Brad1405

I have a 98 Mustang GT with a 4.6. I have had a problem with the car wanting to idle up to 1500 + RPMs and making a loud humming noise. It does this at random times. Sometimes I'm just sitting at idle either in gear or park. I replaced the TPS sensor and the Idle air control valve. Now it has gotten worse. The idle problem has seemed to be solved but the humming noise is bad. Just sitting at idle it doesn't make any noise. If I rev it up it starts making the loud noise when the RPMs begin to fall. It does this just sitting still or while driving. I've had mechanics look at it and we've found that when we restrict the air flow (not completely) to the IAC the noise completely stops like the IAC isn't adjusting properly. It did this though with the old one and the new. I unhooked the new one and attempted to start the engine and the valve seems to be adjusting fine. I also tested the cables that go to it and they show 12.2 volts which is within the 10.5 to 12.5 volts that Ford recommeneds. I've showed this to Mechanics and life long Mustang enthusiast. No one can figure this out. This car is my baby and I really need some help. Thanks!

cliffyk

I am not greatly familiar with the '96 - '98 models, however is the plumbing to the IAC stock? On the '99 - '04 there is a Helmholtz resonator in the IAC feed line that is there to break up standing waves that can create the "honk" even with a good IAC.

Here's what I found when I sectioned my IAC after it developed the honk.

The other possibility is that you got a bad "new" IAC, that does happen now and then...

Brad1405

I'm assuming the resonator your talking about is the plastic box that is in between the IAC and air intake hose and yes it is there. There is nothing inside that box. Is there suppose to be? I managed to make the noise stop by putting a hose clamp on the IAC hose and tighting it down so that it bottle necks to a third of it's original size.

Donnie's GTS

I had the EXACT problem you have described, fixed with a new IAC. Basically rpms would shoot up 14-1700 rpms with a loud humming sound. Basically a quick rev would temporarily fix the problem. At one point I let it go, and my car shut off (assume it went into safe mode.)

0949er

bad iac. My buddy drove a taxi (had the 4.6 in it) with that hum... replaced the IAC and fixed it.

Mr.SVT

what is a IAC

Mr.SVT

Thanks it seem everytime I hear a noise it gones away the next day I'm like wtf

cliffyk

That is the way it starts out, then as the vibration further wears the IAC valve centering vanes it get more frequent and louder--just replace the IAC and be done with it...

Mr.SVT

Can anybody tell me where IAC is located on the car cause it sounds like its coming from the bottom or back of the engine is it a DIY or take to dealer

Airman21

do a search on here for IAC you will find out all you need to know.. It is a DIY job no need to take it to dealer.