90 5.0 gt running rich and cold idle issue

Discussion in 'Fox 5.0 Mustang Tech' started by vpap1, Sep 21, 2009.

  1. Hey all,
    I have a 1990 mustang gt. Has 99 k, has edelbrock intakes, cold air intake, BBK headers, Flowmaster H pipe and mufflers. T 5 tranny. When cold it has a rough inconstant idle untill it warms up. With in the first 2 minutes of driving it may even idle so low it dies. It hold and idle of 850 when warmed up. It also has been running very very rich. Exhaust smells of gas. Also the passenger side of the engine compartment also smells like gas when running. It does not have cats, and is not throwing any codes. Where do I begin.
    Thank You !!!
  2. No codes what so ever? Not one Flash?

    You should at least get the Systems pass code.
  3. Run the codes first. Good place to start
  4. The simple way to dump codes is in the checklist...

    See the "Surging Idle Checklist for help with all your idle/stall problems. Everything you need to find and fix idle problems is in there.

    The first two posts contain all the updates to the fixes. At last count there were 24 possible causes and fixes for surging idle/stall problems. I continue to update it as more people post fixes or ask questions.
  5. OK test wire was cut and I fixed that, thats why I got no codes.

    Codes Found:

    15 ECC prossesor or power to KAM interupted
    67 Nutral/drive switch open
    31 EGR position sensor below circuit minimum volt
    41 O2 circuit system lean
    66 Mass air flow circuit below minimum volt
    91 No o2 sensor switch

    Do I start with repalcing o2 sensor, egr, and mass air flow sensor and see if it runs right then???

  6. Code 15 - No Keep Alive Memory power to PCM pin 1 or bad PCM (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

    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




    CODE: 31 (KOEO) - EVP circuit below minimum voltage. Vref (5 volt reference voltage supplied by the computer) missing or broken wire or bad connection in circuit. Use a DVM to check for 5 volts on the orange/white wire. If it is missing, look for +5 volts at the orange/white wire on the TPS or MAP sensor located on the firewall near the center of the car. Use the black/white wire for the ground for the DVM.
    With the sensor removed from the EGR and still connected, press the plunger and watch the voltage change on the brown/lt green wire. Pull the passenger side kick panel and measure the voltage at the computer. You will need to remove the plastic cover over the wires and probe them from the backside. A safety pin may prove very useful for this task. Use pin 27, EVR input (brown/lt green wire) and pin 46, signal ground (black/white wire) to measure the voltage. The orange/white wire is Vref and should always be 5 volts -/+ .25 volt. Be sure to measure Vref at the EGR sensor to rule out any broken wires or bad connections.
    Measuring the voltage at the computer helps you spot broken wiring and intermittent connections.

    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 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 is for cars with an intact smog pump and cats. If the tube on the back of the heads clogs up the driver’s side, 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. 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.

    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

    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.
  7. Ok friend said to check timing and it was set at 31!!! Turned it back to 15 and doesnt smell like gas as much and idles much better. Car used to have a NOS system which I jsut found out so makes cents. Still misses at idel at times and i bet it just needs a tune up.

    Dumped codes: erased them and ran codes after 4 days of driving and only have theses three now.

    15 ECC prossesor or power to KAM interupted
    67 Neutral/drive switch open
    31 EGR position sensor below circuit minimum volt

    The nutral drive switch works because i cant start the car unless i depress the clutch.

    Progress is good !

  8. The neutral safety switch has nothing to do with the starter operation on a 5 speed car. The clutch safety circuit controls starter operation. The neutral safety switch is used to tell the computer that the transmission is in neutral and it is safe to run the Key On Engine Running diagnostic tests. The Key On Engine Running take throttle control away from the operator. This could cause an accident it the car were driven and the tests were in progress.

    Typical start circuit...
    Diagram courtesy of Tmoss & Stang&2birds

    Check my previous post for the fix for all the items.
  9. Ok update.....

    I took the nitros biggy back chip that was on the computer off and reset it, also changed the plugs because they were grimy and the ground straps were missing!!! No more code 15 or egr code, only code left is 67. Still having cold idle issues and surging .. time to go down that checklist and find the bug ...
    Btw timed it before i pull the nos chip and was running leaner, but now afte the chip is off it is now pig rich again ... guess I gotta reset the timing again. But no more skips in the engine while driving round town, throttle response was great too.

  10. Possible clutch pedal switch. Make sure the White plug is plugged in on the CPS.

    Code 67 is known to cause Idle issues. But its usually in Open and Closed Loop.

    Get code 67 squared away then run the KOER test.
  11. Ok Started probing and here is what I found ...

    My findings with TPS voltage

    Car power on but motor not running … .580 volts
    Car on, motor running at cold idle … .602 volts
    Motor off ignition on WOT … 3.774

    also carb cleaned the TB and the IAC they are clear.
    I twisted the tps to try and adjust it but I could only get as far as .7 volts, I turned down the idle screw and on cold start it died quickly and then ran but felt like it was choking out. But now running around 750 istead of the 1000 of before. The iac is getting its 12 volts too. Running out of options idk what to do next.

  12. Was told to adjust the tsp voltage by the idle screw with motor off ignition on. Set it to .90 volts and car starter right up when stone cold, went to 1300 and creeped down to 1000 rpm after a min it was searching from 1000-1100 up and down ...
    I was also told to clean the air charge valve and I am in the process of doing this too.

    Any thoughts
  13. Could it be the temp sensors??? if it is what voltage am I looking for each???
  14. Go through the "Surging Idle Checklist . It is your best bet to finding and fixing idle problems. It will reduce the time and money you spend chasing rabbits that get you nowhere.
  15. Were is this test wire that was cut your speaking of