Crank no start Lb7

Clement Technician Missouri Posted   Latest  
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2002 Chevrolet Silverado 2500 HD 6.6L (1 LB7) 5-spd (1000/M74)
Crank / No Start

good evening . i have this truck that came from another shop, i do not know what the whole story is , but it came to them as a no start partly disassembled by the owner. any way they did head job and new fuel pump and injectors. and still truck does not start. what i am looking for is some info on the high voltage coming from the fuel control module to control injectors. some scope captures of these injector would be great as well. i think these lb7 high voltage is around 90 volts , but i have not found anything to confirm that. now as far as testing i have done , there are no codes, engine cranks good , relitive compression very even, good fuel pressure, low side is being supplied by a fass fuel system so there is 10 psi at the test port and no air in it. also fuel rail pressure goes right up to 40 MPA cranking. rpm shows 150+. almost no smoke coming from exhaust. Scoped the voltage feed from the ficm to the injectors and the control and ficm is commanding injectors,also amp clamp on the power wire shows a current draw for each injector. so i am pretty sure ficm is trying to control injector. i also have a knock sensor that i hook up to my scope and i placed it on the fuel line going to the injector because you can feel a pretty good click on those lines when an injector fires and i see no difference in the amplitude of the signal with injectors commanded on or off. so i have ficm control , confirmed fuel in the rail. the thing i dont like is the high voltage coming out of the ficm is between 50 to 75 volts , maybe not enough to fire injectors? we happen to have another ficm around the shop for the LB7 i put it in and it does the same thing. i have not yet gone over powers and ground to the module . thanks for your input. 

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Interesting
Jesse Owner/Technician
Pennsylvania
Jesse
 

Hi first you did an excellent job checking everything pretty thoroughly, I read your post twice looking for flaws in testing, I recomend mechanical compression test next , next scope injector signal for tdc accuracy, ficm voltage spec I dont know.

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Clement Technician
Missouri
Clement
 

Hi jesse , thanks for your responds. mechanical compression would be a good idea. i have not done that yet based on the sound of the motor it sound very normal , i know that is relative :) but also the relative compression was very even. but the other thing is even if i had low compression if the injectors were delivering fuel i would think i would see some smoke out the exhaust. that is why i…

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Joel Mechanic
California
Joel
 

If I remember correctly voltage at the injectors is 42v when running. Are the cam and crank in sync? Do you have any codes? What is the current draw on the injectors?

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Clement Technician
Missouri
Clement
 

you could be right on that voltage, i was thinking that the lb7 used higher voltages then the next generation did. i just did a 2004 i think that is the LLY engine and i know that one is around 50 v. as far as the amp draw i don't remember for sure but i think it was like 15 to 20 amps . And no there are no code .

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Christopher Technician
Washington
Christopher
   

LB7 FICM should make 96 volts running. I would recommend performing voltage drop test to battery for FICM power and ground while cranking. You have the pressure. Verify fuel quality API. Lastly, I have found some FASS systems do not play well with early CP3 pressure regulators. Had a 03 LB7 with hard starting with 10 PSI going in. Adjusted pressure down to 3 PSI solved it. Maybe the FPR was…

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Matt Technical Support Specialist
California
Matt
 

I agree with Christopher, 10 psi is too much. Those trucks normally run with negative pressure on the inlet of the injection pump. for a quick and dirty test you could put a hose from the inlet to the factory filter housing (if you’re still running it) into a bucket of fuel, use the hand primer, and I’ll bet that it’ll start. even better, put the bucket of fuel on a table higher than the…

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Clement Technician
Missouri
Clement Update
 

Good evening fellow techs. So I did eliminate the fast fuel system and put a suction line right up at the injection pump this did not correct anything which I didn't think it would because I was getting good high pressure in the fuel rail. I also double checked the current draw at the power supply fuse for the ficm and each injector is pulling around 18 amps, which the fact that it is…

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Jesse Owner/Technician
Pennsylvania
Jesse
 

I really think you should do a mechanical compression test I had a diesel engine this summer that had brand new head the valves were leaking so bad I couldn't get more than 200 PSI compression the head that I had used was from an offshore company what I did was used a compression test adapter in the glow plug hole and the air leakage coming out the intake and exhaust valves was Unreal

