I never knew what a moose was until I put a ShoShop CAI (K&N with heat shield) on last summer. I still only had a moose when I revved the engine to above 4000 rpm while in neutral. And then winter came. The car moosed at startup and while driving if I ever let off of the throttle, especially while idling and turning the wheel, even after the engine was warm. The temperature of the air was more of an effect than the temp of the engine. I tried the TH Fix. It did quiten the moose but I noticed a loss of power. I took the TH Fix out for the summer and was fine with the KN filter. I put the stock air box on for winter as to not have to deal with the moose. It didn't do any good. The MOOSE is back. I have read hundreds of articles about the moose, but most are for contours before 2000. With the 2000 SVT I should already have the new IAC. I will try to clean it as my next action. Are there any other options that I could take other than the TH Fix again? Do I need a new IAC Valve? Some people have said that cleaning or replacing it worsened the problem.

Thanks in Advance

Well, I know for a fact I didn't have the moose until I cleaned my UIM and LIM. I've since tried cleaning my IAC, but I'll have to wait and see if it takes care of the moose completey. It's gone for now, but the weather's been warmenr, too.

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?

Thanks,

That is a great explanation. I will try the IAC cleaning and then try an extended rubber hose. Anybody have a 2000 SVT with an alternate fix?

Missed this thread on first pass, your 'Moose-B-Gone' link caught my eye this evening. Thanks for the comprehensive post, I think I'll have this thread stick around for a while to get a little more exposure.

Very interesting information Roger-thanks! IIRC I installed a throttle body spacer just before the moose showed up. This seems opposite to what others have said that lengthening the distance from the IAC to the MAF improves/eliminates the moose.

Roger: When you refer to "..engine wear, crud buildup...), " is it possible you're referring to valves, valve seats and/or seals, rings or other mechanical wear items? I ask as my car has relatively high mileage (143,000 miles). I am planning an engine transplant next summer so maybe I should just get a set of earplugs for now!

Thanks again.

Try using binder clips to squeeze the IAC supply hose just enough to eliminate the moose. This creates essentially the same effect as the TH fix(limiting air supply) but is much less of a hassle. It will create similar degradation of power as the TH fix but that is still less annoying than the moose and it can be easily removed for warmer weather.

I for one, never experienced a degradation of power with the TH fix. Now my car uses OBD1/EEC4 and seems to accept wild and woolly mods without the dreaded CEL like the EEC5/OBD2 cars. I really hated the throttle hang, although I never did a back to back at the track or dyno so who knows.

Originally posted by Bradness:

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Roger: When you refer to "..engine wear, crud buildup...), " is it possible you're referring to valves, valve seats and/or seals, rings or other mechanical wear items? I ask as my car has relatively high mileage (143,000 miles). I am planning an engine transplant next summer so maybe I should just get a set of earplugs for now!

Thanks again.

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Yes, of course.

Wear and crud affecting compression and intake runner air velocity directly impact intake air reversion.


When a Duratec engine is started and is high idling at 1000 rpm., the cylinders are firing at a rate of 3 times 1000 per minute. This equals 50 reversion pulses per second, or 50 Hz. If the intake system and control system start to resonate, you get a very loud 50 Hz. The exact frequency of the combined systems will vary some. A few people have even reported moosing at harmonic frequencies (multiples of the original at higher RPM's).

Breaking the cycle requires detuning the intake tract and/or feedback mechanism (hence the different resonators and IAC), brute force supression of amplitude (the TH fix), or a homebrewed solution affecting amplitude or resonance or hysteresis.

Cleaning the intake or clearing plugged CAT's would come under the heading of supressing amplitude. Changing the distance between the IAC and MAF, or getting a new and more responsive MAF affects hysteresis. The size and length of the combined intake ducting, as well as the shape and interior surface affects acoustic resonance. Changing the response of the ECU or IAC is out of most of our hands.