98 rough idle

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Feb 13, 2005 | 11:08 AM

bhaven

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Joined: Feb 2005

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98 rough idle

I have a 98 F-150 4.6 with a bad idle and gets worse when put in gear but truck runs great other than that HELP

New plugs new wires same problem no check engine light. ford dealer said with out check engine light it could cost a lot of $ to find. if you give it a little gas it evens out any help

Feb 13, 2005 | 11:33 AM

fordman311

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If it is just rough idle, and hittin the gas helps, it probably the IAC (Idle Air control) sensor. Just finnished writing in another thread about this. Use the forum search tool and look up "IAC" and you'll find all the info you need. When you go to replace it/clean it out cheack my gallery for help, I got some pics of it there. Dont pay anyone else to do it, its real simple and saves the $$. Takes about 15minutes to replace and a couple of bolts. Ususally about $50+ bucks for a new IAC.

Feb 14, 2005 | 01:38 PM

BillyBob69

Lead Driver

Joined: Jul 2003

Posts: 7,404

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From: Wheatland, Mo

What fordman said. The IAC is located at the top back of the intake manifold. Just remove the plastic intake cover, then unplug the harness from the IAC and then remove the two bolts. It pops right off. Best to replace, (Approx $70), but sometimes cleaning helps.

Feb 14, 2005 | 06:16 PM

xallxforxchristx

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Joined: Dec 2003

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From: Arlington, Texas

Would the rough idle be a constant problem? My truck's about to turn over 100,000 and I've felt it once or twice in the past month, but once I restarted it, it stopped. What would I look for to tell whether it needs to be cleaned/replaced?

Feb 14, 2005 | 06:34 PM

fordman311

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The idle is usually its roughest when the engine is colder. The cold seems to aggrivate the problem. If you find youself having to apply a little of the gas pedal to keep it from stalling out, the IAC is probably the problem. I think when i really felt the bad idle and had to apply gas when it got into mid-low 30's (F) here. But a lot of stuff would affect the temperature at which you would really feel it, such as how dirty it is. Just take the IAC off and check to see how dirty it is. I've made a gallery just for IAC help, because so many people want to know the specifics to fixing IAC problem. Racerguy is going to put a link for the gallery under one of the main threads, but you can still view it now.

Mar 4, 2005 | 08:02 PM

xallxforxchristx

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Joined: Dec 2003

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From: Arlington, Texas

Hey Guys, I went to check out my IAC tonight to see if it was clogged or not, but ran into a slight problem. It seems as the IAC is located in a very inconvienient spot...ie. behind the throttle body, next to the fire-wall and under the cowling. Am I looking at the right thing? It looks exactly like what was in your gallery...a shiney tube about an inch or inch and a half long held on by 2 bolts. If I'm looking at the right thing, is there an easier way to get back to it? The engine I have is the 5.4 L V8 in a 98 truck. Thanks for the input.

Mar 4, 2005 | 09:28 PM

RoyalFord

Elder User

Joined: Apr 2004

Posts: 866

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From: North Carolina

I have another post with this same problem... http://www.fordtrucks.com/forums/sho...d.php?t=351325

I found a vacuum leak on the hose that comes from the PCV Valve and connects to the back of the intake. I have heard it is pretty common.