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Low MPG on Chevy Tahoe

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  • #31
    Ken is quite correct in his statement that there would be no possible scientific way to prove any true mileage claim one way or the other, the exception being possibly a dyno and a wind tunnel. No parameter can be identical at all times.

    However, an average study CAN be taken over a period of time, with enough drive cycles over the same(ish) conditions. This is usually sufficient to claim fuel mileages in this type of environment.

    I travel the same roads every day to and from work, about 53 miles each way. I have been doing so for four years now, winter, spring, summer, and fall. I check gas mileage with every tank. Yes, I hit one of three stoplights at any given time, and road conditions, along with my speeds, certainly change from one day to the next. I can tell you with pretty good authority that my car gets 33 miles to the gallon. That is the figure I have derived over a very long time.

    So, for a given week or two, if my mileage drops considerably, I start looking for a problem under the hood/ drivetrain. Or I look at gasoline brands (I do not fill up at the same place all the time). I usually find some little problem that is causing the differential.

    If I get significantly better mileage (doesn't happen), I would just be happy for that week!


    • #32
      Exactly garagemark,
      I do not doubt for a minute that the claims made are legitimate as checked, only that it would be all but impossible for the average driver to substantiate any claim that pulling the little camper give any regular and expected improvement over not pulling.

      Personally, I don't buy it but that is not particularly scientific either


      • #33
        My 98 5.7 1/2 ton 4X4 GMC Suburban with 170,000 miles with gets about 13 mpg on the roads back and forth to town. Maybe 17 on the freeway on cruise control. Less with 5% ethanol.
        Saltmine: I did not know that GMC (or chevy) made a 1 ton suburban. I need one.
        Don't forget leveling the vehicle the same way each time you refuel; just to get rid of one more variable.
        Wish I could afford a new 3/4 ton 4x4 suburban. Great people hauler, stuff bringer home, and trailer puller. I have had this vehicle for 4 years
        I have always been a ford man, but this GMC has been super.
        John Burchett
        in Byng OK


        • #34
          Originally posted by garagemark
          Some of you guys are flat out wrong. A 600 pound trailer is almost nothing to the Chevy, and the pressure differential can indeed raise his gas mileage a tad. A pickup truck gets better mileage with the tailgate UP, not down. The roll of negative pressure created behind the cab with the gate up is better than a smooth airflow across the bed with it down. This WAS done on Mythbusters several years ago.

          The leaking injector is a real possibility. Pooled fuel in a cylinder would cause such a backfire reaction, and could be more pronounced in cooler weather.

          If you don't have an OBD II code reader, get one. Beg, borrow, or steal one. Get one that has real time drive cycle information. Is your vehicle throttle body INJECTED, or throttle body air flow only with individual injectors in the manifold? If the latter, you can tell with the OBD reader which cylinder bank is using more fuel and/or compensating the other bank. It won't tell you which injector, but it can tell you left or right bank.

          Check for vacuum leaks. Use a can of brake cleaner or carburettor cleaner and spray it around fittings, intake, and lines. If you notice a quick drop in RPM, you have found a vacuum leak. Fix it.

          Note: Make sure to do tests with the engine hot and the ECM loop is closed. The OBD reader will tell you if it is or isn't.

          I believe your engine is designed for 210F water temp. 160 may not close the loop, or may cycle the ECM back and forth.

          There is more, but that's a starting point. I just did all these tests on my 6.0? Chevy truck. Ended up as a small vacuum leak causing my issue.

          If you are going to work on cars (1996 and up) the OBD II is simply a must. It can keep you from chasing your tail, check engine light or not.

          I have been using Auto Zone code reader. I can check the codes at Auto Zone and also online at GMC/CHEVY web site. Code was saying I had an air leak I checked all the vacuum hoses and all the air intake stuff under the hood. The air intake seak had to be fixed and a new gas cap fixed the problem. You are correct about the temperature guage it shows 100.....210.....260. When I start the cold engine the needle is down below 100. After a engine runs a few minutes the needle comes up to 160 then suddenly drops to 150 when the NEW 195 thermostate opens and the water starts circulating through the radiator. The temperature guage shows the engine runs at 150 degrees F all the time. Even though the temperature guage reads 150 if I remove the radiator cap the water boils over. The laws of physics never lie water boils at 212 degrees F elevation here is about 650 ft. The guage on the dash is reading wrong but as long at the computer knows the correct temperature that is OK I replaced the temperature sensor to make sure. With the 6 new parts and repairs the Auto Zone code reader says PASS. Gas mileage has come up from 9 to 17.2 on the highway that is much better but still not what I want. We are having warm weather now so I am not noticing any back fire when I start the engine. Auto Zone and online information both show the 5.7L engine on my SUV has 8 injectors. I can raise the hood and see the throttle body but there is NO fuel rail so the injectors must be inside the intake manafold or thorttle body. Auto Zone has a fuel test kit on tool loan I am planning to use it to test the fuel system pressure. If a fuel injector is leaking the fuel system will not hold pressure after the engine is turned off. If the system is leaking it will not tell me which injector is bad. I was told by a mechanic at a local auto repair shop they can hook my vehicle to a diagnostic machine and find out if the engine is running Rich or Lean. If it is running rich they have the option to make it run Leaner up to a certain amount red line area. I am taking it one step at a time. I still may be able to get my gas mileage up a few more mpg. This engine is rated 255 hp at 4600 RPMs if I loose 20 hp to better gas mileage I will never miss that 20 hp. RPMs are about 1800 crusing at 70 mph on the interstate.
          Last edited by gary350; 04-04-2011, 05:37 PM.


