My 1996 Ford Ranger with the 4.0L V6 developed a weird howling noise. It sounded like a moose call or a sick cow. It wasn’t all of the time. Usually in gear at idle or low speed. If I gave it gas, the noise usually went away, and come back at low idle again. It was very annoying.

The problem was a bad Idle Air Control (IAC).

How The IAC Works:

The idle air control valve is located on the intake and is designed to adjust engine air intake at idle which controls the engine idle speed. The PCM receives feedback data from various sensors to send output signals designed to adjust the air passage open or closed which adjusts the engine idle speed. At engine speeds above idle, the valve is not used and does not effect the engine. Engine idle speeds are not adjustable.

Internal air ports are used to regulate the volume of air allowed to pass through the valve. Air passage bypass ports are used to route air past the butterfly and through the idle air control valve.

Because of the massive amounts of air processed through the valve a condition called “coking” develops which can inhibit proper operation of the valve.

(Here you can see the coking in my old IAC)

(Here you can see what a new one looks like)

When an idle air control valve fails it can cause the engine to idle erratically and even stall. Sometimes a weak idle air control motor will can cause engine problems without triggering a MIL (check engine) light. Inspect vacuum lines when servicing the idle air control valve and replace any that are broken or dilapidated.

Symptoms of a Bad IAC:

  • Howling in gear at low idle
  • Rough or fluctuating idle speed
  • Runs but stalls at idle / won’t idle at all
  • Fluttering sound in intake
  • Trouble code P1504, P1505, P1506, P1507 P1508, or P1509


Some people remove their IAC and clean it. It’s not difficult to do.

Unplug the IAC.

Remove the (2) bolts with a 10mm socket..

While holding it so that the motor is pointed up and the ports are at the bottom, spray it out with a good carburetor or throttle body cleaner. Be careful not to get any cleaner in the motor portion of it.

Make sure that the gasket is still good, and re-install the IAC. If the gasket is damaged, you can get a new one from the parts store. They cost about $2.00.

Reconnect the plug.

Replacing It:

I chose to simply replace mine and not bother with cleaning it. Cleaning it may have fixed the problem, but for $50.00, replacing it with one from Autozone (Duralast TV245) was a more reliable option. It only took a couple of minutes to replace it, and the noise was gone.



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