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Thread: Fuel Mileage Poor - Bone Stock SC - Suggestions

  1. #1
    Join Date
    Jul 2002
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    Roanoke, Virginia
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    Fuel Mileage Poor - Bone Stock SC - Suggestions

    My daily driver 89 SC w/ 218K miles has poor fuel mileage..I average 13 miles per gallon which isn't ideal. Given the car owes me nothing, and since it is cheap to insure I accept it. Just looking for some direction as to what I should consider replacing to improve the mileage. My usual commute is mixed highway and city - 20 miles either way per day.

    Also for what it is worth, the fuel mileage took a dive after the head gaskets blew at 200K. I deleted EGR at this time, the EGR sensors were hooked back up and capped off (read/heard this keeps the CE light off...it has for the most part). Would this have any impact? Also there is a small exhaust leak before the O2 sensor where the down tube attaches on the passenger manifold.

    Since it will be asked...here's what has been replaced or done to the car in the last 20K or less.

    New Plugs - Autolite XP103s
    New Wires - Autolite OE Replacements
    New Oxygen Sensors - Standard Brand (el cheapo's off rock auto)
    Cam Sensor - BWD OE Equiv
    SCP Down Tubes
    High Flow Aftermarket Cats
    Air Filter Cleaned (Spectra Cone Filter)
    DIS was re-seated with Artic Blue

    Car runs smooth 95% of the time. Sometimes at start up it run like a 4 cylinder and then it smooths out after 1 min...not really sure why this is...or what the check. Not usually anything that throws a CE light.

    CE Light was on 24/7 - After O2s replaced it went out - now it stays off for 2 or 3 days...comes back on, then goes back off.

    Do any of the sensors never spoken about have impact on mileage...i.e Map Sensor, IAT Sensor on the Rear Plenum?

    -Tim
    Current Collection
    1994 Thunderbird SC - Auto converted to 5spd

    1995 Thunderbird SC - Auto

    1994 Thunderbird SC - 5spd
    1989 Thunderbird SC - 5spd - Daily Driver
    1990 Thunder/Cougar XR7 - 5spd
    1994 Thunderbird SC - 3v Project

  2. #2
    Join Date
    May 2011
    Posts
    825
    I would address the leak before the o2 sensor. It could be pulling fresh air into the system causing a false lean condition thus adding fuel to correct the readings. This done after reading codes. It's my understanding that egr only adds exhaust gas during decelerating. Just how I would approach the problem.

  3. #3
    Join Date
    May 2012
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    257
    Only thing I could think of is doing a compression test, check timing. I'm sure a bad sensor could also be the culprit

  4. #4
    Join Date
    Jan 2003
    Location
    Woodbridge, VA
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    Need to find out what the check engine light is. And I would clean the MAF and install an OE airbox and filter. Also fix the exhaust leak.
    ~Chris
    SCCoA Member #1826
    Bettie Riding jack stands
    SCARlit on drag duty now

  5. #5
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    Quote Originally Posted by 90coug View Post


    Connect E and 1

    Then read the flashes after turning the key to acc

    This can be found on passenger firewall or by air box on later model SCs

    Looks like I will do this on my lunch break just to see what it says to keep this thread somewhat active.

    -Tim
    Current Collection
    1994 Thunderbird SC - Auto converted to 5spd

    1995 Thunderbird SC - Auto

    1994 Thunderbird SC - 5spd
    1989 Thunderbird SC - 5spd - Daily Driver
    1990 Thunder/Cougar XR7 - 5spd
    1994 Thunderbird SC - 3v Project

  6. #6
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    Jul 2002
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    Roanoke, Virginia
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    Okay here is what I have - I crossed these over with the help of http://www.extreme-check-engine-ligh...%20Decoder.htm


    32 - egr positioner sensor below closed limit
    34 - egr valve circuit out of self-test range
    41 - oxygen sensor circuit indicates system always lean
    66 - mass air flow sensor circuit below minimum voltage
    67 - neutral/drive switch circuit open
    91 - no oxygen sensor switching detected

    No check engine light is currently present when the car is running.