          • #35
            If you still have problems with a vacuum leak, make sure that you check the vac-advance diaphragm.


            • #36
              Gary350, the reason you don't see a fuel rail or exposed injectors is because you have an "R" code engine. The "R" engine is a 5.7L V-8 with sequential port fuel injectors. The fuel is delivered into the center of the fuel "spider" where the fuel pressure regulator lives and is distributed out to each individual injector through plastic tubes. Early "R" engines had a single centeral injector and small poppet valves on each individual injector, hence the need for a very high fuel delivery pressure. GM soon discovered that the poppet valves were prone to sticking both open or closed, and they also had a tendency to dribble fuel....which led to all kinds of fuel delivery problems and rough running. To start with, GM issued a set of instructions on how to properly clean the injectors, but the cleaning didn't last. So, as a last resort, they produced an "injector upgrade kit" that replaced the "spider", tubes, and pressure regulator with a set of tiny "Multech" type injectors, a wiring harness and new support assembly for the revised "spider". This kit was made available for both the 5.7L V-8 and the 4.3L V-6 engines to eliminate injector problems. The average Mechanic can install one in about an hour.
              I've installed quite a few, on both the V-6 and the V-8's and the transition is dramatic. To make it even more attractive, the price of the kit and gasket is actually less money than if you went to NAPA and bought, new, stock injectors, tubes, pressure regulator and gasket. I'll look in my files and see if I still have the GM part number for the kit...

              BTW, Bob, GM hasn't used a vacuum advance on any of their engines since the early '80's.
              Last edited by saltmine; 04-05-2011, 02:10 AM.
              No good deed goes unpunished.


              • #37
                As luck would have it, Gary350, I found the part numbers for the V-6. So, I went online and located both the V-8 kit numbers and comprehensive instructions for installing it. Here are the part numbers:

                V-8 MFI Assembly

                V-8 Seal Kit

                V-8 Bracket

                You should be able to handle the installation, but if you need help, let me know, and I'll get you that link.
                No good deed goes unpunished.


                • #38
                  Fuel Injector

                  Hi saltmine-
                  I have a '96 S10 Blazer with a 4.3 Vortec V6. Would this improvement be valid for that model year?


                  • #39
                    Yes, Toolguy, as long as the truck is a '96 or newer. BIG improvement. A 4.3L engine uses a different part number for the kit, and the engine code is a "W" instead of "R"

                    Here are the GM part numbers for the 4.3L:

                    V-6 MFI Assembly

                    V-6 Bracket

                    V-6 Seal Kit

                    Like I told Gary, I have the complete GM Service Bulletin and instructions saved as a "Microsoft Word" document, if you need it. I can also convert the document into a PDF format.

                    Here's a quote from the Bulletin you might find interesting:

                    Some customers may comment on rough idle after start-up, especially if the vehicle has sat overnight. These symptoms may be intermittent. The Service Engine Soon (SES) light may also be illuminated. Current misfire or history misfire codes may be detected with the Tech 2 scan tool.

                    A deposit build-up on the CSFI poppet valve ball and/or seat may cause the poppet ball to stick open or closed. In either case, the specific cylinder will be mis-fueled, resulting in a cylinder mis-fire condition."
                    No good deed goes unpunished.


                    • #40

                      Thanks a lot saltmine! I really appreciate that! I didn't think to ask at the time - do you have a ballpark idea of how much these parts cost? Tens, hundreds, thousands? Yes - I could use the service bulletin and instructions as a PDF. [email protected]
                      Thanks again.
                      Last edited by Toolguy; 04-05-2011, 12:11 AM.


                      • #41
                        My brother in law had one of those GM spiders take a dump on his 5.7L Suburban. The dealer wanted $600 for the part. He bought the aftermarket stainless kit for $350(I think it was Edelbrock).

