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Fuel economy question....again...

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  • #1

    Fuel economy question....again...

    What in the world!! I just stopped to fill up w/ gas, and oh my gosh, I was only averaging 9.76 mpg!! Something has to be totally screwed up, but the car is not showing any obvious symptoms of a fuel problem. I know recently "Injuneer" said the fuel injectors might be a problem, but I haven't checked yet. The car doesn't act like injectors are a problem, actually since the new tranny it has been nice and smooth. The new tranny is a "beefier" one actually w/ steel components and a shift kit now, but I don't think? that would have anything to do with fuel consumption. Is there anything else someone can think of that is an obviuos place to at least start to find a problem? Should I get a fuel system cleaning? I just don't know!
    1996 White T/A, LT-1 auto with only aftermarket Flowmasters, shift kit, and a LG Motorsports G2 cold air kit.....that's it! She can't eat all the 'Stangs but will die trying.

    Currently on the clock: 120K
    Tags: None
  • #2
    Did you notice a significant reduction in mileage per tank or did you when you did you calculations? Just curious...

    When was the last time you changed your filter? Or even popped a thing of fuel injector cleaner...any SES lights?

    Christopher Teng

    1999 · A4 · 3.73's · Auburn LSD · Whisper Lid · K&N · Pacesetter Headers/Y-pipe
    Magnaflow Cat & Catback · MSD Coils/Wires · Bosch +4 Plugs · EGR Bypass
    B&M SuperCooler · 160* Stat · Descreened MAF · SLP CAI · BMR STB & SFC
    Strano Sways · Eibach Springs · Bilstein HD Shocks · Hawk-Pads · Brembo Blanks
    Speedlines · Nitto 555s · Texas Speed Mail Tune

    Lots of Weight Savings · Stubby Antenna · Corbeau TRS · Zaino · 273K

    F-Body Dirty Dozen

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    • #3
      Originally posted by Christopherrr
      Did you notice a significant reduction in mileage per tank or did you when you did you calculations? Just curious...

      When was the last time you changed your filter? Or even popped a thing of fuel injector cleaner...any SES lights?
      My fuel mileage in general has been dropping significantly over the past few months. Last time I wrote about it, I was averaging 14mpg, which was also a drop at that time. No SES lights, and I actually put in a bottle of Prestone (?) injector cleaner last week. Changed the air filter this past summer. My only other thought about this is when the car was being serviced for the VATS problem, they had the computer out and somethin' got messed up then possibly??
      1996 White T/A, LT-1 auto with only aftermarket Flowmasters, shift kit, and a LG Motorsports G2 cold air kit.....that's it! She can't eat all the 'Stangs but will die trying.

      Currently on the clock: 120K

      Comment

      • #4
        do you notice any smoke out the exhaust or any unusual smells from the exhaust. i jwould imagine that if your fuel economy dropped that durastically you would notice a miss or sputter if snapped throttled. i averaged around 9 mpg city driving my car really hard with a 8 psi blower something fairly significant must be wrong. do you know if the odometer is functioning properly it's a stretch but worth a a shot.
        '95 T/A Conv., Green, Tan Top
        '00 T/A Silver
        '86 Chevy RCSB 4WD (LS1 to go in)

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        • #5
          Three things that can contribute to poor fuel mileage:

          -exhaust leaks before the O2 sensors
          -misfires
          -faulty O2 sensors

          All of these can cause the computer to think the engine is running lean, when it really isn't, causing it to add extra fuel that is not required.

          Does your area require oxygenated fuels in cold months? That can cause a drop in fuel mileage.

          Have you checked for leaks in the fuel system?
          Fred

          381ci all-forged stroker - 10.8:1 - CNC LT4 heads/intake - CC solid roller - MoTeC engine management - 8 LS1 coils - 58mm TB - 78# injectors - 300-shot dry nitrous - TH400 - Gear Vendor O/D - Strange 12-bolt - 4.11's - AS&M headers - duals - Corbeau seat - AutoMeter gauges - roll bar - Spohn suspension - QA1 shocks - a few other odds 'n ends. 800HP/800lb-ft at the flywheel, on a 300-shot. 11.5 @ 117MPH straight motor

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          • #6
            I noticed you have a Cold Air kit on your car. When was it put on? If you had to take off the air intake ducting than it can be difficult to fit the air tube back on to the engine without a slight air leak. I had the same problem with my '96. The tube wasn't properly sealed onto the throttle body, air got in after the MAF, the computer read a false lean condition and added more fuel in and milage went down.
            Joe K.
            '11 BMW 328i
            '10 Matrix S AWD
            Previously: '89 Plymouth Sundance Turbo, '98 Camaro V6, '96 Camaro Z28, '99 Camaro Z28, '04 Grand Prix GTP

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            • #7
              Air bypassing the MAF sensor creates a "true" lean condition, and the corrections made by the PCM should not cause significant changes fuel economy, since they are correcting an actual lean condition.
              Fred

