Resonate noise from IAC valve

[bhuff30]

I am getting what I would described as a resonate noise from the IAC valve. At the same time, the IAC isn't controlling the idle speed well... it is unsteady, fast, and even died once. Anyone had this happen before? This is on my 97 GT.

At idle or light throttle conditions, the engine started making a loud hmming type noise. At first, I thought it was an idler pulley bearing making noise as it was about to die. After a while listening and wiggling parts, I narrowed it down to the IAC system. I pushed on the rubber hose between the intake and the IAC valve and the noise went silent... for about a minute. When I closed the hood and restarted the car, it started right back up.

The noise is clearly being generated in that hose between the IAC and intake, but WHY? Will a new IAC valve solve the problem.

I thought the IAC is a stepper type motor instead of a device that opens and closes quickly?

 

[madspeed]

not sure how different they are on the sn cars, but try disonnecting the plug on it and see if the noise stops

 

[Kilgore Trout]

Is the gasket missing or damaged, are the bolts tight?

 

[madspeed]

I pushed on the rubber hose between the intake and the IAC valve and the noise went silent

again, not familar with the sn stuff, but that tells me right theres your problem

 

[bhuff30]

Gasket is good, bolts are tight.

I inspected the hoses and they were all in good shape. Unplugging the IAC, the noise goes away. Squeezing the hose and the noise goes away.

I replaced the IAC and all is fine now.

I was reading that the ford IAC operates at 320 hz. The factory resonator is likely tuned to dampen any noise around that operating range. I suspect the IAC (a replacement part just installed in december) started operating at a different frequency... and that frequency happen to be the resonate frequency of that resonator/hose assembly. This is why the noise changed when I squeezed the hose: that changed the length and thus the resonate frequency.

The noise was literally deafening. Anyone on the road stared at my car, expecting it to explode at any moment when idling. Even inside the car with the windows up, the noise was uncomfortably loud and could not be covered by the radio. Craziest thing I have ever seen!!!

 

[Kilgore Trout]

Interesting...

 

[madspeed]

easy peasy!

 

[Rife96GTconv]

Gasket is good, bolts are tight.

I inspected the hoses and they were all in good shape. Unplugging the IAC, the noise goes away. Squeezing the hose and the noise goes away.

I replaced the IAC and all is fine now.

I was reading that the ford IAC operates at 320 hz. The factory resonator is likely tuned to dampen any noise around that operating range. I suspect the IAC (a replacement part just installed in december) started operating at a different frequency... and that frequency happen to be the resonate frequency of that resonator/hose assembly. This is why the noise changed when I squeezed the hose: that changed the length and thus the resonate frequency.

The noise was literally deafening. Anyone on the road stared at my car, expecting it to explode at any moment when idling. Even inside the car with the windows up, the noise was uncomfortably loud and could not be covered by the radio. Craziest thing I have ever seen!!!

I had the same exact problem you are describing about 2 years ago. The noise started, it got louder, and the car was surging like crazy at idle. I actually was sitting at a light by the local ford dealership and it died. It started back up but I had to hold the revs above 1500 to keep it going. Once the light turned green it died again. I ended up pushing my car into the dealership lot. I replaced the IAC and the problem was fixed. You probably know this already, but just make sure you use the Ford Motorcraft IAC. I had another friend with a 98 that tried using the Advance Auto IAC and it did not fix the problem completely.

96 GT Conv., PI head swap, tuned at Prodyno

 

[sneaky98gt]

I know this thread is a couple months old, but....

I'm having this problem also. However, I know my IAC valve is fine. What's causing the problem is that I don't have the little black box on the hose; with the way my supercharger is, I can't use it without a ton of extra hose which looks very unsightly. So it's just a piece of 3/4 hose, maybe 6-7 inches long, running from the intake to the IAC. With the extra hose and black box, 0 noise. Ever. With just the 6" piece of hose, noise. Every time.

Any idea on how to solve it? I heard someone say maybe zip tieing the hose at a point in the middle down tight to change the diameter and keep it from resonating. I'm gonna try that, but if it doesn't work, any other ideas?

 

[bhuff30]

Very interesting. Well, as you know, the issue is resonance. The IAC is fluttering at the same frequency as the natural frequency of your IAC tube. It probably isn't the best idea to change the frequency of the IAC (but you probably could through programming). So you have to change the natural frequency of the tube.

You could try a slightly longer hose without the black box. For example, instead of a 6" hose, increase it by a random percentage (but avoid double or triple the length incase of other harmonics. Maybe a 10" tube without the black box would work?

You could also try putting a step in the tubing size, for example going from 3/4 to 1" and back to 3/4", but that would be unsightly as well. Or maybe a different resonate box of some sort with an inlet/outlet that matches your setup better? You just need to break up the length of the hose with some kind of change in the geometry, or make the hose a different length.

 

[trinity_gt]

Good post bhuff.

Another option might be a damper in the form of, say, a Helmholtz resonator but I suspect for the frequencies involved the components and volumes would be prohibitively large: The wavelength of 320Hz is over 1 meter (~3 feet) so...

 

[JTrueBlue]

well guys, this looks to be my problem i think,

 

[trombonedemon]

There is a little black colored box that comes stock, does that not dampen the noise??