Need some help. I recently bought an '04 Mustang but am having idling problems. I've looked up some things on the Web but they didn't do anything for me.

I've clean the IAC and it helped some but not much. 1100 rpm to 800-1000rpms after cleaning. In park it stays at 800-1000 rpm. In D it drops to between 500 and 750 rpm but the idle goes up and down it never smoothes out.

It rattles when you smash the gas and rear feels jerky like the differential is worn out or needs oil. Also, when you put it in D without pushing gas you're going 30 to 35 mph.

I need some help or opinions, thanks.

2 Answers 2


With the hunting idle and the car going 30-35 mph without touching the gas pedal, it looks like a classic case of unmetered air making its way to the intake manifold.

One way to confirm this is the problem is to hook the car up to a scan tool and confirm positive long term fuel trim (LTFT) values; with the given symptoms I expect the values to be relatively high (more than +10%).

Another way to confirm it is to block off the air intake entrance and see if the engine continues to run; if it stays running then it is drawing air in from an alternative path.

Typical sources of unmetered air include disconnected vacuum lines, perished hoses that have holes or cracks in them, compromised intake manifold gaskets and cracked air filter housings.

This Q&A may be useful for your diagnosis:

How can I detect vacuum leaks without access to a smoke machine?

  • I have checked for vacuum leaks with carb cleaner sprayed a mist over all intakes but no high idle or low idle just stayed the same idle. And it's hard for me to get it scanned no one in my town has one for me to find out Jul 10, 2016 at 13:58
  • But I will take it to someone who has a coder to tell what code it's reading. I'm just really puzzled it did have a tubro on it and I took it out cause no tubro just the system might look and see if I missed something Jul 10, 2016 at 14:01

Possible, but generally air intake and vacuum leaks will often set an error code in the OnBoardDiagnostics (OBDII) system.

We're assuming you've read the codes and came up with no faults. If you've not tested codes yet, do that first. Tell us what you found.

On many cars like this, there are no error codes set for ignition wiring faults, or spark plug failures. I'm guessing there is a short in one of the spark plug wires. One easy test is to run the car at night, place it in park, open hood in a dark area and look for sparks where the wiring is grounding out. Easy test, but be careful. Keep hands and loose clothing away from front end moving parts. Again, Spark plug wiring failures on that car won't set a code. (and if I've got my model years mixed up, and you've got individual coils on plugs, apologies. Those should set a code in the OBDII system...)

  • 1
    The threshold for triggering a system-too-lean CEL varies across vehicle manufacturers. It's possible for idle-hunting to be present without fuel trims maxing out.
    – Zaid
    Jul 10, 2016 at 7:41
  • @BrianFloyd Zaid. I agree that is possible. But I'd sure like for Brian to speak up and tell us what he found on the OBDII test. He may have already conducted that test, but he needs to tell us those results first.
    – zipzit
    Jul 10, 2016 at 7:46
  • Yes, he needs to get back
    – Zaid
    Jul 10, 2016 at 7:47
  • The 4.6 on that year is COP IIRC.
    – Ben
    Jul 10, 2016 at 13:07

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