hijinxtattoo

This is my first post so a little info on me, I am a tattoo artist and current student at UTI. Have had my current Mustang sense it was new and have had a few others over the years so I know my way around most of them.

My current issue is a howling coming from the air intake only when the throttle plate is closed. I have replaced the IAC and have really narrowed the sound to be coming from the bypass hose that runs from the IAC to the intake in order to bypass the throttle body. I have read this to be a common problem but was unable to find any TSBs on it. Anyway is anyone familar with this and could it just be a crack in the black box that acts as a muffler in the bypass line? I am going to try some different hose configurations tomorrow and see if I can effect the tone of the noise but any suggestions and or help would be nice. Thanks.

cliffyk

If the throttle stop screw preset is incorrect (too far open) the IAC could be bottoming out while the PCM is attempting to control the idle speed--this would cause the noise you describe, even with a new IAC. Here is the procedure for re-setting the throttle stop screw.

The PCM sends a 2.1 kHz variable duty cycle signal to the IAC , that looks like this at idle (with a properly set TB stop):



Because of the inductance of the IAC solenoid's coil, and inertia of the plunger, the plunger does not jump about wildly when driven by this signal however it does vibrate a bit. At 42% duty cycle the IAC is being held open at approximately 1/2 of its total range, allowing the PCM to open or close it as needed to control idle speed.

If the throttle blade is too far open the PCM will attempt to reduce the flow through the IAC, to control the idle speed, by closing the valve (actually by holding it less open). At some point the valve plunger will be nearly closed and vibrating against it's seat--making the noise which is amplified by the resonator box and intake tube/airbox/etc.

hijinxtattoo

THANK YOU cliffyk. That makes a lot of sense I appreciate the technical response also.
Gives me a reason to bust out the good old Oscilloscope and check the signal.

cliffyk

If the duty cycle is quite low (<20%) then it could very well be the TB stop screw setting that is prompting the noise...

hijinxtattoo

Well everything looked pretty normal and I could'nt do much with the noise by adjusting the throttle. I ended up clearing the KAM and it knocked some the tone out of it. Then I reduced a small section of the bypass hose from 3/4'' to 1/2'' at the inlet to the air intake tube. This made a huge impact on softening the howl and didnt effect the idle at all. I am going to drive this car for the next week as my primary car and monitor its driveability.

cliffyk

I wonder if you got a defective IAC, how long ago was it that you replaced i?

Do you have a mechanic's stethoscope?



A very useful tool for tracking down ticks, vibrations, bearing noises, etc. For example you can use it to compare injector sounds and find sticking/sluggish units, or find the source of annoying sounds like the intake howl--that's how I tracked my '03's "honking" back to the IAC...

hijinxtattoo

I replaced the IAC about two months ago and as soon as this started I took it back and replaced it on the warranty. So the IAC on it is at nearly zero miles. When I scoped it the sound dose emanate from the IAC. This has to be an issue in the PCM as far as I can tell at this point. This Stangs mileage is well over 200k and has very few issues throughout its life this far. I am wondering if its worth taking off the Throttle Body Adapter and Intake Plenum for inspection, cleaning, and regasketing?