90 GT Running rich, backfiring?? Help!

Discussion in 'Fox 5.0 Mustang Tech' started by afiveo, Nov 7, 2009.


  1. afiveo

    afiveo New Member

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    I have a 1990 mustang GT. I've had it for 14 years. All stock except K&N panel filter, Mac headers, h-pipe, flowmaster cat back, and a msd blaster coil. 92000 miles. It is undriveable as it is now. It is running so rich the exhaust fumes burn your eyes. Doesn't smoke, just running rich. The idle is surging up and down and when i try to rev it up it pops and backfires.

    I have installed:
    new fuel pump
    plugs, msd wires, cap and rotor
    fuel injectors
    O2 sensors

    Nothing has helped. It will stall when you try to take off normal so I have to rev it up (like a 2 stroke motorcycle) and it just pops and carries on. :shrug: It just started doing it one day while I was driving it. I was going to lauch pretty good from a stop sign and it just fell on it's face. I barely got it home. That was a few years ago and it has been in the garage ever since. Fuel is fresh also. I want to get it back on the road but it runs like crap. I never had any probs with it until this happened.

    Any help would be much appreciated. Thanks
    #1
  2. 65ShelbyClone

    65ShelbyClone Founding Member

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  3. afiveo

    afiveo New Member

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    I haven't tried to get any. I'll have to do a search to see how.
    #3
  4. 65ShelbyClone

    65ShelbyClone Founding Member

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    It's really easy and only involves a paper clip and a test light. Search for posts by jrichker.
    #4
  5. afiveo

    afiveo New Member

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    I have a check engine light so I don't need a test light, correct?

    I have been reading the proceedure and plan on trying in in the morning. Do I just jumper it and then get in the car and count the flashes of the check engine light? That part I am not sure of.
    #5
  6. 65ShelbyClone

    65ShelbyClone Founding Member

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    Yes and yes.
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  7. afiveo

    afiveo New Member

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    Going to go work on it now.
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  8. grinch27

    grinch27 New Member

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    I had the same problem, I don't know if u have an aftermarket chip or not. But I ended up getting my car tuned and that fixed the problem.
    #8
  9. 50foxbody

    50foxbody Member

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    im interested in what your codes may be my buddys 90 lx is doing the same right now let me know what you find thanks and i hope you fix the problem
    #9
  10. afiveo

    afiveo New Member

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    Codes are
    41 System Lean
    91 System Lean
    33 EGR not open properly
    18 Timing
    12 ISC
    21 ECT

    I'm going to see if I can go borrow a timing light and start there. Any other suggestions?
    It does have one of those idle bypass plates on it. Should I remove that and see what it does? It was never a problem before as it's been on there for years.

    Where would the chip be if it had one?
    #10
  11. jrichker

    jrichker StangNet's favorite TOOL SN Certified Technician Founding Member

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    Code 12 -Idle Air Bypass motor not controlling idle properly (generally idle too low) - IAB dirty or not working. Take it off and clean it thoroughly with throttle body cleaner. Clean the electrical contacts with non flammable brake parts cleaner at the same time.


    Code 21 or 116 – ECT sensor out of range. Broken or damaged wiring, bad ECT sensor.
    Note that that if the outside air temp is below 50 degrees F that the test for the
    ECT can be in error. This code may go away as the engine warms up, so don't dump the codes
    on a cold engine


    The ECT sensor has absolutely nothing to do with the temperature gauge. They are
    different animals. The ECT sensor is normally located it the RH front of the engine in
    the water feed tubes for the heater.

    The ACT & ECT have the same thermistor, so the table values are the same

    ACT & ECT test data:

    Use Pin 46 on the computer for ground for both ECT & ACT to get most accurate
    readings.

    Pin 7 on the computer - ECT signal in. at 176 degrees F it should be .80 volts

    Pin 25 on the computer - ACT signal in. at 50 degrees F it should be 3.5 volts. It is
    a good number if the ACT is mounted in the inlet airbox. If it is mounted in the lower
    intake manifold, the voltage readings will be lower because of the heat transfer.

    Voltages may be measured across the ECT/ACT by probing the connector from
    the rear. A pair of safety pins may be helpful in doing this. Use care in doing it
    so that you don't damage the wiring or connector.

