Idle Issues

Discussion in 'Fox 5.0 Mustang Tech' started by Ant85, Sep 20, 2013.


  1. Ant85

    Ant85 Member

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    On cold starts it starts up fine, but will not idle on it's own unless I hold the revs up a little for about 30 seconds, but then when I put it in gear to start going it acts like it wants to bog down and even sounds like it wants to backfire. If I have the RPM's up higher than normal while taking off it does not do it.

    When the car is up to temperature every so often the RPM's drop down close to 500 RPM but never any lower. A quick rev will get it idling back to normal.

    The previous owner never mentioned the engine having a cam, but it does sound like it. I also know that having a vacuum leak can cause the engine to sound like it has a cam. I'm new to the Mustang world, and I haven't been around many, but would having a cam cause the engine to rock at idle? Because mine does. Sitting in the car you can feel the car rocking side to side. Someone told me it sounds like it has a B cam in it. I know videos don't give you the best idea of what something sounds like, but my car doesn't sound like any Mustang on Youtube with a B cam in it.

    I have checked for vacuum leaks and haven't found any. I have cleaned the TB and IAC, then I replaced the IAC because it would not come all the way clean. Also, when I replaced the IAC about a week after cleaning it I noticed a little bit of oil coming out of the holes that wasn't there when I took it off to clean it. Could oil be causing this if it's just sitting in there for a long period of time? Should I keep going with checking for vacuum leaks?? The codes I have are for the 02 sensors, some fuel circuit code, engine coolant temp and one for not controlling idle at high rpm's.
  2. Casey2552

    Casey2552 Member

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    Post your code numbers so we can see exactly what codes they are you say fuel failure if there's an injection problem it's Gona set off o2 sensors


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  3. Casey2552

    Casey2552 Member

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    If you run a balance test it will tell you what cylinders are not contributing by code or if all good it will show 9 codes 1-8 will will indicate the cylinder number not contributing
  4. Grabbin' Asphalt

    Grabbin' Asphalt Advanced Member

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    If you have oil in the iac, go inspet your pvc and the crank case screen under it. if theyre clogged you have back pressure coming out of the valve cover hose leading to the throttle body. Your throttle body should be cleaned again afterwards. The pvc/ crankcase screen are cheap to fix. Follow the LARGER vaccum line going down to the rear of the intake. Easier to get to from passenger side, for me anyways:nice:
  5. 7991LXnSHO

    7991LXnSHO Active Member

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    Use the surging idle checklist jrichker has posted. You should find your answer there. No shortcut, just good info.
  6. Ant85

    Ant85 Member

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    Codes are
    41 - System Indicates Lean -passenger side(R), No o2 Sensor Switching Detected. always lean -passenger side(CM)
    91- o2 Sensor Circuit Indicates Lean -driver side(R), No o2 Sensor Switching Detected. always lean -driver side(CM), Shift Solenoid 1 Circuit Failure (O)
    96- Fuel Pump Circuit Open-Battery To PCM (CM,O)

    There's also a code for the coolant temp. I forgot the number for that one though.
  7. Ant85

    Ant85 Member

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    I knew buying this car was going to be some work. Luckily it isn't my daily driver lol. It seems like every time I fix one thing, something else goes wrong right away. I kinda feel like not wanting to fix anything else because the next thing that goes wrong could be a bigger issue.

    It seems like there's so many different issues that could be causing this idle issue, and it seems to be getting worse now that the temp is getting colder. It's gotta be a vacuum leak.. Or maybe the intake leaking. I don't know. I've spent so much time trying to check every line possible or look for anything noticeable but I can't find anything. Since winter time is around the corner maybe I'll just give up on this because I won't be driving it much in the winter(heater core has been bypassed)
  8. Grabbin' Asphalt

    Grabbin' Asphalt Advanced Member

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    Fuel pump relay should be under driver seat, if not there passenger fender near maf sensor
  9. jrichker

    jrichker StangNet's favorite TOOL SN Certified Technician Founding Member

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    Fix your codes first. You can ignore the code 96 until you get the idle problems fixed.

    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 21-Sep-2013 to clarify reference to where RH & LH sides are viewed from

    Code 41 is a RH side sensor, as viewed from the driver's seat.
    Code 91 is the LH side sensor, as viewed from the driver's seat.

    Code 172 is the RH side sensor, as viewed from the driver's seat.
    Code 176 is the LH side sensor, as viewed from the driver's seat.

    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.

    Disconnect the O2 sensor from the harness and use the body side O2 sensor harness as the starting point for testing. Do not measure the resistance of the O2 sensor , you may damage it. Resistance measurements for the O2 sensor harness are made with one meter lead on the O2 sensor harness and the other meter lead on the computer wire or pin for the O2 sensor.

    Backside view of the computer wiring connector:
    [​IMG]

    87-90 5.0 Mustangs:
    Computer pin 43 Dark blue/Lt green – LH O2 sensor
    Computer pin 29 Dark Green/Pink – RH O2 sensor
    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.

    91-93 5.0 Mustangs:
    Computer pin 43 Red/Black – LH O2 sensor
    Computer pin 29 Gray/Lt blue – RH O2 sensor
    The computer pins are 29 (LH O2 with a Gray/Lt blue wire) and 43 (RH O2 with a Red/Black 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 29 (LH O2 with a red/black wire) and 27 (RH O2 with a gray/lt blue wire). Use pin 32 (gray/red wire) 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. Using the Low Ohms range (usually 200 Ohms) you should see less than 1.5 Ohms.

    87-90 5.0 Mustangs:
    Computer pin 43 Dark blue/Lt green – LH O2 sensor
    Computer pin 29 Dark Green/Pink – RH O2 sensor
    Disconnect the connector from the O2 sensor and measure the resistance:
    From the Dark blue/Lt green wire in the LH O2 sensor harness and the Dark blue/Lt green wire on the computer pin 43
    From the Dark Green/Pink wire on the RH Os sensor harness and the Dark Green/Pink wire on the computer pin 43

    91-93 5.0 Mustangs:
    Computer pin 43 Red/Black – LH O2 sensor
    Computer pin 29 Gray/Lt blue – RH O2 sensor
    Disconnect the connector from the O2 sensor and measure the resistance:
    From the Red/Black wire in the LH O2 sensor harness and the Red/Black wire on the computer pin 43
    From the Dark Green/Pink Gray/Lt blue wire on the RH Os sensor harness and the Gray/Lt blue wire on the computer pin 29

    94-95 5.0 Mustangs:
    Computer pin 29 Red/Black – LH O2 sensor
    Computer pin 27 Gray/Lt blue – RH O2 sensor
    From the Red/Black wire in the LH O2 sensor harness and the Red/Black wire on the computer pin 29
    From the Dark Green/Pink Gray/Lt blue wire on the RH Os sensor harness and the Gray/Lt blue wire on the computer pin 27

    There is a connector between the body harness and the O2 sensor harness. Make sure the connectors are mated together, the contacts and wiring are not damaged and the contacts are clean and not coated with oil.

    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.


    Surging Idle Checklist

    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 134,000 hits, which indicates it does help fix idle problems quickly and inexpensively.
    Last edited: Sep 26, 2013
  10. Casey2552

    Casey2552 Member

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    I had similar codes and my egr was deleted with no simulator or tune , I put a cheap simulator in and cured all of them and turned engine light off but still don't understand how that lead to lean codes in my case but there gone now
  11. 7991LXnSHO

    7991LXnSHO Active Member

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    Ant85, hang in there, you will run out of things to fix. Or you will eventually be fixing the problems a second time and it will be easier. Either way, I hope you get to enjoy driving it.

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