              381ci all-forged stroker - 10.8:1 - CNC LT4 heads/intake - CC solid roller - MoTeC engine management - 8 LS1 coils - 58mm TB - 78# injectors - 300-shot dry nitrous - TH400 - Gear Vendor O/D - Strange 12-bolt - 4.11's - AS&M headers - duals - Corbeau seat - AutoMeter gauges - roll bar - Spohn suspension - QA1 shocks - a few other odds 'n ends. 800HP/800lb-ft at the flywheel, on a 300-shot. 11.5 @ 117MPH straight motor

              Comment

              • #8
                I just don't know. If this car doesn't throw a SES light, I'll probably have a hard time finding the prioblem. Driving it around today, it's just as smooth as a baby's butt , no abnormal smells except every now and then a fuel odor, but I usually smell that anyways in the colder weather. I don't think (?) there is any exhaust leak because I don't hear any louder noise, but maybe that don't mean anything. Our 300M has an exhaust leak that you can hear real well when starting it up and giving it a little gas. I am going to take it into the shop this week for a "check-up" on the tranny, and I will ask the guys there about it. ONLY other thing I can think of is where I am getting my gas now is poor quality, and I only run 93 in it. Who knows!!
                1996 White T/A, LT-1 auto with only aftermarket Flowmasters, shift kit, and a LG Motorsports G2 cold air kit.....that's it! She can't eat all the 'Stangs but will die trying.

                Currently on the clock: 120K

                Comment

                • #9
                  Is it possible that someone could be syphoning your gas? Or is that not possible on an f-body?
                  2006 Saturn Ion Redline
                  2003 Mits. Eclipse Spyder

                  Comment

                  • #10
                    Its possible to siphon fuel from a 4th Gen F-Body

                    Check the coolant temp sensor as well. As temperatures drop, low coolant temp at cold start causes significant mixture enrichment... just like the choke on an old catb'd engine. If the coolant temp sensor isn't showing the correct temperature, and reading a bit lower than actual temperature, that could reduce fuel mileage.

                    When were the O2 sensors changed?

                    An exhaust leak can sound like a small "tick" when you accelerate. Won't necessarily be that noisey.
                    Fred

                    381ci all-forged stroker - 10.8:1 - CNC LT4 heads/intake - CC solid roller - MoTeC engine management - 8 LS1 coils - 58mm TB - 78# injectors - 300-shot dry nitrous - TH400 - Gear Vendor O/D - Strange 12-bolt - 4.11's - AS&M headers - duals - Corbeau seat - AutoMeter gauges - roll bar - Spohn suspension - QA1 shocks - a few other odds 'n ends. 800HP/800lb-ft at the flywheel, on a 300-shot. 11.5 @ 117MPH straight motor

                    Comment

                    • #11
                      Originally posted by Injuneer
                      Its possible to siphon fuel from a 4th Gen F-Body

                      Check the coolant temp sensor as well. As temperatures drop, low coolant temp at cold start causes significant mixture enrichment... just like the choke on an old catb'd engine. If the coolant temp sensor isn't showing the correct temperature, and reading a bit lower than actual temperature, that could reduce fuel mileage.

                      When were the O2 sensors changed?

                      An exhaust leak can sound like a small "tick" when you accelerate. Won't necessarily be that noisey.
                      Thanks for the siphon info: I'll be sure to use it (hee, hee)
                      Anywho, I've never had to replace the 02 sensors..yet. I just assumed if they were faulty, they would trip the SES light. But what do I know, I literally learn somethin' new about my car everyday! I'll try to listen for anything else, especially a "ticking" sound. Haven't heard that yet though.
                      1996 White T/A, LT-1 auto with only aftermarket Flowmasters, shift kit, and a LG Motorsports G2 cold air kit.....that's it! She can't eat all the 'Stangs but will die trying.

                      Currently on the clock: 120K

                      Comment

                      • #12
                        Well, silly as this may be, but I have whole-heartily come to the conclusion that someone probably siphoned (sp?) gas out of the tank. Since the new tranny and torque converter, and 3 tank fill-ups after my poor mileage thread, I have been averaging 18mpg+ around the city!! I do believe I was losing gas mileage right before the tranny/torque conv. went, but all is better than new now. I'm getting better gas mileage now then I've ever had before. I just love happy endings
                        1996 White T/A, LT-1 auto with only aftermarket Flowmasters, shift kit, and a LG Motorsports G2 cold air kit.....that's it! She can't eat all the 'Stangs but will die trying.

                        Currently on the clock: 120K

                        Comment

                        • #13
                          Originally posted by 'STANGETR
                          Well, silly as this may be, but I have whole-heartily come to the conclusion that someone probably siphoned (sp?) gas out of the tank. Since the new tranny and torque converter, and 3 tank fill-ups after my poor mileage thread, I have been averaging 18mpg+ around the city!! I do believe I was losing gas mileage right before the tranny/torque conv. went, but all is better than new now. I'm getting better gas mileage now then I've ever had before. I just love happy endings
                          you should buy a locking gas cap then.

                          edit: or at least thats what i would do
                          2006 Saturn Ion Redline
                          2003 Mits. Eclipse Spyder

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