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Clement Technician
Missouri
Clement
 

Well maybe I should 😄 you have a good point. The main reason I have not done that is because I do not think I have an adapter for the glow plug. But here is my theory, first off most low compression issues in a diesel would produce a lot of smoke because there is not enough compression to ignite the fuel properly. And I literally have very little smoke almost zero. Secondly if it would be a low…

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Jesse Owner/Technician
Pennsylvania
Jesse
 

in that engine it would not even ignite ether , i forgot that you had it started on ether , and i noticed that you did have 40 mpa rail pressure which is high enough , if i remember correctly i have seen them start with about 3500 psi, hey wait a minute , my son had disassembled his 5.9 injectors and shimmed them to much and it would not inject fuel while cranking, Aa local diesel shop owner…

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Clement Technician
Missouri
Clement
 

When I got to the point of realizing it was injectors ,I'm like no can't be,they've just been replaced supposedly brand new Bosch. So we talked with the shop that installed them and they for some reason bought them from an online company they've never dealt with before which might be fine but it does put some question marks on the quality of injectors that are in it. But when I put the pieces of…

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Christopher Technician
Washington
Christopher
 

Last year another shop installed 8 Bosch injectors obtained online to their shop truck. Three of them were faulty right out of the box. We put in a set of injectors from GM. Fixed.

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Ryan Owner/Technician
British Columbia
Ryan
 

These early Duramax engines run around 90 volts and 18 amps. From my scope capture it can very slightly. As you have already pointed out, if you have rail pressure don't sweat the supply side at the moment. I believe FASS runs 8-10 psi on the Duramax engine. This really is not much different than the 5.9L Cummins runs to the CP3 pump on those engines. I have been don't this road a few times…

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Clement Technician
Missouri
Clement
 

Thank you for your input. As far as the fuel, I am trying to start it on a 5 gallon bucket of known good fuel. I did out of curiosity dig out an old diesel compression set that we have and found an adapter that worked in the glow plug and it pumped the gauge right up to about 450 PSI. I'm almost done installing the new injectors so hopefully today I will see what that does .

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Clement Technician
Missouri
Clement Update
 

So this truck proved to be a real challenge. After installing a set of injectors the thing still would not start. Nothing changed in fact. Still had injector control good amp draw good high pressure fuel but still very little smoke. So I began investigating deeper. I noticed a black juice coming out of the exhaust pipe seams. I started pulling pipes apart and saw the insides of the pipes were…

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Christopher Technician
Washington
Christopher
 

So the reluctor wheel on the cam is held on by six bolts with a pin to index the wheel to the cam gear. The bolt pattern would allow it to be bolted on 180 out but the pin is there to prevent this. Maybe the pin was lost or broken and the wheel installed 180 out by someone else? I cannot imagine the gear itself being 180 out or moving as there is a robust key where the gear fits to the cam. I…

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Clement Technician
Missouri
Clement
 

Yes I did go over wiring and see nothing switched around. Earlier on through the testing I was scoping the command coming from the ECM commanding injector number one and also watching number one right down at the injector to make sure the ECM was commanding the right one and that the ficm was controlling the right one. And it was. It really appears like either somehow the ECM got scrambled or it…

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Christopher Technician
Washington
Christopher
 

You must have an alternate firing order camshaft. Diesel power freaks do this to take the pressure off the crank during the 1-2 firing stroke. This is the biggest reason modified duramax engines break the crankshaft. Dig around on the internet. See if this makes sense to you.

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Jesse Owner/Technician
Pennsylvania
Jesse
 

Is it possible to install the reluctur wheel upside down? I find it interesting that when you switched control wires that the engine started. Also when people do firing order switch I think its usually 4 - 7. Good luck!!

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Clement Technician
Missouri
Clement Delay
 

Well DN would like me to close this post. I wish I could give a final verdict, but for now it's at a stand still. So far the customer has not given approval for any farther investigating. He is currently out of town for a few weeks so I do not know yet what will happen with this truck. if we ever do come up with a final verdict I'll be glad to post it.

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