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Thread: loss of MPGs after valve adjustment???

  1. #1
    Lifetime Member LB72004's Avatar
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    Default loss of MPGs after valve adjustment???

    I know this is not necessarily an EFI Live or tuning question but i thought that with your guys smarts on how these engines work you would be able to help me out.

    ok, so here it is, a few months back i replaced my injectors (The infamous LB7s). while i was in there i decided to adjust the valve lash, based on some other forums recommendations as it would help quiet down the engine and make it smoother running. well it did, but i now it seems that i get worse mileage. (i know, internet mechanics)

    the valve was already close to stock (.012~.013) and seemed fine but i was curious how much quieter the engine would run with them tightened up. well i adjusted them down to .010 as this was the max recommended and said to make the smoothest, quietest engine.

    anyways, after installing the new injectors and making the valve adjustment the engine runs great. balance rates all below 1, spools great (maybe too well. will explain later). the only issue is that it seems that my MPGs took a dump. i used to get between 430 to 450 miles to a tank taking the same rout to & from work every day for almost every fill up for over a year. now for the last few month since the adjustment i have a hard time breaking 320 miles to a tank. that is a drop from about 16~17 MPGs down to about 12 MPGs. highway MPGs are also not much better

    my theory is that the exhaust valve is now opening up earlier and letting the exhaust gases out sooner. this would lessen the amount of time the combustion pressure would be able to work on the piston. this seems to be backed up by the fact that i can now hear the turbo running, small amount of turbo whistle, all of the time ever so slightly when cruising around 60-65. i never was able to hear it before unless i pressed the accelerator down more, supplying it with more fuel to speed up. it also seems to spool faster then before and smoke a little bit more too. all while seeming to make slightly less HP on my butt dyno.

    things i have done to try and remedy it:
    -raise timing in the cruise area. probably went dangerously high on the test as i slowly worked my way up to about 9 degrees at 20 mm3 and 2200 rpm (about 500 pw at 115 Mpa). didn't seem to help much so returned it to the previously good values.
    -raised and lowered fuel Mpa. got as high as cruising around at 150 Mpa. now running at 65 Mpa. thought that i might be able to get the fuel to finish burning by the time the valve has opened up.
    -currently playing with pilot injection. running with the events closer together right now. not impressed yet but still too early to tell.

    any ideas guys? do you think the valves are too tight? should i dig back into it and readjust the valves or just wait until the injectors go bad again. almost 100 miles less per a tank could add up to quite a large number $$$ by that time.

    my next step will be logging boost during steady cruise to see if it is higher then my logs before the change

    anyways, i am open to suggestions to tuning ideas that may get me by until i can restore the valves to stock.

    thanks for any help and sorry for the long read

    Jason

    PS: this may make a good discussion on valve lash and MPGs. why some engine get good mileage while other don't, etc.
    2008 Chevrolet 3500HD DRW LTZ
    285/75/17 Nitto Dura Grapplers, 4.56 gears, Front and rear ARB lockers, Efi Live tuning by me, Raptor 160 LP, Cognito 4-6 NTB Lift, 4" Exhaust, Built Trans by Mike L.

    Sold - 2004 LB7 Duramax, EFI Live with DSP5 Tune
    stage 4 SC transmission, 265/70/16 tires

  2. #2
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    This is interesting but I would suspect injectors and fuel quality before the .003" you tightened up valve lash. Did you do something else to the truck around the same time, tires, intake anything else?

    Edit- just saw your sig any of that stuff on or removed before/after the injectors/valve lash?

    Also thinking about the valve lash maybe its a tad to tight on the exhaust side, everyone uses feeler gauges differently. It could be possible the exhaust valves could be held slightly of there seat when at full temp causing lost exhaust energy. Not saying for sure but a thought, .001-.002 could be enough to loose compression and lost energy from combustion.
    Last edited by rcr1978; September 3rd, 2012 at 04:33 AM.
    04 LB7
    2010 LMM
    2015 LML

  3. #3
    Lifetime Member LB72004's Avatar
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    thanks for the reply

    the only thing i did was, while the last set of injectors where on there way out, i installed the second CP3 to help hold rail pressure until i could get new injectors. i haven't increased the PW any from before. do you think two CP3s require that much energy to turn?

    in my reading it seams that the valve lash will increase as the engine warms up, but this is from the same source of my valve lash adjustment. if it was leaking past the exhaust valve would hot starts be hard as it would make it more difficult to build up the pressure in the cylinder or is this offset by the extra heat already in there???

    most of the fuel i have gotten from the same place but several times i filled up at other stations. the fuel filter is a rancor double coalescing type with less then 5000 miles on it.

    several others on the duramax forums seem to be running .011 just fine but few have tried .010. i wonder if i was to go the other way .013~.014 i could get 25 MPG on the highway
    2008 Chevrolet 3500HD DRW LTZ
    285/75/17 Nitto Dura Grapplers, 4.56 gears, Front and rear ARB lockers, Efi Live tuning by me, Raptor 160 LP, Cognito 4-6 NTB Lift, 4" Exhaust, Built Trans by Mike L.

