Fuel mileage ??

Discussion in 'Chevy Suburban Forum (GMC Yukon XL)' started by Treadnmud, Jan 26, 2012.

  1. Treadnmud

    Treadnmud New Member

    I recently purchased a 2WD 99 C1500 Suburban with a 5.7L, 4L60e, and 3.73 rears. It has about 137,000 on it and it appears to be very well maintained. The previous owner told me that it pretty much gets 14 hwy or city. I figured it should have got a little better on the highway. Some friends of mine are claiming high as 19 in theirs.

    A recent family vacation from east Texas to the Florida panhandle proved the horribly consistent 14 mpg. I actually got as high as 14.2. While in Florida, it developed a very hard 1-2 shift and the CEL came on. When I got back to Texas, I hooked my scanner up and got a P1870 I think it was. Anyways, wound up putting a reman valve body in and now everything is shifting fine, torque converter locking up and it cruises 70 at 2000 rpms. I haven't got to drive it enough to determine the mpg's yet. About how bad does the torque converter not locking up affect the mpg?
  2. Dantheman96

    Dantheman96 New Member

    14 mpg is kind of low. I get 15mpg on the highway in my K1500 and about 12 in the city. I think you should be getting upwards of around 20mpg highway with your ride.
  3. murdog94

    murdog94 New Member

    Depending on what you had in it for a load, along with speed etc.. 15 is really all they get with the 5.7L they will get a bit more but most people average around that kind of milage, however the torque converter not locking is usually good for around 2mpg in the burbs.
  4. Treadnmud

    Treadnmud New Member

    I figured the tc lockup would have a pretty good impact on it. 2 mpg more? I'll take it!

    The idle is a little unstable so I think I'll check the plugs and give the injectors a good cleaning. See if that'll help any.
  5. koyote

    koyote New Member

    I can't tell you much, but I'm getting 14 in my '89 4wd 5.7 suburban, so I suspect yours is a bit low.

    Driving habits make a difference, of course, I probably average a good 1mpg plus extra in mine just because I barely touch the brakes (takes some long range attention!), accelerate nice and easy, and don't push 70mph.
  6. murdog94

    murdog94 New Member

    I would also take a VERY close look at the cap and rotor. the 5.7L vortec is known for issues with them. And 15-16 is about the best my buddy gets with his 2000 burb exact same model as yours (almost the same color lol)
  7. vncj96

    vncj96 New Member

    when chevy switched to the vortec 5.7 from the old tbi 5.7 the fuel economy took a hit, my parents had a 92 K1500 and we pulled 20mpg (70 MPH too)on a trip to texas (from MN)and carrying five people and their gear for a 2 week trip. I bought my 99 K1500 and have NEVER pulled more then 16mpg, and i have done everything I can to try and get better MPG. But for a 2wd you should be getting atleast 16 MPG on the hwy and about 13 in town
  8. Treadnmud

    Treadnmud New Member

    I'll do that. Thanks. Any recommendation on the brand or style of rotor? Such as brass or aluminum contacts?
  9. murdog94

    murdog94 New Member

    Nope since they both corrode quickly it is a minor defect with the Vortec motors.
  10. Treadnmud

    Treadnmud New Member

    Well, before the valve body job, this burb averaged a very consistent 14 mpg whether in city or highway across 40 gallons of 87 octane. I don't suppose running a better grade fuel would help in the long run.

    ---------- Post added at 10:42 AM ---------- Previous post was at 10:36 AM ----------

    I did check the plugs. I am guessing these are the IR plugs everyone is talking about. They are are AC brand, however, the contacts on the electrode appears to have a bunch of tiny bumps on it. Strange.... Never seen anything like this, but then again I'm a diesel mechanic! Ha!
  11. murdog94

    murdog94 New Member

    Nope 87 is going to give you the same milage. As to the plugs yeah the are actually most likely the platinums that are recommended for that motor, and the stuff on the tip is most likly carbon etc. Just gap them (to ensure they arent all worn out) and if they gap well and have a nice browned appearance then put them back in.. Another thing i did actually forget to recommend is a new fuel filter.. The injectors are not really serviceable in the 96-99 5.7L Vortec unless you want to pull the top of the motor off, and if you arent getting a misfire code then i doubt they will need it.
  12. phoebeisis

    phoebeisis New Member

    My 1998 2wd 1500 Suburban gets 21 mpg on long-1500 miles one way-all highway trips New Orleans>Flagstaff AZ
    I set the cc to 67 mph on the interstate- but probably actually average 62mph because of road work(45 mph) and various towns and cities-
    and the 60 mph speed limit on Hy 287 in TX.
    Get 12-14 mpg in pure city short trip driving.
    You should do at least 17 mpg at 65-70 mph
    I have the 3.42 rear end-probably helps FE a bit-the elevation on that trip also helps-less air drag
  13. Treadnmud

    Treadnmud New Member

    The fuel filter looks fairly new, definitely not the OE filter. I will probably try running the engine off a pressurized injector cleaning solution before I pull any injectors. If they are dirty, I will be able to tell after that. Haven't checked the fuel pressure yet, still looking for my fittings.

    The plugs didn't have the classic light brown look that I am used to seeing. I would say more black and appeared to possibly be running a tad rich. I don't think the o2 sensors have been changed and I haven't looked at the MAF sensor yet.

    What else I am I missing that I should check?
  14. murdog94

    murdog94 New Member

    I would recommend replacing the fuel filter (usually should be done every 20k miles) plugs i would go ahead and put a new set in when you do the cap and rotor. Make sure you check the PCV valve, and if you are feeling froggy i would pull the TB and clean it up with Throttle Body cleaner (Carb cleaner is not good for TBs) Get a can of maf cleaner and clean it up (never hurts) also air filter of course.
    And now you are talking about the injectors, i will say this. it is a central injector with poppets, with as i said requires pulling the upper intake off to service, and really there isnt much you can do to clean them up. As to the O2 sensors, if they arent kicking a code that means they are working in the proper range and dont need to be replaced.
    Also you only need a shrader vavle to check the fuel pressure (on the intake) you need to have at least 55PSI at idle, and the vortec motors you have to have 58 PSI for the motor to start. I would do a seafoam treatment before you change the plugs, also make sure you are running the corrent weight of oil and change it around 3-4K miles (on the 350 it seems to hurt the milage with dirty oil at least on the 8 of them that i have owned)
  15. Treadnmud

    Treadnmud New Member

    Sounds good. I have been out of he gasoline engine circuit for a while now. I guess things have changed a little. Lol. Running 5w30 I think in it and change it regular. Yeah, gonna give this thing a good cleaning and go from there.

    Came home to Mississippi for a funeral this weekend. The fuel mileage APPEARS to have improved after replacing the valve body.

    I have noticed that sometimes I have to crank the engine twice to get it to start. Could be low fuel pressure. No codes though.
  16. murdog94

    murdog94 New Member

    Check the Fuel pressure, but there is a chance that the Fuel pressure regulator is going down hill. But verify since it isnt fun to change.
  17. janikphoto

    janikphoto New Member

    Bad. The most efficient way of driving is with the converter locked. It is unlocked so the vehicle can stay in drive at a stoplight and not stall. However, the physical attributes of using fluid to transfer energy from the engine to the wheels is much more inefficient than having it hard-locked together. This is a dumbed down version of how an auto trans works, but it gets the basic point across.

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