LBZ head scratcher

I've got an LbZ giving me fits. It's having hard starts (extended crank time). So far I've:
Replaced both 1/2" hoses from fuel filter to engine valley, the 3x 5/16 hoses in the valley (suction to pump and return hoses), rebuilt filter head, new AC Delco filter.
Bypassed filter head= no change
Checked injector return on each bank= both sides had almost no return at idle
Shimmed PRV= no change
Finally, Friday before we left, I wired up a makeshift lift pump. It definitely helps. It's starting in about 1.3-1.5 seconds, but with a Duramax that a lot of rotations.
Any advice would be appreciated.


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Do you have something to log the rail pressure while cranking?

Yes, Rob. I got my V2 from you. As soon as it builds rail pressure, it fires. It will sit at ~216 then shoot up and fire pretty quickly. I watched balance rates today. There are a couple in the -2 to -3mm range.


We emptied the fuel tank today, because it had a few gallons of gas in it. Filled with diesel, additive and drove it. The LP system seems like it's definitely bleeding off, takes 8-10 pumps to get hard at the filter head. But then, it still doesn't crank instantly, just quicker than not priming at all.

I've never heard or seen this before but can the CP3 bleed off the LP fuel to the return side? I'm thinking the gasoline damaged something, just can't pinpoint what.


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Yes the cp3 can. Very rare but with gas in the system I've seen crazier stuff.

With a lift pump you're still having extended crank?

When you say you checked the return rates you mean mechanically checked them or just threw the v2?

I put the return from each bank in a catch can. I may do the return test once more to verify.


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Put the make shift lift pump right to the cp3 to rule out the rest of the system. If that works fine, then take the lift pump off and put the suction side of the cp3 into a can of fuel without the lift pump, it'll stiil run fine if the high pressure side is good.

If either of these fail, you have a bad cp3 and/or injectors.

Thanks Rob. After getting all the gas out of the system, the truck is doing way better. I went ahead and installed a lift pump. Truck fires right up now, even if you don't give the LP pump time to run.
Only issue now is the IAT 2 sensor circuit.. Everything looks fine as far as wiring, going to try a new one.


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