First Time Poster, Age Old Question re: 454 engine!

Discussion in 'Chevy Suburban Forum (GMC Yukon XL)' started by ZR WON, Feb 13, 2007.

  1. ZR WON

    ZR WON New Member

    Hey all!

    New to the forum, have a question re: our 1988 454 Suburban. This is a CA truck with no rust, transplanted to Indiana about 2 years ago.

    The truck is in very good shape, pulls anything we've thrown at her, but my GOD, the gas mileage is awful. I'm not talking 10mpg bad, I'm talking 5mpg!!!! This mileage was verified last weekend on a 400 mile trip to Chicago and back, pulling a 24' Haulmark enclosed trailer 65-70mph. I know of people who get 8mpg with a 454 in a 30' motorhome running the same speed!!!:eek:

    A little more on the vehicle: 180K miles, new Mark V 454 crate engine installed at 130K. 4.10 gears, TH400 trans(no OD), Hummer H2 wheels 265/65/16 Goodyear Fortera tires and I'd say a 2" body lift. Cruise RPM at 65 is 3100rpm, which is right at the engines power peak.

    I've heard about all kinds of people getting 12mpg pulling with 454s and all that stuff, but I don't necessarily buy it. I can't believe however, that this engine is so inefficient that it only gets 5mpg pulling. Surely there is a happy medium here!!! I can't imagine the fuel injectors could possible spray that much fuel in the damned motor!!!lol!! I think that 8mpg would be very reasonable pulling a 24' trailer with a 3300lb car. That would put the cruise range at pretty close to 300 miles(vs. the 200 mile range currently), which is pretty reasonable.

    Are there any things that might be contributing to the seemingly excessive consumption, or given the stats, is this about all I can hope for mileage-wise?

    As an aside, the truck does have an issue when it's cold outside, it runs very rough and rich, but based on the recommendations from a mechanic, I ran a 't' fitting to the MAP sensor to an additional vacuum source and that seems to have diminished the time it runs rough.

    Any and all ideas would be appreciated!

  2. 84fiero123

    84fiero123 Epic Member 5+ Years 1000 Posts Platinum Contributor


    When you are towing a trailer you have not only the bad aerodynamics of the truck but also the trailer to cope with.

    5 mpg does sound a little low, but then when was the last time it had a full tune up?

    Wires, plugs, rotor button, cap, oil change (fuel efficient oil), transfer oil change, both if you have 4 wheel drive.
  3. TrailLeadr

    TrailLeadr Epic Member 5+ Years 1000 Posts

    It actually doesn't seem out of line for that vehicle to be getting 5mpg, based on the info you provided.:eek::eek: Mostly because the TH400 tranny's gearing has you running so high in your cruising gear. If you had OD, you would be running about 2300rpm at 65mph, instead of the 3100rpm that you currently run. Since you don't have OD, you could change out the rear end from 4.10 to 3.73, and you'd see an increase in mpg. The truck wouldn't pull with as much confidence, but if you're not looking to pull 10,000lbs you won't notice.
  4. 67Cowboy

    67Cowboy New Member

    Pulling Life Style

    Well after 30 years of hauiling livestock and hot rods I can honestly say the options are limited on pump gas. The best I ever had was a 396/400th with 3.73 in a 1 ton dooley that got 15mpg (empty). A good fuel induction system with a lean idle circuit and a clean filter helps a lot, also I use full synthetic oil. Also the right temperature range spark plugs with the proper tip and cooling system thermostat (i also use a water wetter). I also love big tube headers with flow through mufflers. Also I've actually gotten more miles per gallon and power by running 91+ octane. Proper air pressure in the tires is another factor. If you do a google or dogpile search on fuel milage you will be supprised, I recently found something called Aqua-tune* that I'm looking into and another thing called Geet* and alcohol injection and pcv-jar.
  5. tbplus10

    tbplus10 Epic Member Staff Member 5+ Years 5000 Posts Platinum Contributor

    I'd recomend a gear change to 3.73's, dump the lift if you dont need it, and switch to an overdrive transmission if the budget can take it.
  6. Cableguy

    Cableguy Epic Member 5+ Years ROTM Winner 1000 Posts

    I'm feeling the same thing as Tim 4:10's are a bit aggressive and the lack of Overdrive are not helping your case. Other than a big tune up and some aftermarket goodies for big block breathing i.e. air intake,exhaust etc.
    BTW welcome Brian. :biggrin:

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