    -Tim
    Current Collection
    1994 Thunderbird SC - Auto converted to 5spd

    1995 Thunderbird SC - Auto

    1994 Thunderbird SC - 5spd
    1989 Thunderbird SC - 5spd - Daily Driver
    1990 Thunder/Cougar XR7 - 5spd
    1994 Thunderbird SC - 3v Project

  7. #7
    Join Date
    Jul 2015
    Posts
    19
    Quote Originally Posted by Tim Groth View Post
    Okay here is what I have - I crossed these over with the help of http://www.extreme-check-engine-ligh...%20Decoder.htm


    32 - egr positioner sensor below closed limit
    34 - egr valve circuit out of self-test range
    41 - oxygen sensor circuit indicates system always lean
    66 - mass air flow sensor circuit below minimum voltage
    67 - neutral/drive switch circuit open
    91 - no oxygen sensor switching detected

    No check engine light is currently present when the car is running.

    -Tim
    32 and 34 are because the EGR delete fix the exhaust leak and test again after a couple days and it should clear 41 and maybe clear 91 the other two I'm not sure about could just be a loose connection

  8. #8
    Join Date
    Feb 2003
    Location
    Tinton Falls, NJ
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    1,660
    EGR will help with fuel mileage, but only by 1mpg or so, and only during a steady cruise situation like on the highway, so I doubt that is your problem. The exhaust leak before the O2 sensor however definitely will drop your mileage substantially. O2 sensor can only detect oxygen, and they infer from that how much fuel they need to add or take away. If the amount of oxygen in the exhaust goes way down, the computer figures that there is too much fuel and it needs to lean it out. On the other hand, in your case the oxygen sensor is sensing a lot of oxygen all the time. Since it thinks all this oxygen came from the combustion chamber, it assumes there is nowhere near enough fuel, and will keep adding fuel until it reaches the limits of the adaptive programming. This will not only cause poor fuel mileage, but you will be down on performance, and it can foul out plugs due to the rich condition. Fix your exhaust leak, and I bet your mileage will come back up to the 18-20 range.

  9. #9
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    825
    Ya kinda what I said in post number two. Seen it happen.

  10. #10
    Join Date
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    Well engine is now running like a top. Took a minute to install an S-Port on the car this weekend and found a handful of vac leaks that were from the plastic Christmas tree that mounts behind the inlet plenum. The plastic must have become week causing 2 of the teets to break off. a little JB Weld and patients later those areas were fixed.

    Mileage is now 24/25 per gallon. Engine runs so smooth its hard to tell its running...no engine lights.

    Should always keep in mind a vac leak always should be the first spot to look.

    -Tim
    Current Collection
    1994 Thunderbird SC - Auto converted to 5spd

    1995 Thunderbird SC - Auto

    1994 Thunderbird SC - 5spd
    1989 Thunderbird SC - 5spd - Daily Driver
    1990 Thunder/Cougar XR7 - 5spd
    1994 Thunderbird SC - 3v Project

  11. #11
    Join Date
    Jul 2006
    Location
    Escondido CA
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    468
    Mileage is now 24/25 per gallon. Engine runs so smooth its hard to tell its running...no engine lights.



    -
    What total mileage are you getting on a full tank?

  12. #12
    Join Date
    Jul 2002
    Location
    Roanoke, Virginia
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    Quote Originally Posted by sccrewzer View Post
    Mileage is now 24/25 per gallon. Engine runs so smooth its hard to tell its running...no engine lights.



    -
    What total mileage are you getting on a full tank?
    I haven't filled it completely until this morning so I can't fully report on that, I will update once this tank runs dry.

    This weekend I drove from Roanoke VA to Martinsburg WV for the annual Joel Bender Memorial Nats aka Summer Blast. When I started the trip, I used what was left from a full tank on Wednesday. Trip showed 110 miles and hadless than half a tank. A normal full tank here the last few weeks would go 225 to a tank...Needless after the repairs and the trip to WV I made it to 299 prior to filling up. At that point both check fuel lights were on.

    I put in 10 gallons of full in WV and made it 255 miles so again that's where the above math comes from.

    -Tim
    Current Collection
    1994 Thunderbird SC - Auto converted to 5spd

    1995 Thunderbird SC - Auto

    1994 Thunderbird SC - 5spd
    1989 Thunderbird SC - 5spd - Daily Driver
    1990 Thunder/Cougar XR7 - 5spd
    1994 Thunderbird SC - 3v Project

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