    Sold - 2004 LB7 Duramax, EFI Live with DSP5 Tune
    stage 4 SC transmission, 265/70/16 tires

  4. #4
    Lifetime Member anarchydiesel's Avatar
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    I definitely would suspect the injectors before the valve adjustment in regards to your drop in mileage.

    Anarchy Diesel Tuning/ Advanced Injection Diesel Technologies
    EFI Live Cummins Beta Tester
    Duramax and Cummins Tuning
    423-506-7427

  5. #5
    Lifetime Member LB72004's Avatar
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    But this is with new injectors.
    2008 Chevrolet 3500HD DRW LTZ
    285/75/17 Nitto Dura Grapplers, 4.56 gears, Front and rear ARB lockers, Efi Live tuning by me, Raptor 160 LP, Cognito 4-6 NTB Lift, 4" Exhaust, Built Trans by Mike L.

    Sold - 2004 LB7 Duramax, EFI Live with DSP5 Tune
    stage 4 SC transmission, 265/70/16 tires

  6. #6
    Lifetime Member THEFERMANATOR's Avatar
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    Quote Originally Posted by LB72004 View Post
    But this is with new injectors.
    New doesn't always meen good. Are tehse stock injectors or oversized? If your running 2 CP3's with stock injectors, I would think you would have alot of parasitic draw on teh engine that isn't needed. If it is oversized injectors now, were the old ones oversized as well. MANY variables here to consider, but I wouldn't think the valve lash would hurt it.
    1995 GMC 2500 SUBURBAN powered by 01 DURAMAX/ALLISON, little of this, a little of that,
    DIAMONDEYE 4" exhaust, CORSA muffler, AFE stage 1 dry filter, custom tuning by me, KENNEDY single pump and pump rub kit.

  7. #7
    Lifetime Member LB72004's Avatar
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    Stock injectors before and after. I guess I could put the stock belt on and see if there is any change.
    2008 Chevrolet 3500HD DRW LTZ
    285/75/17 Nitto Dura Grapplers, 4.56 gears, Front and rear ARB lockers, Efi Live tuning by me, Raptor 160 LP, Cognito 4-6 NTB Lift, 4" Exhaust, Built Trans by Mike L.

    Sold - 2004 LB7 Duramax, EFI Live with DSP5 Tune
    stage 4 SC transmission, 265/70/16 tires

  8. #8
    Lifetime Member LB72004's Avatar
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    Removed the 2nd CP3 and put the stock belt back on. Now I just need to run some fuel through it and see what kind of figures I get.

    if this ends up being the cause I am going to look into attaching an ac compressor clutch or similar to the fuel pump and activate it with a boost switch or something. That way it won't be running all of the time. But turns on when I want it to.
    2008 Chevrolet 3500HD DRW LTZ
    285/75/17 Nitto Dura Grapplers, 4.56 gears, Front and rear ARB lockers, Efi Live tuning by me, Raptor 160 LP, Cognito 4-6 NTB Lift, 4" Exhaust, Built Trans by Mike L.

    Sold - 2004 LB7 Duramax, EFI Live with DSP5 Tune
    stage 4 SC transmission, 265/70/16 tires

  9. #9
    Lifetime Member THEFERMANATOR's Avatar
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    If your running stock sticks, then a stock CP3 should be able to keep up with it just fine to most any sane amount of pulsewidth provided your injectors are in good shape and on the tight side(very low return rates).
    1995 GMC 2500 SUBURBAN powered by 01 DURAMAX/ALLISON, little of this, a little of that,
    DIAMONDEYE 4" exhaust, CORSA muffler, AFE stage 1 dry filter, custom tuning by me, KENNEDY single pump and pump rub kit.

  10. #10
    Lifetime Member minytrker's Avatar
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    I installed new injectors on my LB7 dually and adjusted my valves earlier this year also. My valves were all really loose. I went from 18-20mpg to 15-16mpg now. I talked to several diesel shops and they said thats normal and the mileage should go up with 15-20,000 miles. It really sucks losing mileage BUT the truck runs so much smoother and quieter now.

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