3? This sounds more like fuel restriction than failed cp3 to me. Or can you hook a hose up to the prime pump and feed it from a separate fuel supply to eliminate restriction. When my cp3 went, it didn't have problems until you ramped pressure up, but under 15k it was fine. It started up fine and everything, but it would pour fuel out the cp3 return line when ypu ramped it up.

I did hook a hose up to the prime pump and fed it from a jug of fuel with identical results.
I don't have a fitting for the fuel test port but the local chev dealer said it had 19 psi so obviously they tested it with key on.

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