Help! Lbz won’t run under 1200 rpm

[Unlimitedauto]

Hello, first time posting but I’ve been a lurker for quite some time. I’m working on an 06 lbz all stock other than cold air intake. I received this truck with the motor completely disassembled in the bed. I found out it’s an lmm block with the rest off the lbz. I assembled the motor and was very thorough. Installed with new cp3, injectors, fuel filter housing. The issue at hand is the engine will not idle. There are no codes. I can get it to start and run if I hold it over 1200rpms and it sounds great. If you let it drop lower it starts fuel knocking like crazy until it dies. The actual and desired fuel pressure are well within spec and do not drop when it dies. Ive tested return rates on the new injectors and they are good. I’ve ran clear lines to verify it’s not sucking air. I’ve even vacuum tested the entire fuel system broken down into sections and all are ok. We drained the fuel and filled with fresh diesel.
We’ve compression tested, all cylinders ok. Checked the valve adjustment again. I know I’ve done more but at this point I’m out of ideas. Also it has no tuner. The ecm is stock.
Any thoughts, ideas or crystal balls would be greatly appreciated.

 

[TheBac]

Moved to LBZ section.

 

[Unlimitedauto]

Thanks I wasn’t sure where to post.

 

[gmduramax]

What injectors were used

 

[Unlimitedauto]

Thank you for the reply, all 8 oem GM injectors. I forgot to mention the cp3 and injectors were replaced after the motor install. The problem was present before the injector and cp3 replacement and they made no difference.

 

[juddski88]

Thank you for the reply, all 8 oem GM injectors. I forgot to mention the cp3 and injectors were replaced after the motor install. The problem was present before the injector and cp3 replacement and they made no difference.

Sounds like if you ruled out hydraulics, I'd be looking into the individual injector connectors, and the injector wiring harness.

 

[Unlimitedauto]

Juddski88- do you think I could have a wiring issue with no codes? I just inspected every injector connector and as much wiring as I could. Also when I had the engine out I inspected the engine harness as best as I could. Nothing obvious was found.
Thanks!

 

[juddski88]

Juddski88- do you think I could have a wiring issue with no codes? I just inspected every injector connector and as much wiring as I could. Also when I had the engine out I inspected the engine harness as best as I could. Nothing obvious was found.
Thanks!

A common problem is loose terminals in the harness connectors at the injectors. Not so common with the lbz and newer but it still happens. Normally it should code, but it might not. Try unplugging each injector one at a time to see it the rattle stops. If you have a good diag tool, use a dvt function to shut each one off.

 

[Unlimitedauto]

I do have a snapon Apollo scan tool which will disable injectors individually, however there is no difference with any of the cylinders cut out and I can’t run a cylinder contribution balance test because it won’t idle.
Is it possible I’m chasing the wrong area? Could this be a boost issue? Or does it sound more fuel related?
Again thanks for the reply!

 

[JoshH]

Does the engine run clean and smooth at or above 1200 rpm? Are the injectors brand new from GM or did you get them used or from a second hand source? Does the engine still have a functioning EGR valve on it?

 

[Unlimitedauto]

Yes it runs smooth like there’s no problem at all if it’s over 1200 rpm. The injectors are new from the dealership and the egr is there and functioning. I cleaned it thoroughly before install as well.

 

[JoshH]

Can you unbolt the cast intake pipe that runs across the top of the EGR valve and see if the engine will idle with the intake open like that? Just be careful nothing falls down in there while it is running.

 

[Unlimitedauto]

Doing that now. I’ll let you know how it goes

 

[Unlimitedauto]

Pulled that intake piece and it made no difference. Any other ideas?

 

[JoshH]

Do you have another rail pressure sensor you can swap in or a different ECM you can test it with?

 

[dmaxfireman]

what is your rail pressure vs commanded?

 

[dmaxfireman]

I wonder if the ecm was tuned for larger injectors and will not run correctly due to now having factory injectors. How have you verified that the ecm is stock?

 

[Unlimitedauto]

Seriously thanks for the help guys,
Sorry I forgot to mention, the ecm is new. When I got the truck it had no ecm so I ordered one pre programmed from all computer resources. I thought maybe the new one was faulty and they sent me another. Still the same issue. I have also swapped the rail psi sensor with a good known sensor and no change. Same with the fprv. I’ve wiggle tested all the connectors relating to fuel components. My actual fuel pressure varies but is over 40 MPa with the desired pressure right there with it. Also the injector flow rates were updated when the injectors were replaced as well as the ecm replacement.

 

[2004LB7]

Any difference if you unplug the maf? Whats the coolant temp show?

 

[Unlimitedauto]

Haven’t tried the maf sensor. I’ll give it a shot and the coolant temp is accurate. I haven’t ran it up to operating temp yet but it does start out at ambient air temp and will rise accordingly.