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Thread: LQ4 will fire but lose injector pulse immediately

  1. #1
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    LQ4 will fire but lose injector pulse immediately

    I have a 2005 LQ4 with a 2bar speed density tune that will fire up right away when cranked but will putter out after barely 1 second. VATS is off. I lose injector pulse according to the scanner. Fuel pump relay output stays on. Any ideas??

    I previously had the engine running well (several years ago) with stock MAF tune. Since then this is what has changed hardware-wise.

    - eliminated MAF by snipping plug but retaining the 2 IAT wires, wired to a new IAT sensor
    - changed injectors from stock to 52lb/hr, and re-wired injector plugs to USCAR
    - went from a 2005 DBW pedal to a 2004 DBW pedal (reused 2005 TAC module)

    The IAT sensor reads accurately, I'm pretty confident in the rewiring of the injector plugs (crimped not spliced/soldered and I read good voltage to injectors), but the pedal has me suspicious. The scanner throws no DBW codes, it reads ~20% just sitting there and 100% at WOT.. but could the mis-matching model year pedal & TAC module be the culprit?

    Anything look horrendously suspicious in my tune? I've attached the tune, scan log, scan config file, and a screenshot of the scan log for quick reference...

    Thanks!!
    Joe
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  2. #2
    Advanced Tuner Montecarlodrag's Avatar
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    Why is your MAP reading 105 KPa at all times? There is always vacuum at cranking and startup, but your log isn't showing any.

    Do you have a 2 Bar MAP installed
    is it properly configurated in the tune?

  3. #3
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    Thanks for the response. It appears that I didn't delete the 1bar PID and insert a 2bar PID in the scanner, which I will now do and test.

    I do have a 2bar MAP in there, engine will not fire at all with it disconnected, but does do this fire-then-die with it plugged in.

    Also, only 1 code is being thrown at all, the P0622 F-circuit DTC.

    Thanks,
    Joe

  4. #4
    fire and die sounds like vech anti theft system(vats)
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  5. #5
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    you didnt have the head off for a vavle job by chance?
    My setup is a 356ci with a 260/268 (212/218 at .050 lift) duration cam with aluminium corvette heads and flat top pistons running 11.3 comp. ratio. with tuned port injection and vortec crank pick up and dizzy running a 411 pcm and 60lb bosch injectors, 1.6 ratio roller rockers. For transmission its a 4l65e built with the monster in a box mega ss kit. All in a 92 chevy ext cab 4x4 pickup with a 98 cab now installed with a third door! running only e85

  6. #6
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    VATS is turned off, and no I didn't have the head off... out of curiosity what would that've possibly meant?

    I realized I have a 2.5bar Cobalt sensor in it. Ordered the 2bar sensor. Even with the tune being calibrated for a 2bar sensor but receiving a signal from a 2.5bar signal.. would that be enough to prevent it from continuing to run?

    I also noticed no drop in pressure AKA computer reading manifold vacuum during cranking or during the 1second run time, even with the 2bar PID on the scanner.

    Will pick up the proper 2bar sensor tomorrow and try again, then report back.

  7. #7
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    Sorry I was thinking of a different engine the quad four to be exact. If ur machine shop screws up and puts intake spring on the exhaust and exhaust on the intake then it will star for few seconds And dies
    My setup is a 356ci with a 260/268 (212/218 at .050 lift) duration cam with aluminium corvette heads and flat top pistons running 11.3 comp. ratio. with tuned port injection and vortec crank pick up and dizzy running a 411 pcm and 60lb bosch injectors, 1.6 ratio roller rockers. For transmission its a 4l65e built with the monster in a box mega ss kit. All in a 92 chevy ext cab 4x4 pickup with a 98 cab now installed with a third door! running only e85

  8. #8
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    Quote Originally Posted by frojoe View Post
    I have a 2005 LQ4 with a 2bar speed density tune that will fire up right away when cranked but will putter out after barely 1 second. VATS is off. I lose injector pulse according to the scanner. Fuel pump relay output stays on. Any ideas??

    I previously had the engine running well (several years ago) with stock MAF tune. Since then this is what has changed hardware-wise.

    - eliminated MAF by snipping plug but retaining the 2 IAT wires, wired to a new IAT sensor
    - changed injectors from stock to 52lb/hr, and re-wired injector plugs to USCAR
    - went from a 2005 DBW pedal to a 2004 DBW pedal (reused 2005 TAC module)

    The IAT sensor reads accurately, I'm pretty confident in the rewiring of the injector plugs (crimped not spliced/soldered and I read good voltage to injectors), but the pedal has me suspicious. The scanner throws no DBW codes, it reads ~20% just sitting there and 100% at WOT.. but could the mis-matching model year pedal & TAC module be the culprit?

    Anything look horrendously suspicious in my tune? I've attached the tune, scan log, scan config file, and a screenshot of the scan log for quick reference...

    Thanks!!
    Joe
    looks like your injector table is zeroed out. i don't think its going to work like that. Im pretty ssure you need to add values to it. not sure why its zeroed out but you need to look at that.
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  9. #9
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    Thanks for the response, to be honest I've spent so much time looking at the tables and different tunes, my eyes kindof fuzzed over. I can't remember what I started with or what I changed. When I get home tonight I will compare my injector offset tables to a couple other LQ4 tunes (including stock and my previous running tune) and see if there should be numbers in there.

    Cheers!
    Joe

  10. #10
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    Not sure why I had the offset table zero'd out but I copied over the offset values from the stock tune and tried to start the motor again, knowing it wouldn't be perfect for the injectors but maybe it'd be enough to get the engine to run for a couple more seconds... nothing. Same behavior as before, injectors pulse, it catches for half a second, and the injector pulse goes dead and the engine putters out.

    See attached chart/log. I have the proper 2bar sensor in it now, but why does my MAP not register any vacuum or change at all during cranking or the brief fire?
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  11. #11
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    Vats

    even with the VATS shut off in the tune on my brothers truck it did that same thing till i just turned the key to run position for ten minutes or so then it fired right up? i would put stock MAP back in it just to see if starts!
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  12. #12
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    Ok thanks, i'll try that!

  13. #13
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    I'm still confused as to why it has this behavior of barely starting then stopping with both the new speed density tune and the old stock(ish) MAF tune.. could it be that it I haven't zero'd out the right MAF stuff and it's not liking that for SD, and on the old tune with no MAF it's having a hard time kicking over to SD?

    Also, any thoughts on why none of the MAP sensors I've tried in it register any pressure difference on the scanner during cranking/fire? Just read that the MAF PID has to be removed for the 2bar MAP PID to register properly.. will try this and see how goes.
    Last edited by frojoe; 03-27-2012 at 01:53 PM.