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Thread: '04 LQ4 swap - not starting

  1. 01-29-2012 #1
    Doublehead
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    '04 LQ4 swap - not starting

    So I am trying to start but it only cranks.
    The fuel pump does run for 2 secs after I switch on.
    When I try to start, the fuel pump does not run. Fuel pump is from the same truck as the engine.
    The injectors do get 12V positive but the PCM does not give ground.

    I disabled VATS and had a segment swap done (using 4L60E instead of 4L80E).
    I also removed all smog equipment from the harness and disabled it in the PCM.

    Does anyone know what can be bothering me?

    I was thinking, maybe VATS hasn't been disabled. Maybe some ground trouble or some sensor (MAP ?)

    The engine is a LQ4 from a '04 GMC Sierra 2500 HD.

    Thanks!
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  2. 01-29-2012 #2
    Doublehead
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    I have the DBW unit (pedal and module) from the donor truck.
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  3. 01-29-2012 #3
    sjgrimsley
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    Are there any dtc's? When I have problems with my setup I always forget to look for dtc's. Sounds like vats is still there.
    2008 Crystal Red Metallic Base Coupe A6 FE1. FlexFuel conversion, 418 LS3, CamMotion 230/234 117+0 cam, 11.5:1 CR, full Z06 exhaust, custom cold air box built around stock Z06 air bridge. Running Amsoil Signature Series 10w-30 and Amsoil Signature Series Fuel Efficient Synthetic ATF. Tuned by me with AEM 30-0333 WB thru HP Tuners.
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  4. 01-29-2012 #4
    Doublehead
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    I have been watching them but I get none.
    I tried unplugging the MAP, the throttle assembly, and the TB and I did get codes on them.
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  5. 01-29-2012 #5
    Doublehead
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    Is there any way to see the signal from the Crankshaft sensor?
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  6. 01-29-2012 #6
    sjgrimsley
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    That's the next thing I was gonna suggest was the crank sensor. Open the scanner and watch the rpm's while cranking.
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  7. 01-29-2012 #7
    Doublehead
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    Well I do not see any RPM's. Guess I'll check the crank sensor wiring next if someone doesn't have any further suggestions!
    It's weird though I do not get any errors. Maybe that's just because the PCM doesn't even know it's bad because it never knows I'm cranking the engine!
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  8. 01-29-2012 #8
    sjgrimsley
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    Check out the wiring and there's a possibility the sensor could be bad too. You'd be surprised what decides to go bad during a swap lol.
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  9. 01-29-2012 #9
    C5XJEDIX
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    scanner controls ;fuel pump on ;start car. if no run .crank sensor no signal.(i would check grounds) if it starts (i would check grounds)BTW if i knew what this was going in i could give better detail where to look for the grounds!if its a vette theres three grounds to block! one on each lower side !and one on drivers side head!and cranks senor code usally trigger after throttle is given while cranking.ive been sent some long blocks with 58x wheels not 24x!

    feel free to bash if this is bad info!
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  10. 02-02-2012 #10
    sjgrimsley
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    The PCM runs the pump for two seconds at key on and it sees no RPM. It doesn't command the pump back on until it sees RPMs. If he isn't seeing any RPM in the scanner while cranking, the PCM will not fire the injectors or coils regardless of if the fuel pump is running or not. I'm not sure what would happen if there was a 58x rink on the crankshaft. I would imagine it would throw a crank signal code for receiving the wrong signal type.
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  11. 02-06-2012 #11
    Doublehead
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    Changed out the Crank sensor and it fired right up and runs like a tiger!
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  12. 02-06-2012 #12
    sjgrimsley
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    Thumbs up

    Awesome!
    Off topic, but do you have a video of it? I'd like to see.
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