The moose, moosing, foghorn, loud humming, IAC ???
Let me give you my slightly over the top explanation.
Have you ever held a microphone too close to the speaker on an amp?
Did you get a squeal? A hum? A throbbing sound?
Did the sound change when you adjusted the volume? The tone? Moved the microphone?
This is called positive feedback oscillation.
Our cars can also do this, we call it moosing.
Moosing is dependent upon acoustic phase and amplitude, and the timing of the electro-mechanical feedback response.
Primary acoustic waves come from airflow reversion in the intake tract. The amplidude of these is dependent upon many things including static timing, RPM, exhaust scavenging, exhaust backpressure, intake velocity, compression, blowby, and intake air density. Secondary acoustic waves are produced when the IAC changes position causing a change in air pressure upstream of the IAC. Next we have the electro-mechanical feedback which consists of the MAF and engine speed sensors, the ECM, and the IAC which is the output device.
Under normal circumstances the ECM uses the IAC to maintain engine speed by controlling airflow into the engine. If the engine speed is too high the ECM closes the IAC to slow it down, and if the engine speed is too low the ECM opens the IAC to speed it up. At the same time acoustic waves are changing the short term airflow past the MAF. Fluxuations in the airflow past the MAF cause fluxuations in the A/F mixture, which affect engine speed. Small changes in the mixture affects engine speed even more when the engine is cold. If the two feedback systems coincide to reduce or increase the engine speed too much, overshoot will occur. If the acoustic resonance of the intake duct and the electro-mechanical feedback resonance overlap, uncontrolled oscillations between increasing and decreasing overshoot will occur. This is typical moosing and it usually goes away as the engine warms up and becomes less affected by the exact mixture.
Ford has made two attempts to fix the moosing situation. First they experimented with different resonators between the IAC and the main intake duct. These are meant to change the resonant frequency of the duct to be outside of the moosing frequency, and dampen the amplitude of the acoustic wave before it affects the MAF sensor. The second attempt was to change the response speed of the IAC so that again, it would be outside of the moosing frequency.
Why do SVT's have this problem so much more often than ATX's? The SVT has both more cam overlap which causes stronger reversion pulses, and an MTX which is lighter and allows faster change in engine speed.
Why does the TH fix usually stop moosing? By limiting the IAC's ability to increase airflow it also limits the positive overshoot and thereby dampens the acoustic amplitude below the threshhold of oscillation.
The cure for moosing is to break the pattern of positive feedback. On the upstream end this may come from a change in the resonator design, a change in the IAC response speed, a change in the MAF response speed (they tend to slow down as they get older), the TH fix, or changing the distance between the MAF and IAC. On the downstream end everything from the intake manifold size and cleanliness to the exact exhaust configuration has some effect.
Since Ford has fixed it so that these cars do not moose when new, degredation of some component or components is responsible. You have the option of finding and fixing the responsible part (CAT's, IAC, MAF, engine wear, crud buildup...), reverting any modification that caused it (exhaust changes, CAI), or doing modifications to overcome it. Although the TH fix is popular and simple, it would be best used for limiting TH. A couple of people have had success in killing the moose by replacing the IAC supply hose/ resonator assembly with a couple of feet of coiled up plain rubber hose. Who can say what will work best for you.
I cannot help but note here that the resonator design on most other cars are both more elaborate and better integrated between the IAC supply and the main intake duct. Coincidence?