    Here's the table :

    50 degrees F = 3.52 v
    68 degrees F = 3.02 v
    86 degrees F = 2.62 v
    104 degrees F = 2.16 v
    122 degrees F = 1.72 v
    140 degrees F = 1.35 v
    158 degrees F = 1.04 v
    176 degrees F = .80 v
    194 degrees F = .61
    212 degrees F = .47 v
    230 degrees F = .36 v
    248 degrees F = .28 v

    Ohms measures at the computer with the computer disconnected, or at the sensor with the sensor disconnected.

    50 degrees F = 58.75 K ohms
    68 degrees F = 37.30 K ohms
    86 degrees F = 27.27 K ohms
    104 degrees F = 16.15 K ohms
    122 degrees F = 10.97 K ohms
    140 degrees F = 7.60 K ohms
    158 degrees F = 5.37 K ohms
    176 degrees F = 3.84 K ohms
    194 degrees F = 2.80 K ohms
    212 degrees F = 2.07 K ohms
    230 degrees F = 1.55 K ohms
    248 degrees F = 1.18 k ohms

    Diagram courtesy 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 - Engine Information

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

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

    Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs
    http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif

    Vacuum diagram 89-93 Mustangs
    http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxFordVacuumDiagram.jpg


    Code 18 - SPOUT out or wiring fault - look missing SPOUT jumper, short to ground or open circuit in SPOUT wiring going back to the computer. Possible bad TFI.

    The SPOUT enables the distributor to advance the timing beyond the base setting. Without a working SPOUT, you are locked down to the timing you set with the timing light.

    Checking the SPOUT wiring
    Tools needed:

    DVM or Ohmmeter
    10 MM wrench
    Phillips screwdriver.
    6 ft 16 or 18 gauge jumper wire with alligator clips on each end.

    Anytime you make resistance checks, the circuit under test must not be powered on. If it is powered up during the test, your readings will be wrong and you may damage the meter. Do not touch the probe tips when making resistance measurements. If you do, your readings may be inaccurate.

    Remove the SPOUT connector from the ignition harness by the distributor. Remove the passenger side kick panel and loosen the 10 MM bolt that secures the computer wiring connector. Use your DVM to measure the resistance between ground and pin 38 (yellow/lt green wire): you should see and infinite open circuit. Keep the DVM connected to pin 38 (you may need a helper at this point). Insert the DVM lead you previously had connected to the body ground into the computer side of the SPOUT connector. You may need some extra wire to extend the test lead, so have some 16-18 gauge insulated wire handy. If you extend the test lead, remember that the quality of the connections may affect your readings. You should see less than 2.0 Ohms: more than that indicates a wiring problem

    The following is a view from the computer side of the computer wiring connector.
    [​IMG]


    Diagrams courtesy of Tmoss & Stang&2birds
    [​IMG]

    See the following website for some help from Tmoss (diagram designer) & Stang&2Birds
    (website host) for help on 86-95 5.0 Mustang wiring Mustang FAQ - Engine Information Everyone should bookmark this site.


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

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

    Complete computer, actuator & sensor wiring diagram for 88-91 5.0 Mass Air Mustangs
    http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif

    Complete computer, actuator & sensor wiring diagram for 91-93 5.0 Mass Air Mustangs
    http://www.veryuseful.com/mustang/tech/engine/images/91-93_5.0_EEC_Wiring_Diagram.gif

    Complete computer, actuator & sensor wiring diagram for 94-95 5.0 Mass Air Mustangs
    http://www.veryuseful.com/mustang/tech/engine/images/94-95_5.0_EEC_Wiring_Diagram.gif

    Vacuum diagram 89-93 Mustangs
    http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxFordVacuumDiagram.jpg

    HVAC vacuum diagram
    http://www.veryuseful.com/mustang/tech/engine/images/Mustang_AC_heat_vacuum_controls.gif

    TFI module differences & pinout
    http://www.veryuseful.com/mustang/tech/engine/images/TFI_5.0_comparison.gif

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


    Code 33 - Insufficient EGR flow detected.
    Look for vacuum leaks, cracked vacuum lines, failed EGR vacuum regulator. Check to see if you have 10” of vacuum at the EGR vacuum connection coming from the intake manifold. Look for electrical signal at the vacuum regulator solenoid valves located on the rear of the passenger side wheel well. Using a test light across the electrical connector, it should flicker as the electrical signal flickers. Remember that the computer does not source any power, but provides the ground necessary to complete the circuit. That means one side of the circuit will always be hot, and the other side will go to ground or below 1 volt as the computer switches on that circuit.
    Check for resistance between the brown/lt green wire on the EGR sensor and pin 27 on the computer: you should have less than 1.5 ohm.

