High Idle after IAC replacement & Loud Humming

[bhamilton]

Ok, a little back story:

It's a 2003 XLT w/ 4.6 & 124k on it.

I had a rough, surging idle to the point of stall on a few occasions. Truck ran fine while moving. I checked all the hoses, didn't find any leaks, and came to the IAC. Replaced it with one from CarQuest. Surging Idle went away. Great! But...

As soon as started the truck after replacing the IAC, I noticed that the truck would idle around 1100 RPMs for roughly 30 seconds to a minute, then drop down to the normal 650-700 RPMs. Also at this time, there was a loud "humming" sound. It was definately air rushing through something, or something like that. I had read another user talk about a humming sound after IAC replacement, so I wasn't alarmed by this, thinking it would go away.

Problem is, neither the high idle or the humming have gone away. What happens now is when you start the truck, it sits at 1100 RPMs for half a minute or so, has some very slight surging, then drops down to 650ish and holds steady.

When you start to drive, everything seems fine. When driving through the city at around 35-40mph, the RPMs stay up around 1100-1200rpms and actually hold you at that speed...I don't slow down at all.

When I go to stop at a light, the truck, actually pushes me and I have to use a lot of pressure on the brake pedal to stop. Then, just as I'm about to come to a complete stop, the RPMs drop down to normal and I lurch to a stop because of the pressure I'm having to put on my brake pedal.

In addition to all this, the loud humming continues and is becoming more prevalent. Only happens at idle or while coasting. I can get it to go away when I step on the gas. If I tap the gas, it goes away. Sometimes it stays away, sometimes it comes right back.

So...now I'm trying to figure out what the heck is going on with the truck.

I did not disconnect the battery, but I've driven almost 2000 miles. I would think the computer would have re-learned by now.

Could it be the TPS?

Any help would be greatly appreciated. Thanks so much.

 

[toms89]

It sounds like your IAC settings are way off in the computer. Surprised no CEL. Though you have 2000 miles on it the first thing I would is disconnect battery and reset. Kinda like rebooting your computer. Let us know results and go from there.

Also check for vacuum leaks or leaks between your mass air and intake. This would cause issues with the IAC functioning as well.


.

 

[rburch]

I had the whining issue before i replaced my IACV. I would guess that the IACV being stuck open at a certain point was causing it. If i would pinch the IACV hose till it was almost closed it would stop. Changed it and never had a problem since!

 

[bhamilton]

It sounds like your IAC settings are way off in the computer. Surprised no CEL. Though you have 2000 miles on it the first thing I would is disconnect battery and reset. Kinda like rebooting your computer. Let us know results and go from there.

Also check for vacuum leaks or leaks between your mass air and intake. This would cause issues with the IAC functioning as well.


.

Ok, I'll give this a shot. I hadn't tried this because I thought the computer was supposed to re-learn on the fly. All I do is disconnect the negative battery cable for a few minutes, right? How long should I leave it off?

Thanks.

 

[rburch]

You are correct. it does learn on the fly. But in the instance that a issue effects driveability you want to do this to clear any DTC's and the truck will learn faster buy starting fresh.

 

[bhamilton]

OK, quick update. I disconnected the battery for about 5 hours. Driving the 15 miles home, it seems to be a bit better, but I'm not sure. I'll drive it more this weekend and see how it does.

The humming sound is still there. I've narrowed it down and it's a resonance in the air intake tube. I have a K&N intake on it and that's where the humming is coming from - I just don't know what's causing it.

Anybody have any ideas?

Thanks.

 

[rburch]

This also happened to me! Went to O'Reilly's (I hate them SOB's but i have a friend that works there so i can get a sweet deal!) when the whining started and got a IACV. Put it on in the parking lot it was still doing it! Went in and got Kenny (worked with him at this O'Reilly's when i was 19) showed him the truck and he said "you must have got a faulty IACV". So he gave me another IACV and i put it on and the whining was gone!

So take the IACV back and get another one!

Take the pos battery cable off and turn the key to the on position for 10 min.

Re-attach battery cable and start truck.

Let idle for 15 min. (to re-learn idle, air fuel mixture etc. etc.)

You are then supposed to drive for 100 miles but i usually just drive around town for about 15 min and hit the hwy for about another 15 min.

It does take the 100 miles to re-learn your driving habits and shift points!

 

[bhamilton]

Update:
Disconnected battery for a good 5-6 hours last week. Driven close to 200 miles and problem remains. Seemed to get better at first, but now is back to the way it was, with both the high idle and the humming.

Called CarQuest today to see if I could exchange the IAC and they said yes. Hoping they were right and I can get a new one because I think I got a bum part.

I'll let you know what I find. If anybody has any other idea, please let me know.

Thanks!

 

[bhamilton]

Problem solved. The replacement IAC was bad. Replaced it again with one from Napa and it works fine now.

Thanks to everybody to offered advice!