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Low MPG on Chevy Tahoe

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  • Low MPG on Chevy Tahoe

    There is NO activity on the GMC Yukon Chevy Tahoe forum. I signed up months ago and there has been no replies and no new threads in months.

    I use to own a 1997 GMC Yukon I ordered it from the factory with a fuel economy chip. It was getting 18.5 mpg in town and 24 on the highway when I sold it with 200,000. miles on it.

    Now I own a 1999 Chevy Tahoe. This is identical to the GMC Yukon only difference is I get poor gas mileage it probably has NO fuel economy chip. I bought it used with 95,000. miles on it. It was getting about 17 to 18 mph when I first bought it then the mpg went down down down. The service engine light came on and Auto Zone pulled code on the computer and it said I have an air leak.

    Gas mileage was down to 9 mph. A new gas cap bought the gas mileage up to 11 mph. I disconnected the battery cable over night and put it on again next morning and the service engine light went off. The heater has not getting very hot in the winter and the thermostate guage reads 120 degrees. I replaced the old thermostate with a new 195 degree thermostate and gas mileage came up to 14 mpg the dash guage was reading 140 degrees. Next I replaced the temperature sensor that tells the computer how hot the engine is the dash guage now reads 160 degrees and gas mileage came up to 15.1 mpg in town and 17.2 on the highway. Next I put in a new high CFM air filter no change in the mpg. New sparks plugs did not change MPG either. The engine now has 108,000. miles on it.

    Engine back fires when I start it. I park in the garage all night next morning it back fires. I stop at the grocery store or a restaurant for 45 minutes engine back fires when I start it. Back fire is small now but in colder weather the back fire was louder.

    What else can I do to get my mpg up much higher???

    I would love to have 24 mph like the GMC had.
    Last edited by gary350; 04-02-2011, 10:40 PM.

  • #2
    You checked all the vacuum lines, especially to the PC Valve, have you replaced that?

    A hundred thousand miles the plug wires are due, oxygen sensor/s, things are so interconnected that so many single things effect other components.

    Curious what engine is it 4.3?

    I have a 3.4 that suddenly dropped in MPG, bad injector not closing.


    • #3
      Originally posted by Ken_Shea
      You checked all the vacuum lines, especially to the PC Valve, have you replaced that?

      A hundred thousand miles the plug wires are due, oxygen sensor/s, things are so interconnected that so many single things effect other components.

      Curious what engine is it 4.3?

      I have a 3.4 that suddenly dropped in MPG, bad injector not closing.
      The tag under the hood says 5.7 liter.

      How does a person find a bad fuel injector?

      I have been out of work 5 years and have no money. The dealer wants $85 and hour and no quanantee they can fix it in an 8 hour shift. That would be $680 plus sales tax I have no money to pay that. I need to do the work myself. I checked the plug wires in the dark there is no arcing to the engine or other metal parts and the engine runs smooth.


      • #4
        You likely can get away with out the wires, the oxygen sensor can effect mileage quite a bit and a stuck PCV can cause issues, they are cheap, I always used AC PCV's in GM when I was in the auto repair business.

        The engine codes actually pointed to the bad injector.
        The mileage went from 26-28 highway to I kid you not 4-5mpg.

        5.7 is a V8, it's not likely you will be getting any where near 24mpg.

        Presume you are practicing good driving habits, tires a couple pounds over recommended, no long warm ups etc

        Another thing and I know it's hard to swallow when you are out of work but I have had good success in using the premium fuel, MPG increases paid for the difference and it has fuel conditioners in it that help keep a cleaner engine.


        • #5
          Originally posted by Ken_Shea
          5.7 is a V8, it's not likely you will be getting any where near 24mpg.

          Heh, I'd be amazed if I could verify the 17mpg he's currently claiming, 24mpg is just BS.

          All the "economy" chips in the world will not change the fact that you are pushing a land yacht down the road. A vehicle of this size requires a certain amount of energy to move simply due to the laws of physics, all the wishing and hoping in the world can't change that number.


          • #6
            Well, that's pretty blunt, unfortunately also pretty factual.


            • #7
              Ethanol in the gas your buying?

              When I get stuck with it my milage drops 20% or better.
              I just need one more tool,just one!


              • #8
                Why not swap it for a smaller car, do you need a 6l V8?
                In this part of the world not so many people drive V8s , gas is $2.20 per litre about $9.00 for a US gallon. Alot of 1 and 2 litre cars on the road.
                My neighbours diary says I have boundary issues


                • #9
                  I've had several 5.7 GMs and 18/20 MPG is as good as it gets, if your using ethanol it gets worse. Wide fat tires will kill you or at least your mileage. IF you can't stand those numbers get a rice burner.

                  I would rather have tools that I never use, than not have a tool I need.
                  Oregon Coast


                  • #10
                    Ethanol has been driving the gas mileage seekers crazy for a few years now.
                    What's funny is listening to people trying to figure out why their mileage is in the toilet.

                    Ethanol has a remarkably low energy content when compared to gasoline.