                        I was delivering to Delphi, asked the receiver what they made there. He said, "the fuel spiders for GM 5.7L. I told him what we had done. He told me to wait a minute, left, came back and handed me a Walmart bag. He said he looked at the cost analasys of that part. It cost them $68.00 to make the entire setup-right here in the good ole USA. I told him I couldn't take it, he said,"[email protected]$*Sh#% they shouldn't tell you to keep your GM vehicle GM, then make it so you can't afford to.

                        It's funny how the dealer could justify almost 900% markup on a part that only has one middleman(the distribution center)

                        gary350, if you find the spider to be defective, and I can find it in the shed, let me know. You can have for the shipping. If you want the Walmart bag it came in, that will cost you $599 extra-it's one of the old white ones you can't see


                        • #42
                          Well, here's the shocker. The whole injector upgrade kit, from GM, retails for $368.50 (MSRP).
                          You can buy the same, identical kit from "GM PartsDirect" (online) for $259.08. I imagine there's tax and shipping costs involved, but it's nowhere near the $600 the dealer quoted. The entire job only pays 1.4 hours labor, and even at $100 an hour....well.....Somebody is getting tubesteaked...

                          The V-6 injector kit is a bit less: $351.02 (MSRP) and $179.43 from "GM PartsDirect". Again, the labor is 1.4 hours for the installation.

                          The last kit I personally bought was about 8 years ago, for a 4.3L V-6 and at the time it sold over-the-counter for $150 at a GM dealer in Phoenix.
                          I installed the kit in 45 minutes, myself.

                          Here's the GM partsDirect URL:

                          BTW, the Edelbrock kit is the original, stock CMFI spider, or the marine stainless steel spider(with poppet valves). GM (Delphi) is the only one who makes the factory drop-in upgrade kit.

                          On an additional note, Lazarus, when I worked at a GM dealer out on the "Left Coast", it was common practice for dealer parts departments to "double-net" all of the parts they sold. Basically, they would look at what they had to pay for a part....and double it.
                          Last edited by saltmine; 04-05-2011, 02:35 AM.
                          No good deed goes unpunished.


                          • #43
                            "saltmine" On an additional note, Lazarus, when I worked at a GM dealer out on the "Left Coast", it was common practice for dealer parts departments to "double-net" all of the parts they sold. Basically, they would look at what they had to pay for a part....and double it.

                            Yeah, go to a city like Virginia Beach and let them see a military base permit on the front bumper. My guess is they think members of the armed forces make as much money as government contractors.

                            Hasn't been any problems with the Edelbrock system. Do you know if the upgrade will work with it, should he ever have any issues?


                            • #44
                              Saltmine are these parts for the Chevy Tahoe?

                              Do I get these at Auto Zone or the dealer?

                              Wonder how much they cost?

                              I SOLD the GMC it was having transmission problems. I wish I had it back it was getting much better mileage than the Chevy Tahoe.

                              V-8 MFI Assembly

                              V-8 Seal Kit

                              V-8 Bracket

                              I use to work for a company where we made parts for Ford, Chrysler and GM. The parts cost us from 40 cents to $2 to make depending on which part. I checked with the dealer just to see how much the parts were selling for prices were $198 up to about $300. That should be illegal they know you have to have it so what can you do but pay the price.
                              Last edited by gary350; 04-05-2011, 09:56 AM.


                              • #45
                                I've never worked with the Edelbrock system, but Edelbrock usually stands behind their products pretty well. If they don't perform properly, Edelbrock won't put his name on them.

                                All of the part numbers I listed were GM part numbers, Gary. If you can find a decent auto parts store, they should be able to cross the part numbers over to their product line. But, I wouldn't recommend it. The parts listed are for a V-8 Tahoe.
                                When I needed a spider assembly and injectors for a 2000 Blazer the County owned, I called NAPA auto parts (our usual supplier) and was told the stock replacement parts (not upgraded) were going to be roughly 30% more than the GM kit, and would leave the Blazer with the same potential problem it already had. I went with the kit, and got a "cookie" from my boss for saving the County money...
                                Another thing I forgot to mention earlier was the fact that the fuel pump was starting to lose pressure, too. But the "Multech" type injectors and regulator didn't care, they perform perfectly all the way down to 30 PSI without any issues.
                                That's how you determine which injectors are "the good ones". "Multech" injectors are electronic, and have both a fuel line and an electrical harness leading out to each injector. The stock, troublesome poppet valve injectors just have a plastic line to each one.

                                Yeah, it should be illegal. But what can one do. You get what you pay for.
                                Last edited by saltmine; 04-05-2011, 11:57 AM.
                                No good deed goes unpunished.