'91 GT running rich. No codes. 10-12mpg, help

Discussion in 'Fox 5.0 Mustang Tech' started by txstang347, Feb 24, 2011.


  1. txstang347

    txstang347 Member

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    My fox is running really rich. I just sold the beater and $3+/gal + 10-12mpg is killing me. I tried pulling codes and came up with nothing except "11". Both KOEO and KOER checked out fine. I was really hoping to get a code to help me out. Exhaust fumes are pretty bad considering I have cats. No smoke, but carbon build up is nice and black. Shorties, bassani catted X, bassani catback.

    I'm going to replace the O2 sensors, clean the MAF, and check for vacuum leaks tomorrow. Anyone have any ideas or have the same symptoms before?
    #1
  2. Kdubslugga

    Kdubslugga Active Member

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    those are all really good places to start! Do you have access to a vacuum pump? If so check the egr valve for proper function. An easy way to check is remove vacuum line to the egr valve while the car is running and supply vaccuum to it, car should immediatley stumble. If it doesnt you have a blockage somewhere in the egr system or a bad egr valve which could cause ur problems!!
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  3. txstang347

    txstang347 Member

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    Yup, got a vacuum pump in that rat's nest I call a garage somewhere. Good tip, I will try that out, thanks!
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  4. txstang347

    txstang347 Member

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    Question though, wouldn't that throw a code "33 EGR EGR did not open / respond to test"?
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  5. Mustang5L5

    Mustang5L5 Car used in adult film "Highway Gangbang-InDaButt" SN Certified Technician Founding Member

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    You should have codes

    Just to check, you got three "11" codes right? One for KOEO, CM and KOER?
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  6. txstang347

    txstang347 Member

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    I agree, I should have codes on most issues that would cause a rich issue, but I have none. :bang:

    I ran the KOEO again and got "11-10-11".
    The KOER test went through the cylinder test dumped an "8" for the cylinder id test and the gave an "11" and that was it.

    I left the reader on for about 3 minutes wondering if it was done and ran the check twice. The first time it got stuck on code "8" and never gave me an "11". :shrug:
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  7. txstang347

    txstang347 Member

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    Also, side note:
    I got a code "21" for the coolant sensor temp being out of range, but the motor wasn't quite warm yet so I'd imagine that's why I got that code. I ran the car up to temp, ran the test again and everything came out fine, no code "21" when it was warm.
    #7
  8. jrichker

    jrichker StangNet's favorite TOOL SN Certified Technician Founding Member

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    Testing the O2 sensors 87-93 5.0 Mustangs
    Measuring the O2 sensor voltage at the computer will give you a good idea of how well they are working. You'll have to pull the passenger side kick panel off to gain access to the computer connector. Remove the plastic wiring cover to get to the back side of the wiring. Use a safety pin or paper clip to probe the connections from the rear. The computer pins are 29 (LH O2 with a dark green/pink wire) and 43 (RH O2 with a dark blue/pink wire). Use the ground next to the computer to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle.

    Note that all resistance tests must be done with power off. Measuring resistance with a circuit powered on will give false readings and possibly damage the meter. Do not attempt to measure the resistance of the O2 sensors, it may damage them.

    Testing the O2 sensor wiring harness
    Most of the common multimeters have a resistance scale. Be sure the O2 sensors are disconnected and measure the resistance from the O2 sensor body harness to the pins on the computer.

    The O2 sensor ground (orange wire with a ring terminal on it) is in the wiring harness for the fuel injection wiring. I grounded mine to one of the intake manifold bolts

    Make sure you have the proper 3 wire O2 sensors. Only the 4 cylinder cars used a 4 wire sensor, which is not compatible with the V8 wiring harness.

    Replace the O2 sensors in pairs if replacement is indicated. If one is weak or bad, the other one probably isn't far behind.


    Testing the MAF
    The MAF body and sensor are designed to match each other. Therefore you can't swap sensors between different part number MAF bodies and maintain proper calibration. The assembly is designed to match the computer’s internal program, and swapping a different MAF can upset the computer's calibration.

    There are three parts in a MAF: the heater, the sensor element and the amplifier. The heater heats the MAF sensor element causing the resistance to increase. The amplifier buffers the MAF output signal and has a resistor that is laser trimmed to provide an output range compatible with the computer's load tables.

    The MAF element is secured by 2 screws & has 1 wiring connector. To clean the element, remove it from the MAF housing and spray it down with electronic parts cleaner or non-inflammable brake parts cleaner (same stuff in a bigger can and cheaper too).

