Bad Idle Surge on '88 Fox w/Mods

Discussion in 'Fox 5.0 Mustang Tech' started by dangrego, Sep 12, 2009.

  1. is 4.55 bad then? what is under the throttle body? the idle air bleeder? thanks!!
  2. No 4.55v is good, thats the reference voltage that the sensors use. If it was zilch, then there would be a problem
  3. eng2.jpg
  4. interesting, well, I actually took my TB off to see where that idle air screw was and it doesn't have one.....even more interesting is that when I took the TB off, a fair amount of coolant came with it. This would be the 3rd time I had to repair a headgasket in that car in 2 years. I don't know why it keeps blowing...maybe the block is warped?
  5. Code 67 - clutch not depressed (5 speed) or car not in neutral or park (auto) or A/C in On position when codes where dumped. Possible neutral safety switch or wiring problem. This code may prevent you from running the Key On Engine On tests. You can generally ignore this code, since it has no effect on engine performance.

    The computer wants to make sure the A/C is off due to the added load on the engine for the engine running tests. It also checks to see that the transmission is in Neutral or the clutch depressed (T5, T56, Tremec 3550 & TKO). This prevents the diagnostics from being run when the car is driven. Key On Engine Running test mode takes the throttle control away from the driver for several tests. This could prove hazardous if the computer was jumpered into test mode and then driven.

    The NSS code 67 can be bypassed for testing. You will need to temporarily ground computer pin 30 to the chassis. Computer pin 30 uses a Lt blue/yellow wire. Remove the passenger side kick panel and then remove the plastic cover from the computer wiring connector. Use a safety pin to probe the connector from the rear. Jumper the safety pin to the ground near the computer.

    Code 95 Key On, Engine not Running - the following test path is for 86-90 model Mustangs.

    The 95 code is because at one time or another, the fuel pump relay hiccupped and didn't provide power the pump when the computer told it to run. Sometimes this is a one time thing, other times it is a no run or runs poorly condition.

    Suspect items are bad fuel pump relay, corrosion in the fuel pump relay socket, inertia switch, wiring damage , corroded connector contacts.


    Using the diagram, check the red/black wire from the fuel pump relay: you should see 12 volts or so. If not, check the inertia switch: on a hatch it is on the driver’s side by the taillight. Look for a black rubber plug that pops out: if you don't find it, then loosen up the plastic trim. Check for voltage on both sides of the switch. If there is voltage on both sides, then check the Pink/black wire on the fuel pump relay: it is the power feed to the fuel pump. No voltage there, check the Orange/Lt blue wire, it is the power feed to the fuel pump relay & has a fuse link in it. If there is good voltage there & at the Pink/black wire, swap the relay.

    Some Mass Air conversions neglect to run the extra fuel pump wire, and they always have a 95 code. See Mustang Mass Air Conversion | StangNet for more information on the Mass Air wiring conversion.

    Code 29 - Vehicle Speed Sensor (VSS) is an electronic sender mounted on the speedo pickup gear on the trans. It works the cruse control for both 5 speed and auto trans cars. The VSS is used to tell the computer to speed up the idle as you slow to a stop. This helps keep the engine from stalling when you slow down for a stop sign or stop light.
    Check to see if the electrical connector is plugged into it. Clean the connector & contacts with non flammable brake parts cleaner prior to replacing the sensor, as that may fix the problem. The sensor cost is under $30 and it is easy to replace.

    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

    Fuel pump, alternator, ignition & A/C wiring

    Computer,. actuator & sensor wiring

    Fuse panel layout

    Vacuum routing

    Fix the MAF code 66 first and see if your idle/surge problems clear up The MAF plays a major part in metering the air/fuel mixture. If you have a cold air induction kit, be sure to clock the MAF in 20 degree increments and check the idle. Go in a full circle and not just 3 or 4 locations when you clock the MAF.
  6. coolant is going to come out of it. the egr spacer uses coolant to heat up. that is fine. it does not mean your head gaskets are blown.
  7. yah, but i bipassed the coolant from going to the isn't like stock-there's no coolant line anyhwere near the TB anymore, it should be dry as a bone. :(
  8. Code 95 may be because of the Mass air Conversion. Which will happen when the Fuel Pump Monitor( FPM, or PIN 19 at the ECM) is not hooked up.

  9. If you are going to adjust the idle screw, you need to unplug the IAC first. Then set a LOW idle, reset the computer and plug the IAC in.

    The computer controls idle in these cars. SO if you adjust the screw, the computer will attempt to drop idle by closing the IAC. That's why you set the minimum required idle to keep the car running, and let the car control idle with the IAC.

    If you want to bypass this, unplug the IAC totally and then set the idle screw.

    With that said, how is your IAC? New? Is it clean and well lubed?
  10. hi, the IAC is only a year old and clean...are they supposed to be lubed? :)
  11. If you let the car sit over night. Pull the connectors from the act and the ect, put an ohm meter across the connections on each sensor, if they don't give the same reading something is wrong with one of the sensors.
  12. Replace the sensor and let us know. The IAC is irrelevent right now. For the IAC to be considered you have to make sure nothing else is affecting the Idle. "Cough" "Cough" ECT sensor......"Cough"
  13. Why not check the sensors first?
  14. Well, here are the results of my resistnace test (battery was already disconnected all night and day):

    ECT 24.5
    ACT 25.5
    Both harness connectors 19.0

    Is this significant data at all?
  15. oh, what i said earlier about coolant coming out of the TB when i took it off, there shouldn't be any there since i dont route coolant through it anymore, but i once did and what came out must just be old stuff that sat in there....not worried about a head gasket anymore.
  16. Have you deleted the EGR? WHy did you remove the coolant?
  17. yep, EGR is gone. since i rigged up my own homeade heating system, i decided, it was just too much trouble cutting and splicing into my heater hoses to run coolant through the TB. i don't think it makes a huge difference, but hey, if i can get the BIG BUGS worked out of this car, then, yah, i will probably do a lot of tweeking and making things right.
  18. If the ect is reading coolant and the act is reading air and both are the same temperature they should have the same resistance. I'm not sure if that's within tolerance or not. I assume you typed those numbers correctly.

  19. I figure it to be ~ 4%, they should be O.K.
  20. yah, i think so, not sure what to do next....thinking extreme like new MAF sensor or new computer and have somebody that knows what their doing for sure wire it up. possible i could have hooked up the new computer funky?