LB7 no start / electrical gremlins - Chevy and GMC Duramax Diesel Forum
01-04.5 LB7 Duramax Powertrain Discussion of components that are directly involved in the power production and all that is needed to get and keep the truck moving . Engine , Transmission, Etc.

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  • 1 Post By Orndorff52
 
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post #1 of 7 (permalink) Old 11-09-2019, 12:34 AM Thread Starter
 
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LB7 no start / electrical gremlins

Ok gents, Iím stumped... hear me out on this and give me any pointers you may have. Mother in Law has an Ď03, which suddenly wouldnít fire the other day (approx 50 dog ambient temp, so not too cold). It turns over just fine. Hereís the details:

- batteries less than 1yo, each with good connection, 12v, 88% charge, 100% health according to battery tester.
- all fuses checked and found to be intact.
- crank rpm at 180-190rpm, but slightly lopey.
- fresh oil change and filter since issue started. No dilution or growth of oil.
- FASS fuel system added 12mo ago when truck started cutting out under load. No issues since. I changed the fuel filter and water separator today, primed, and bled system to the bleed screw on the engine.
- while cranking, rail pressure is 1400kPa / 74psi Max
- performed fuel return test today and got less than 2oz / 100ml of fuel in 15sec of cranking.
- only consistent code is U1000 - Class 2 Communication Failure
- mother in law recently (before the no start issue) started using a new key fob because sheís too stupid or lazy to change the batteries in the OEM one. I replaced the battery and am using the OEM key fob during testing.
- instrument cluster will intermittently not spool up upon turning of the key. No gauges respond (ie fuel level, voltage, etc). The only lights that come on are chk eng and glow plugs. The gauges will also intermittently not go limp when key is shut down and key removed (see photo). Other times, it spools up as it should. I am unable to establish a pattern as to why.

Alright, let me hear it! What do yíall think? At this point, my mind is going down a faulty computer module route... or possibly a CP3 (I suspected it was getting weak last year when I added the FASS lift pump), or possibly a Fuel Pressure Regulator.
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post #2 of 7 (permalink) Old 11-09-2019, 12:35 AM Thread Starter
 
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Oh, and the truck is bone stock except for the FASS, a mechanical pyrometer, and BD exhaust brake.
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post #3 of 7 (permalink) Old 11-09-2019, 02:31 PM
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Fuel pressure regulator wonít cause a no start, when mine went bad atleast. Just makes it idle like crap. And a weak CP3 will usually still start but then have no balls. So i wouldnt suspect either one of those. Was the truck hot or cold when you couldnt start it? When my fuel pressure relief valve went bad, my truck would actually not start when it was hot but still start cold. I assume yours was cold though. I would lean more towards electrical because you have a lift pump and have no fuel leaks so the CP3 is getting fuel, its highly unlikely the CP3 had a complete failure just sitting.


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post #4 of 7 (permalink) Old 11-09-2019, 02:38 PM
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The U100 code is the key. Somethings not letting the ecm receive all the inputs it needs.
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post #5 of 7 (permalink) Old 11-10-2019, 12:51 AM
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Exclamation Say whaaaat?

Welcome to the Forum!

Although you spelled out the trucks details in your first post, itís important to have all that 411 in a signature line (see User Control Panel) so readers know all the truckís basics in each post and donít have to refer back.

Also, you seemquite wise in the ways of LB7ís and the tests you have performed are informative.

A couple of observations:

ďwhile cranking, rail pressure is 1400kPa / 74psi MaxĒ

That kind of rail pressure at cranking will simply not do and you need somewhere around 5,000 psi to wake up the FICM and get the injectors on line.

Iím too lazy to look up total return rate specs, but the test you did seems to indicate the internal ball and seat in the injector return circuits are working properly. Double check me on that, since failure of even one injector return circuit will certainly blow off all your rail pressure.

ďNot making oilĒ is obviously good too, since that rules out leakage around the return plumbing. Cracked injector bodies can also blow off rail pressure, but most of those old Bosch sticks are long out of service. Did you do a ďpaper towel testĒ to make sure no fuel in the oil?

For diagnosing low rail pressure, search is your friend and donít hesitate to use a search engine, which Iíve sometimes found to even uncover posts here on the Forum that didnít come up under our own Forum search, or were too buried in multiple key word hits to sort through.

Frankly, Iím more concerned about the multiple electrical symptoms that have presented, and would make every effort to address those before even attacking the no-start. While I canít see a connection between low rail pressure (which is pretty much a mechanical issue involving various elements of the fuel delivery system) there may be electrical interactions Iím not of that can cause low RP. Just canít recall reading of any.

Quite often, multiple seemingly unrelated electrical issues can be traced back to a single sketchy ground, cable or connector and Iíd spend time using the time-tested ďwiggleĒ system, along with a careful visual inspection to narrow your search.

And regarding the key fob, unless Iím mistaken, ours only accesses the panic alarm and doors, and unless there is some connection to an anti-theft function, I donít see the key itself being part of the problem.

And WOW, does your MiL know of your low opinion of her?

You seem to be a smart guy and have checked all the ďlow hanging fruitĒ boxes, and Iím confident you will solve this problem.