    See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host)

    http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif

    http://www.veryuseful.com/mustang/tech/engine/images/88-91eecPinout.gif


    EGR test procedure courtesy of cjones

    to check the EGR valve:
    bring the engine to normal temp.

    connect a vacuum pump to the EGR Valve or see the EGR test jig drawing below. Connnect the test jig or to directly to manifold vacuum.

    Do not connect the EGR test jig to the EVR (Electronic Vacuum Regulator).


    apply 5in vacuum to the valve. Using the test jig, use your finger to vary the vacuum

    if engine stumbled or died then EGR Valve and passage(there is a passageway through the heads and intake) are good.

    if engine did NOT stumble or die then either the EGR Valve is bad and/or the passage is blocked.

    if engine stumbled, connect EGR test jig to the hose coming off of the EGR Valve.
    Use your finger to cap the open port on the vacuum tee.
    snap throttle to 2500 RPM (remember snap the throttle don't hold it there).
    did the vacuum gauge show about 2-5 in vacuum?
    if not the EVR has failed

    EGR test jig
    [​IMG]

    The operation of the EGR vacuum regulator can be checked by using a test light applied across the wiring connector. Jumper the computer into self test mode and turn the key on but do not start the engine. You will hear all the actuators (including the EVR vacuum regulator) cycle. Watch for the light to flicker: that means the computer has signaled the EGR vacuum regulator successfully.


    Code 41 or 91 Three digit code 172 or 176 - O2 sensor indicates system lean. Look for a vacuum leak or failing O2 sensor.

    Revised 24-Sep-2009 to correct computer pin description for 94-95 5.0 Mustangs

    Code 41 is a RH side sensor,
    Code 91 is the LH side sensor.

    Code 172 is the RH side sensor
    Code 176 is the LH side sensor

    The computer sees a lean mixture signal coming from the O2 sensors and tries to compensate by adding more fuel. Many times the end result is an engine that runs pig rich and stinks of unburned fuel.

    The following is a Quote from Charles O. Probst, Ford fuel Injection & Electronic Engine control:
    "When the mixture is lean, the exhaust gas has oxygen, about the same amount as the ambient air. So the sensor will generate less than 400 Millivolts. Remember lean = less voltage.

    When the mixture is rich, there's less oxygen in the exhaust than in the ambient air , so voltage is generated between the two sides of the tip. The voltage is greater than 600 millivolts. Remember rich = more voltage.

    Here's a tip: the newer the sensor, the more the voltage changes, swinging from as low as 0.1 volt to as much as 0.9 volt. As an oxygen sensor ages, the voltage changes get smaller and slower - the voltage change lags behind the change in exhaust gas oxygen.

    Because the oxygen sensor generates its own voltage, never apply voltage and never measure resistance of the sensor circuit. To measure voltage signals, use an analog voltmeter with a high input impedance, at least 10 megohms. Remember, a digital voltmeter will average a changing voltage." End Quote

    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.


    Testing the O2 sensors 94-95 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 43 (LH O2 with a red/black wire) and 44 (RH O2 with a gray/lt blue wire). Use the metal 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.


    If you get only code 41 or 91 and have changed the sensor, look for vacuum leaks. This is especially true if you are having idle problems. The small plastic tubing is very brittle after many years of the heating it receives. Replace the tubing and check the PVC and the hoses connected to it.
    A secondary problem with only a code 41 or 91 is for cars with an intact smog pump and cats. If the tube on the back of the heads clogs up, all the air from the smog pump gets dumped into one side. This excess air upsets the O2 sensor calibration and can set a false code 41 or 91. The cure is to remove the crossover tube and thoroughly clean the insides so that there is no carbon blocking the free flow of air to both heads.
    #11
  12. afiveo

    afiveo New Member

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    OK, I got the timing set at 14 degrees. I removed and cleaned the IAC and re-installed it. Removed and cleaned EGR also.