    Changes in RPM causes the airflow to increase or decease, changing the voltage output.. The increase of air across the MAF sensor element causes it to cool, allowing more voltage to pass and telling the computer to increase the fuel flow. A decrease in airflow causes the MAF sensor element to get warmer, decreasing the voltage and reducing the fuel flow. Measure the MAF output at pins C & D on the MAF connector (dark blue/orange and tan/light blue) or at pins 50 & 9 on the computer. Be sure to measure the sensor output by measuring across the pins and not between the pins and ground.

    At idle = approximately .6 volt
    20 MPH = approximately 1.10 volt
    40 MPH = approximately 1.70 volt
    60 MPH = approximately 2.10 volt

    Check the resistance of the MAF signal wiring. Pin D on the MAF and pin 50 on the computer (dark blue/orange wire) should be less than 2 ohms. Pin C on the MAF and pin 9 on the computer (tan/light blue wire) should be less than 2 ohms.

    There should be a minimum of 10K ohms between either pin C or D on the MAF wiring connector and ground. Make your measurement with the MAF disconnected from the wiring harness.

    Diagram courtsey of TMoss & Stang&2birds

    [​IMG]


    See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring Mustang FAQ - Wiring & Engine Info

    Ignition switch wiring
    http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif

    Fuel pump, alternator, ignition & A/C wiring
    http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif

    Computer,. actuator & sensor wiring
    http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif

    Fuse panel layout
    http://www.veryuseful.com/mustang/tech/engine/images/MustangFuseBox.gif

    Vacuum routing
    http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxFordVacuumDiagram.jpg
    #8
  9. txstang347

    txstang347 Member

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    Ok, it's been a while and the exhaust fumes have settled down quite a bit. Maintenance I've done since first post in order to get my MPG up from 10 mpg:

    All Motorcraft parts:
    1. New rotor cap/button
    2. New plugs
    3. New PCV valve
    4. New fuel filter (which was a PITA with the IRS in there now BTW!)
    5. New O2 sensors (Bosch)
    6. Cleaned MAF sensor
    7. Cleaned and oiled K&N filter
    8. Replaced vacuum hoses
    9. New gas cap

    So, after all that I've gone from 10mpg to a best of (approx):
    203 miles/13 gallons = 15.6 gallons on street driving
    230 miles/13 gallons = 17.6 gallons highway

    That's a pretty significant change, but still don't think I'm getting the most out of it and think the MPG are still kinda low for a mostly all stock 5.0. Anyone have any other ideas? Would new plug wires, coil really make that much of a difference?

    I was thinking about checking for a leaking fuel injector? Would there be an obvious symptoms of a leaking injector? I have none right now other than low MPG. Starts right up, no bog, runs fine. Picked up some 19lb'ers with 50k miles on them and adj FPR. Just nervous about breaking another old bolt while pulling the intake like I did in my timing chain cover last year.
    #9
  10. Mustang5L5

    Mustang5L5 Car used in adult film "Highway Gangbang-InDaButt" SN Certified Technician Founding Member

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    Did you ever perform a cylinder balance test?
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  11. txstang347

    txstang347 Member

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    Yup, everything checked out fine.
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  12. jrichker

    jrichker StangNet's favorite TOOL SN Certified Technician Founding Member

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    The O2 sensor heaters have their own ground (HEGO ground) coming from the computer. This is different and separate from the O2 sensor ground. It is an orange wire with a ring terminal on it. It is located in the fuel injector wiring harness and comes out under the throttle body. It gets connected to a manifold or bolt on back of the cylinder head.
    #12
  13. txstang347

    txstang347 Member

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    I'm not following why you mentioned this, JR. Should I be checking this? If so, what am I looking for?
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  14. jrichker

    jrichker StangNet's favorite TOOL SN Certified Technician Founding Member

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    You are looking for a loose orange wire coming out of the passenger side engine fuel injection harness. It is supposed to be grounded to the engine block or firewall to insure proper O2 sensor operation.

    O2 sensors for 5.0 engines are 3 wire. Sometimes the parts guys will sell you 4 wire O2 sensors, which won't work without a re-wire.
    #14
  15. Dave2000GT

    Dave2000GT Active Member

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    Any improvement since this last post ?
    My old 92 Vert 5.0 could hit 27mpg freeway, but the notch I just got is stuggling to make 10 mpg so I'm looking for ideas / solutions. Thanks
    #15
  16. txstang347

    txstang347 Member

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    Turned out to be a combination of needing a tune up, which I did on the road to finding the real issue, which turned out to be a torn fuel neck filler grommet. The fuel was evaporating and spilling out while full during driving. It finally busted open at a gas station when I overfilled and leaked out all over the floor. It was pretty funny watching everyone freak out and run away. Finally at a steady 18/22 mpg.

    10mpg is awful. Since you just picked her up I'd start with the tune up basics and record improvements as you go.
    #16

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