Hello, Lately my truck has been having trouble with it's idling speed. Here's what happening. I usually go to school at about 8:00am. After i start my truck it goes to about 1500-2000rpm then drops to about 500rpm and does it again and again untill it stops at about 600-700rpm. It was especally bad today. When it got to 500rpm it ran rough and i thought it was going to stall. I gave it some gas and got it up to 2000rpm, then let up but it slowly went to 500rpm(ran rough). I did this a few times. I finally put it into reverse and it ran normally. I backed out of the driveway and put it in drive. It ran fine. It did not idle weird when i left school. The only time it has any problem is in the morning when it is in park. I did some research and think that the IAC valve might need to be replaced. It seems to match the symptoms. But I don't know a lot about this sort of thing. I was wondering what you guys think.


Thanks

 

Before you do replace it, unhook the battery for 15 minutes to 1/2 hour.

Then hook it back up, start the truck, and let it idle for 15 minutes. If the problem goes away I wouldn't worry about it.

Don't have the AC on during that first 15 minutes.

After that, at any time really, idle it for another 15 minutes with the AC on.

After a PCM reset (which unhooking the battery that long forces) the truck relearns it's idle strategy, which may have gotten corrupted or something.

It's not the most likely thing, perhaps -- but it's free to try and doesn't hurt anything.

 

After that, I would take a look at the IAC. Cleaning it may help, sometimes replacement is the permanent cure.

 

So far, I don't think anyone's had luck with cleaning (if I'm remembering properly). They almost always end up replacing it.

 

The check engine light is not on. But it used to be, a long time ago. I checked the code and it was something about a wrong fuel to air mixture.

 

If thats the case then possible vacuum leak. How many miles are on your truck? Not sure on the ford intake manifolds but with GM we have the same problem when cold only, once it warms up the issue is gone. The intake gaskets are the cause. Others chime in and confirm.

 

my truck has about 115,000mi on it. I forgot about the posability of a vacume leak. That would make sense too. It stalled on me twice last year during the winter. I will try and look for a leak.

 

That is a good suggestion.

 

The best way I know of to look for a leak is when the issue is present to spray carb clean around the intake gasket area. Be careful when doing this as the cooling fan is going to try and blow it everywhere. Try and get as close as you can to the gasket area. If the engine revs up high or it smooths out then you found your problem. Let us know if it works for ya.

 

I was reading about how to replace an intake gasket in my shop manual. It looks like it would be hard to replace. I really hope that is not what is wrong.

Today when i started it, it reved and dropped to 500rpm then leveled off at 1000rpm. It seemed to be a lot better today.

I just reset the pcm and ill see how it does tomorrow morning. I'll let you know if it works. Thanks guys.

 

Heres a little test.
When idleing press the brakes, if the idle drops then the brake booster may be leaking vac internally

 

Mine does the same exact thing in the morning i cant figure it out it has been doing it for a while now only in the morning though really. It really doesnt hurt anything though

 

By upper intake gasket do you mean the gasket in between the black plastic air intake and the metal intake manifold. I might be able to replace that one without too much trouble.

Now i'm almost totally convinced that it is a gasket. It only has trouble in park in the morning. I guess a leak wouldn't have as much effect in other gears? But i've still got to wait to see if reseting the pcm has helped.

 

When you put it in gear the PCM compensates for the extra load you just put on the engine and raises the idle some. That my be why its being deskised (sp?) when you put into gear.

 

I just went out to see if it would work. I started it up and It worked just fine. Maybe reseting the pcm has done it, but i'll still have to see how it does for a while. It is susposed to be cold tomorrow morning, That might have some effect. I'll let you know how it turns out.Thanks guys.