                    Gasoline burns at an ideal 14.7 : 1 ratio. Ethanol (by itself) burns at 7 : 1 for the same power.
                    Almost half.(twice as much fuel for a given amount of air.)
                    Adding ethanol to gasoline can easily drop your fuel economy drastically.

                    Example: A 2009 Ford Taurus, with average equipment and care usually gets about 22-24MPG (on the highway). If that same Taurus is a "flex fuel" car, and you can find ethanol, without gas in it, the gas mileage drops to 12-13 MPG (highway). Using Gasohol (10% ethanol by volume), the same car will deliver 16-18MPG. Assuming your Tahoe was getting 18MPG, it probably never will again as long as the Federal Government forces oil companies to add ethanol to their fuels.
                    Which I find amusing. It actually costs more to process corn into ethanol than it does to refine crude oil into gasoline.

                    On a serious note: some things can be done to improve your fuel economy. Properly inflated tires is a good place to start (if you're driving around on 24" wheels with skinny "rubber band" tires a foot wide, forget it)
                    Thermostat. The hotter an engine runs, the more efficient it is. If your temperature gage is laying around on "COLD" most of the time, you're wasting gas. Same goes for coolant sensors for your computer. If the computer thinks the engine is cold, it will richen the mixture to compensate. Oxygen sensors. An old or "lazy" oxygen sensor can waste plenty of gas...they fail reporting "LEAN" to the computer (except on Jeeps)...Vacuum leaks: More oxygen sensors, injectors, and catalytic converters have been replaced because of vacuum leaks than you'd ever believe. Lift kits, luggage racks, piles of junk in the back also cost fuel economy. 100lbs of useless junk can drop gas mileage 1%...think about it.
                    The correct grade and viscosity of oil helps. I flushed the transmission in my car and replaced the fluid with full synthetic ATF. The gas mileage increased by 1 MPG.
                    Get your gas at one place. You never know if the owner is "doping" the storage tanks with extra ethanol...We caught a guy locally. The Department of Weights & Measures tested his storage tanks and found he had 65% ethanol in them..Yes, he was fined.

                    On the subject of injectors: I worked on cars and trucks professionally for over 45 years, and only recall replacing 5 injectors the whole time. Most injector replacements are from mis-diagnosis. Don't believe me? Go to your nearby wrecking yard, and see how many injectors are missing off of the vehicles there...
                    Last edited by saltmine; 04-03-2011, 03:43 AM.
                    No good deed goes unpunished.


                    • #11
                      When it "backfires" on start up , is it in the exhaust, or in the intake ?


                      • #12
                        Perhaps his 24mpg ..was showing on the computer at a set speed ..which is possible ..

                        mpg tests should only be done brimming the tank ...doing a set mileage ..then brimming it again and noting the amount put in ..and doing some calculations.

                        ALL THE BEST.MARK


                        • #13
                          Originally posted by Uncle O
                          When it "backfires" on start up , is it in the exhaust, or in the intake ?
                          I really can not tell for sure. I can hear a small back fire sound but I never see any smoke. It does not do it all the time and now that I have changed a lot of parts and the weather is warmer it is not doing it as much as it was. The weather is warmer now than it has been every time we have some cold weather move through the engine starts back firing on start up again. The problem seems to have something to do with cold temperatures.

                          I have tire pressure at 32 psi.

                          Gasoline around here is 10% ethanol.

                          I was reading online about different types of fuel and how much power each one produces. Diesel fuel produces more power than gasoline and kerosene produces more power than diesel. I did an experement and put 1 pint of kerosene in a full tank of gas. Gas mileage went up 1/2 mpg and the engine starts and runs the same as before. That was my first experement so I don't put much faith in just 1 experement I need to do more experementing. I hope the kerosene will cancel out the low mpg effects of ethanol.

                          I turn off the engine every time I stop at a RED traffic light. I timed the traffic lights in town, some RED lights are 45 seconds and some are 3 minutes. This is saving me a LOT of fuel. I am getting about 4 extra days of driving on a tank of gas. Instead of filling up every 8 days I now fill up on day 11 or 12. There are 18 traffic lights between my house and Lowe's and Home Depot. Average time parked at those 18 traffic lights on a 1 way trip to Lowe's is about 15 minutes, round trip is about 30 minutes. How much fuel does an engine burn if the vehicle is parked and the engine idles for 30 minutes??? If I drive around town and stop at the same number of traffic lights every day for a whole week that is equal to having the vehicle parked with the engine running for 3 1/2 hours. How much fuel would that waste???
                          Last edited by gary350; 04-03-2011, 07:22 AM.


                          • #14
                            You own a re-badged Chevy Suburban and bought it expecting to get "good" mileage? Suburban owners buy them for their size, capacity and towing ability, do you really need it? 100k on platinum plugs and premium wires is about average, time for a replacement. Downstream O2 sensor usually fails first. Best to replace them both, simple wrench work. Diagnostic codes rarely identify specific components correctly, usually points to a faults at a system level.


                            • #15
                              The first and cheapest thing to do is to start replacing every single piece of vacuum hose. It's old enough and gets hot enough under that hood that it's all pretty porous by now.