Best

2003 GMC Sierra 2500HD SLE ExtCab CA Emissions; 155k; Allison 1000 w/Transynd X2 Flush & Fill; 8' bed; AFE/CAI; MBRP 4" Exhaust; Side Tint, 33% VLT, rear, Limo; Smittybilt Stainless Side Nerf Bars; Sprayed-in Bed Liner; MadJack Fuel Filter Access Door; Regular Used Oil Analysis (latest available on request), Blackstone Labs; EGT/Boost Gauges; Dual Chrome Dipsticks, JC Whitney; Stainless Steel fuel filter head bleeder screw; New WP and Hydroboost; Billet Aluminum Fuel Filter Housing Spacer; Professionally repaired instrument cluster (new steppers and lamps); Does anybody ever read this s%#t?; Injectors under warranty @ 70k; 2nd set DIY @ 155k
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post #6 of 7 (permalink) Old 11-10-2019, 10:56 PM Thread Starter
 
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Quote:
Originally Posted by stevewaclo View Post
Welcome to the Forum!

Although you spelled out the trucks details in your first post, itís important to have all that 411 in a signature line (see User Control Panel) so readers know all the truckís basics in each post and donít have to refer back.

Also, you seemquite wise in the ways of LB7ís and the tests you have performed are informative.

A couple of observations:

ďwhile cranking, rail pressure is 1400kPa / 74psi MaxĒ

That kind of rail pressure at cranking will simply not do and you need somewhere around 5,000 psi to wake up the FICM and get the injectors on line.

Iím too lazy to look up total return rate specs, but the test you did seems to indicate the internal ball and seat in the injector return circuits are working properly. Double check me on that, since failure of even one injector return circuit will certainly blow off all your rail pressure.

ďNot making oilĒ is obviously good too, since that rules out leakage around the return plumbing. Cracked injector bodies can also blow off rail pressure, but most of those old Bosch sticks are long out of service. Did you do a ďpaper towel testĒ to make sure no fuel in the oil?

For diagnosing low rail pressure, search is your friend and donít hesitate to use a search engine, which Iíve sometimes found to even uncover posts here on the Forum that didnít come up under our own Forum search, or were too buried in multiple key word hits to sort through.

Frankly, Iím more concerned about the multiple electrical symptoms that have presented, and would make every effort to address those before even attacking the no-start. While I canít see a connection between low rail pressure (which is pretty much a mechanical issue involving various elements of the fuel delivery system) there may be electrical interactions Iím not of that can cause low RP. Just canít recall reading of any.

Quite often, multiple seemingly unrelated electrical issues can be traced back to a single sketchy ground, cable or connector and Iíd spend time using the time-tested ďwiggleĒ system, along with a careful visual inspection to narrow your search.

And regarding the key fob, unless Iím mistaken, ours only accesses the panic alarm and doors, and unless there is some connection to an anti-theft function, I donít see the key itself being part of the problem.

And WOW, does your MiL know of your low opinion of her?

You seem to be a smart guy and have checked all the ďlow hanging fruitĒ boxes, and Iím confident you will solve this problem.

Best
Thanks so much for the helpful info! I considered making a signature, but this isnít my truck that Iím working on. I own a 2011 Duramax, and would never own an LB7 because Iíve worked on this one enough to know what a PITA they are (did the injectors on this truck 3.5yrs ago). I actually only own my Duramax because itís such a quiet and comfortable ride... Iím a Cummins man through and through, and my 2001 dodge is my true workhorse. 😂😂

A little update: I removed all the usual crap to get down to the FPR, and ran a resistance test in it (3 ohm). I then cranked the engine over with the FPR and FICM unplugged. I actually had assistance turning the key today, so I used this opportunity to check the fuel primer while the engine was cranking and was able to verify that it was still very difficult to press (ie there was obvious pressure behind it under cranking conditions.

With the FPR and FICM unplugged, FRP shot up to an average of 9700psi and peaked at 19800psi under continual monitoring during crank, which tells me the FPR is the culprit. Furthermore, being that the ohm test didnít show anything too crazy, itís the mechanical component of the FPR thatís fouled up and not the solenoid. What say you?

With OEM FPRís as expensive as they are, Iím just replacing the CP3 also while itís under the knife. I ordered a Bosch Reman and started tear down tonite, and am using this opportunity to check all wiring, grounds, etc to address the possible electrical component of this issue. I own the factory wiring diagram supplement as I referenced it while chasing some other electrical gremlins in this truck awhile back.

Thanks again for the advice... truly appreciated!
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post #7 of 7 (permalink) Old 11-10-2019, 10:59 PM
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Quote:
Originally Posted by Orndorff52 View Post
Thanks so much for the helpful info! I considered making a signature, but this isnít my truck that Iím working on. I own a 2011 Duramax, and would never own an LB7 because Iíve worked on this one enough to know what a PITA they are (did the injectors on this truck 3.5yrs ago). I actually only own my Duramax because itís such a quiet and comfortable ride... Iím a Cummins man through and through, and my 2001 dodge is my true workhorse.



A little update: I removed all the usual crap to get down to the FPR, and ran a resistance test in it (3 ohm). I then cranked the engine over with the FPR and FICM unplugged. I actually had assistance turning the key today, so I used this opportunity to check the fuel primer while the engine was cranking and was able to verify that it was still very difficult to press (ie there was obvious pressure behind it under cranking conditions.



With the FPR and FICM unplugged, FRP shot up to an average of 9700psi and peaked at 19800psi under continual monitoring during crank, which tells me the FPR is the culprit. Furthermore, being that the ohm test didnít show anything too crazy, itís the mechanical component of the FPR thatís fouled up and not the solenoid. What say you?



With OEM FPRís as expensive as they are, Iím just replacing the CP3 also while itís under the knife. I ordered a Bosch Reman and started tear down tonite, and am using this opportunity to check all wiring, grounds, etc to address the possible electrical component of this issue. I own the factory wiring diagram supplement as I referenced it while chasing some other electrical gremlins in this truck awhile back.



Thanks again for the advice... truly appreciated!


Good luck with that CP3 they are not a fun thing to do


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