    Now with the key on engine off I am getting these codes:
    15 No Keep Alive Memory to PCM pin 1 or bad PCM (what's a PCM????)
    33 EGR did not open/respond
    66 MAF low signal

    With the engine running, I am getting these codes:
    41 System Lean - Fuel Control
    33 EGR
    #12
  13. afiveo

    afiveo New Member

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    I took it for a short ride down the road. It is actually driveable but still runs like crap. It is popping in the airbox while accelerating. Still seems to be missing a little also.
    #13
  14. jrichker

    jrichker StangNet's favorite TOOL SN Certified Technician Founding Member

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    Go back to the code test paths I posted and work your way through them. You will find the problem if you are careful and don't skip any steps.

    Code 15 - No Keep Alive Memory power to computer pin 1 or bad computer (Memory Test
    Failure). The voltage to the Keep Alive Memory (KAM) is missing (wiring problem)
    or the KAM is bad. The KAM holds all of the settings that the computer "learns" as
    it operates and all the stored error codes that are generated as a result of
    something malfunctioning while the engine is running. Use a voltmeter to check
    the voltage to the pin 1 on the computer - you should always have 12 volts. No
    constant 12 volts = bad wiring. If you do always have the 12 volts, then the KAM is
    bad and the computer is faulty.

    If the computer has to "relearn" all the optimum settings every time it powers up,
    the initial 5-30 minutes of operation may exhibit surges, poor low speed performance,
    and rough idle.

    Note that some aftermarket chips will cause code 15 to set. Remove the chip,
    clear the codes and retest.


    Before replacing the computer, remove the battery ground cable for about 20
    minutes. This will clear all the codes. Retest after several days of running. If the 15
    code is gone, then don't worry about it. If it is still there, then you get to do some
    troubleshooting.

    Wiring diagrams for the proper model years are next…

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

    Diagram courtesy of Tmoss & Stang&2birds
    [​IMG]

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

    http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif

    http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif


    If you disconnected the MAF while cleaning and testing, the code 66 may have popped up.
    Code 66 MAF below minimum test voltage.
    Insufficient or no voltage from MAF. Dirty MAF element, bad MAF, bad MAF wiring, missing power to MAF. Check for missing +12 volts on this circuit. Check the two links for a wiring diagram to help you find the red wire for computer power relay switched +12 volts. Check for 12 volts between the red and black wires on the MAF heater (usually pins A & B). while the connector is plugged into the MAF. This may require the use of a couple of safety pins to probe the MAF connector from the back side of it.

    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.

    If you have a K&N flat panel filter or other filter that requires oiling, excess oil may coat the MAF sensor element and cause problems.

    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.


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

    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
    #14
  15. afiveo

    afiveo New Member

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    I have an Ammeter....not the same as a Voltmeter??? Sorry for the dumb question.
    #15
  16. jrichker

    jrichker StangNet's favorite TOOL SN Certified Technician Founding Member

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    Ammeters measure current (volume of flow). Voltmeters measure voltage (pressure in a fluid system).

    For the electrically challenged, there is hope, and it is even free...

    Try Automotive Online Instruction for a beginning course. The course is very good and best of all, it is FREE!!!

    I recommend you follow the Google search trail - automotive electrical course online - Google Search


    Here's a book that will get you started with how the Ford electronic engine control or "computer" works.

    Ford Fuel Injection & Electronic Engine Control 1988-1993 by Charles Probst :ISBN 0-8376-0301-3.

    It's about $25-$35 from Borders.com see Amazon.com: Online Shopping for Electronics, Apparel, Computers, Books, DVDs & more . Select books and then select search. Use the ISBN number (without dashes or spaces) to do a search

    Use the ISBN number and your local library can get you a loaner copy for free. Only thing is you are limited to keeping the book for two weeks. It is very good, and I found it to be very helpful.
    #16
  17. afiveo

    afiveo New Member

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    I just spend $90 on a timing light today and it looks like i will be spending more to purchase a voltmeter. lol. Geesch.

    I pulled the computer and it doesn't seem to have a "chip". Would it would be between the computer itself and the harness plug if it did?
    #17
  18. jrichker

    jrichker StangNet's favorite TOOL SN Certified Technician Founding Member

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    Most of the chips plug into the computer diagnostic connector on the end of the computer opposite the main wiring harness connector.
    #18
  19. afiveo

    afiveo New Member

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    Thanks for all your help so far.

    I guess what I have is a multi-meter. Would I set it on AC or DC? Craftsman Model 82180.
    #19
  20. 65ShelbyClone

    65ShelbyClone Founding Member

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    Use the DC setting. If it has a range selector, put it on a voltage window of 20.
    #20

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