89 Gt. Rough Idle And Weird Shift, Help Me.

Discussion in '1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk-' started by Derek Kyle, Aug 19, 2014.


89 GT. Rough idle and weird shift, help me.

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  1. Hi everyone,

    So I just bought an 89 Gt from my mom who no longer Dove him. He sat for I'm not sure how long and was lucky enough to get started and ran at idle maybe once a week. First thing I noticed was a SURGING IDLE, until he warmed up. (Almost like a mis fire).

    First thing I did was put new oil/filter and transmission fluid/filter, spark plugs/wires and distributor cap on. After the tune up, he shifts funky (1/2 is quick and smooth, the 2/3 is at like 4000 rpms. And 3/4 is about the same) and continued idle weird. I fidgeted with the throttle position sensor, and spayed some throttle body cleaner in the throttle body and part of the idle air control valve to clean some gunk ( sprayed and cleaned where i can reach without disassembling the throttle body or IAC Valve). Now the idle seems better but low at operating temps, and the shifting seems to improve when "whitey ford" is all warmed up.

    This is my first mustang and I'm excited to get him running good. Any suggestions to what's going on here. I figured it may be the TPS, IAC, or EGR, but idk if they would affect the shifting.

    Also gas mileage is pretty terrible right now.
    Thanks for any suggestions,
  2. on the new mustangs the ford company actually recommended using thicker oil for heating purposes.
    just a little heads up
    shifting could also be a little rough because the car may just be running a little cool at the beginning
    plus it could also affect gas millage horribly
    okay man hope that helped or maybe not Im sorry
    BTW im very happy for you first mustang that is a true beauty !
    bye hope to hear some updates soon
  3. You guys with idle/stall problems could save a lot of time chasing your tails if you would go through the Surging Idle Checklist. Over 50 different people contributed information to it. The first two posts have all the fixes, and steps through the how to find and fix your idle problems without spending a lot of time and money. I continue to update it as more people post fixes or ask questions. You can post questions to that sticky and have your name and idle problem recognized. The guys with original problems and fixes get their posts added to the main fix. :D

    It's free, I don't get anything for the use of it except knowing I helped a fellow Mustang enthusiast with his car. At last check, it had more than 159,000 hits, which indicates it does help fix idle problems quickly and inexpensively.
  4. Where is it?
  5. There is a problem with the link which will be fixed shortly. The same problem affects a number of the other tech notes in the Technical index thread
    #5 jrichker, Aug 20, 2014
    Last edited: Aug 20, 2014
  6. What if the "surging idle checklist" was pinned next to "best hi-po tranny" and all that crap? I bet you you'd have to tell people about it less.
  7. That is a great idea, but I don't manage the board. That is in the very capable hands of the moderators. At this time the checklist along with a number of excellent technical resources are in the "Technical Thread/How-To Index" sticky.
  8. I checked the check list that thats where i started getting possible culprits to the problem.

    No more surging idle! :) yay!

    Also, i coded my car and go 66c error code. The booklet said vane air flow sensor... Is that the same as mass air flow? I unplug with the car running and nothing changes..

    So the big issue im having is the shifting and and fuel consumption, feels like the big guy is lacking power.

    Thanks everyobe for your input,
  9. Code 66 or 157 MAF below minimum test voltage.

    Revised 10-Feb-2014 to add 95-95 Mustang code 157 and 94-95 ECC diagram

    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.

    Computer connector for 88-93 5.0 Mustangs
    Diagrams courtesy of Tmoss and Stang&2Birds

    ECC Diagram for 88-90 5.0 Mustangs

    ECC Diagram for 91-93 5.0 Mustangs

    94-95 Diagram for 94-95 5.0 Mustangs


    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. Changes in RPM causes the airflow to increase or decrease, 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.

    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).

    89-90 Model cars: 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.

    91-95 Model cars: Measure the MAF output at pins C & D on the MAF connector light blue/red 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 pins A or B. Make your measurement with the MAF disconnected from the wiring harness.

    Actually MAF pins C & D float with reference to ground. The signal output of the MAF is a differential amplifier setup. Pins C & D both carry the output signal, but one pin's output is inverted from the other. The difference in signal between C & D is what the computer's input circuit is looking for. The difference in the two outputs helps cancel out electrical noise generated by the ignition system and other components. Since the noise will be of the same polarity, wave shape and magnitude, the differential input of the computer electronically subtracts it from the signal. Then it passes the signal on to an Analog to Digital converter section inside the computer's CPU chip.

    See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring http://www.veryuseful.com/mustang/tech/engine/

    Ignition switch wiring

    Fuel pump, alternator, ignition & A/C wiring

    Computer,. actuator & sensor wiring

    Fuse panel layout

    Vacuum routing
  10. Get a new maf. They're not all that much at your parts store.