Constant Idle Issues

Discussion in 'SN95 4.6L Mustang Tech' started by gore2727, Mar 25, 2014.

  1. 99 Mustang GT 4.6L

    I am continuing to have idle issues and is worsening. My car seems to idle at about 700. When coming to a stop, it will jump down to 500 and stutters. The lights on the dash seem to dim also.

    List of what I've replaced:
    - Power Steering Pump(leak)
    - Fuel Filter
    - Spark Plugs
    - Battery and Cables
    - IAC
    - Alternator
    - TPS
    - CAI
    - Throttle Body
    - EGR Valve
    - Serpentine Belt
    - Ignition Switch
    - and others

    I still can't pinpoint the issue. Thinking it could be a vacuum leak but checked, didn't find anything.

    Thoughts about what it could be:
    -Fuel Pump(low pressure)
    - Vacuum Leak
    - Bad Ground
    - Bad AC
    - Bad O2 Sensor
    - Bad Coil Packs

    If it is a vacuum leak, can you help explain all the hoses that need replaced(still novice).

  2. Manual or automatic?

    If automatic and the problem only occurs coming to a stop, the problem may be a lock up torque converter not releasing as it should.
  3. Its a manual. The stuttering just started but idle has been the main issue.
  4. Thanks for no help people.
  5. Do the CEL is on or off?
  6. Did you use a Motorcraft IAC? It matters. These cars don't like anything but MC when it comes to IACs.
  7. Duralast if I remember right.
  8. I picked up and replaced my IAC three months ago, it couldn't have gone bad already.
  9. Yes it could go bad if there's a large amount of blow by from your motor's PCV system.

    Note the usual cause of a low idle is:
    • Vacuum leak between the MAF and throttle body butterfly. This includes the IAC itself and it's connecting lines.
    • Dirty or bad MAF.
    • low fuel pressure. Confirm the intake vacuum reference line is connected and leak free.
    • excessive EGR flow.
    • dirty or clogged fuel injectors
    • ignition problems
    • poor cylinder power balance (fuel, ignition, or base motor).
    If moving along in neutral what are the idle RPM's? They should be around 1000 RPM's.

    Do you have access to an ODB2 scanner that is capable of monitoring operational data? It would really help to "know" what the PCM thinks the fuel pressure, MAF flow, TPS, and IAC duty percent is.

    Here's some more information on how to trouble shoot IAC/idle issues.
  10. I've been thinking vacuum leak or low fuel pressure. I didn't a pressure test a fuel months back and was in spec. I've cleaned the maf(still could be bad). Fuel injectors was another concern. My base RPM at idle is 650 - 700. It was idling better before installed a CAI. That says a leak. I was still have idle issues with the stock intake. Could be a bad ground. The odometer goes dark and flickers.

    Things I think:
    - Fuel Pump
    - Coil Packs
    - Vacuum Leak
  11. As far as what you think could be wrong. If the fuel pressure is OK, then IMO it's unlikely to be a bad fuel pump.

    If the coil pack are bad, likely there will be other symptoms such as misfire or poor fuel mileage.

    If fuel injectors are suspected, consider an injector cleaning and flow test service such as Once cleaned and tested they are as good as new. Cheaper than new.

    The car moving in neutral idle of 650-700 is too low. Should be 1000 RPM's when moving.

    If it idled better before new CAI, to me that says something happened during the CAI install. Did you change the "index" of the MAF? Some CAI's have the MAF too close to the air filter. This results in incorrect MAF readings.

    It can also cause incorrect MAF readings if the o'clock (index) of the MAF is changed from the stock position.