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poor fuel mileage after lm7 swap

This is a discussion on poor fuel mileage after lm7 swap within the General Help forums, part of the LSx Technical Help Section category; Hey guys I'm a newbie to this forum. I have a problem with my lm7/4l60e swap into my 1981 Chevy ...

  1. #1
    Junior Member haulin c10's Avatar
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    poor fuel mileage after lm7 swap

    Hey guys I'm a newbie to this forum. I have a problem with my lm7/4l60e swap into my 1981 Chevy short wide. I was hoping to get the fuel mileage like these stock motors are supposed to get but mine is only 8 miles per gallon. The combo is out of a 2005 Silverado. All stock no mods the computer has been reflashed for the swap. I did my own wiring harness. It turned out great. The motor runs excellent shifts good and drives good. I like the power it has. But the fuel mileage is killing me in gas. The fuel pump is the walbro 255 with the corvette filter/regulator combo. My exhaust is stock with a flow master. No duals. In the future. 3:73 gears.

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    Welcome!


    What size tires are you running? Stock converter? Any signs of the engine running rich?

    Our 6,200 lb. Suburban pulling our 7,000 lb. trailer manages 9.3 mpg... so you must have something off kilter.

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    Junior Member haulin c10's Avatar
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    Thank you for welcoming me. The tire size is 245/70/17. Stock converter. No aftermarket mods. It is running rich. I finally got the check engine light to come on the codes are p0332 and p0327. Knock sensor codes. Would that make my truck runs rich. And it seems to cut out while driving for a split second.

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    Spaz is My Mentor SMWS6TA's Avatar
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    custom wire harness, did you include at the min the front O2 sensors?


    What gear ratio in the rear? tall gears = crappy mileage.

    Do you know anyone that can data log what the motor is doing? You might not have the correct fuel trim tables, etc.....


    Our best tuner on here is Frost. He specializes in custom and mail order tunes. I as well as many others here have a tune from him.


    He might be your best bet.

    TunedByFrost.com - Professional Tuning For Your LSx Powered Car or Truck

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    +1 for a Frost tune. He has a quick turnaround time and is reasonably priced.

    If it is running rich, you have a couple of possibilities:

    1. Ensure that your fuel pressure at the rail is around 58 psi.

    2. Any exhaust leaks above the front O2 senders will cause them to read "lean" and result in the PCM dumping in more fuel.

    3. Older O2 senders lose their efficiency - a set of Bosch P/N 13111 senders are the ticket. As an added bonus, they come with a nice long harness and work great with headers.

    4. A general tune up could be in order -- plugs and wires, air filter, clean the MAF (especially if you have an oiled cotton gauze style air filter)

    5. The knock sensors may be pulling timing if they are acting up, which would affect your mileage under light cruise conditions. Recommend stock AC Delco replacements as members have had issues with parts store brands.

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    Junior Member haulin c10's Avatar
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    It got 3:73 gears. I'm going to try to get a scan tool today if I can to see whats the data on it. I fairly new plugs and wires on the motor all dealership idems. That's all I use. Ill have to check for exhaust leaks too. The harness is the factory one. I did it myself. I got the information off of lt1 swap. Very useful info there. This has been driving me nuts. I'll keep you guys posted with what's going on with it. Thank you for your inputs

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    Curious to know what you find...

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    Spaz is My Mentor SMWS6TA's Avatar
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    had an idea, did the tuner install the programing for the active fuel management?


    LM7's did come with that, right?

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    Depends -- some did, but I am not sure of the variant. Our Suburban has "DOD" - displacement on demand and it deactivates half the lifters when it kicks into 4 cylinder mode. I think "AFM" came just after 2007... but I am not 100% on that.

    OP - I forgot to mention that on a highway drive, our Suburban will turn just over 17 mpg, with 3.73's out back.

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    Junior Member haulin c10's Avatar
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    I got my knock sensors ordered yesterday. I had it hooked up to the scan tool while driving and at cruise it runs rich while driving. Fuel trims was in the negative now I'm starting to notice a real low idle on it when I come to a stop and it almost stalls but it catches itself I did have an exhaust leak but it was behind the o2s. Got it all welded up. I also ordered a blazer gas tank I'm tired of the saddle tank cause my gas gauge won't stay still. Keeps bouncing around everywhere. I don't have the AFM setup. When I had the reflash done it was for a stock setup. I'm going to do more checking tonight when I get off of work. Thanks for your inputs. I'll keep you guys posted.

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    What are the LTFT's running at? Has the throttle body been cleaned? The PCV systems on these engines tends to suck oil into the intake and things get gummed up with the oil mist flying around in there. If it is gunked up around the blade it'll adversely affect idle.

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    Spaz is My Mentor SMWS6TA's Avatar
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    I'd like to see some pics of this C10. Looks like a bad ass ride.

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    Member jb3golf's Avatar
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    Personally I would try fixing the knock sensor issues first....address the codes .....then let us know....good luck bud.

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    Junior Member haulin c10's Avatar
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    I'm going to fix my codes first like jdgolf said and go from there. Mine pcv valve is brand new. I cleaned everything that can be cleaned. Lol

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    Junior Member haulin c10's Avatar
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    I'll get some pics up tommorow. Its not bad. Needs a little bit of work.

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    Member jb3golf's Avatar
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    Quote Originally Posted by haulin c10 View Post
    I'll get some pics up tommorow. Its not bad. Needs a little bit of work.
    Cool....did you re check your fuel pump assembly? Did you make sure no wires are crossed? You said you installed a high flow pump, right?

  17. #17
    Junior Member haulin c10's Avatar
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    Yes I checked and made everything was minus to ground and plus to the relay. I'm 58 psi. At the rail.

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    Hey I read your post and have same symptoms with 8-9 mpg and very low idle and no codes. Did you ever figure it out ? Thanks

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