New to the forum & gotta say this has been a lot of help to me so far. I have a 2000 f-150 with a 4.2 V-6 and ~102 k miles. Everything is stock. Had a major vacuum leak, rubber pcv elbow at the manifold. Replaced the elbow & pcv valve. CEL gone and things are running much better/smoother now. Problem is that when I come to a stop, red light or stop sign, the idle will drop after a few seconds down to ~500, then jump up to ~800-900 then settle back down to a normal ~650-700. If I am sitting there awhile it will do this constantly. It's driving me crazy! I also changed the air filter, cleaned the MAF and cleaned the TB. No joy! Any help or insight would be greatly appreciated.

TIA,

yagi_boy

 

Have you cleaned or replaced the IAC (Idle Air Control). Sits on top of the throttle body; one electrical connection & two 10 mm bolts (I think they're 10 mm). Easy to do. Take it off spray it down good with throttle body cleaner, work it around with some q-tips or swabs & work the plunger. While you've got the throttle body cleaner, shoot some down into the IAC hole going down into the throttle body. Worked for me.


The 3rd picture in this link shows the IAC:
https://www.f150online.com/forums/sh...t=IAC+cleaning

 

Thanks for the quick reply. No, I haven't done anything with the IAC. Guess that's next as soon as the weather cooperates. Do you need replace a gasket when it's removed?

 

I didn't use one, but quite a few people recommend a new gasket. I'd clean the IAC first (and see how bad it is before you start cleaning). If it's got a lot of crap in it- that's probably your problem. Try not to damage the gasket & you should be able to reuse it. If this fixes it, but the problem resurfaces after a month or so, then you can try buying a new IAC (I'd get a new gasket then).
The IAC may not be your problem, but if you've already got the tb cleaner it doesn't really cost anything to clean the IAC. Are you pretty sure you've got all the vacumn leaks fixed?

 

OK, I cleaned the IAC. Not too bad at all. Very little carbon build-up. Cleaned both the IAC and the ports into the manifold. They are all clean and shiney now. The gasket was good so I just put it back. Idle seems a little better. Still drops down some then surges up a bit then back to normal when stopped with it in drive and my foot on the brake. It constantly does this like it's cycling. Doesn't drop as low nor surge nearly as high as before. When the weather warms up some I plan on pulling the upper intake, cleaning everything I can get my hands on and changing out all the vacuum lines. Also noticed that the main fuel feed line appears to be dry rotting. Guess I'll change that as well.

Thanks for the advice on the IAC. Again, it appears to have helped ...

 

EDIT: reread your first post & saw you already changed the elbow & pcv. Check if you have another elbow/pcv on the other side. At least one of these truck engines have two pcv valves(I'm not sure if it's the 4.2).

While you're changing out the vacumn lines you might want to go ahead and change the pcv valve(s) (I think there are 2 but I didn't change mine so I don't know for sure). I think some people also had a problem with an L-shaped plastic elbow (vacumn line?) cracking causing them some idle problems(you might want to do a search here to see if that might be applicable). Based on the age of your truck, I would replace the things that are probably near the end of their service life, and after you've done all that- if you still have a problem you can either use some propane to check for any other vacumn leaks or try buying a new IAC. Sometimes they do go bad.
I'm thinking it's either a vacumn leak or IAC. If there are any other maintenance items that you haven't taken care of, do those- they may be contributing to the problem. Fuel filter, plugs, etc. I also had new plug wires put on mine @139,000 miles (engine was rebuilt then due to hydrolock problem).

 

I agree that it's probably a vacuum leak. The 4.2 only has 1 pcv and that's been changed. The other side is just a breather. I had that out and cleaned it as well. I am planning on changing the spark plugs and wires in the spring or as soon as the weather warms up. Can't complain any about the truck. No major problems since I bought it used in 2003. I've kept up pretty well with all the routine maintenance and had 2 transmission flushes in the last 5 years. I'm hoping that I can keep this truck for many more years and get at least 200k miles on it. Gotta love a Ford truck ...

 

I changed all the same parts you did. and I was still having your same problem. It turned out to be the hose that is just past the pcv. It connects to a y fitting just past the pcv. Just buy hose by the foot and change it. Ben