Allison 5sp: 2007 LLY prime issues

ikeG

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Apr 19, 2011
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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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DAVe3283

Heavy & Slow
Sep 3, 2009
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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.
 

ikeG

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Apr 19, 2011
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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

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Apr 19, 2011
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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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DAVe3283

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Sep 3, 2009
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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

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Apr 19, 2011
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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

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Oct 15, 2014
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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

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Feb 16, 2009
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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

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Apr 19, 2011
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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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THEFERMANATOR

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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

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Apr 19, 2011
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Western PA
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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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