We bought the pickup in KC and drove it to Utah. It ran great the entire drive. After we we here for about six months it started idling rough. I replaced the Idle Air Control valve and all was well. Add another six months and now the idle will fall to 500 rpm then pop to 1500 rpm. It almost seems like a vacuum leak but I can't hear any air leaks when it it running. It will sometimes stall when the rpm drops. Any ideas on where I should start my diagnostics from the forum's Ford experts?

VTC solenoid perhaps?

First thing I do with a rough or ranging idle is to clean the crap out of the butterfly/throttle body.

Check for any stored codes (if no check engine light nothing is active). Go for the basics. Five things make a gas engine run: air, fuel, spark, valve timing, and compression (lots of subsets, but you get the idea). One of those is off (I realize this is a broad brush but always start with the basics).

Something as random as a pinched fuel line could cause what you describe. Does it happen cold, or warm, or all the time? The more information you can provide the easier it is to troubleshoot.

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Originally posted by mbinky:
Something as random as a pinched fuel line could cause what you describe. Does it happen cold, or warm, or all the time? The more information you can provide the easier it is to troubleshoot.

Never cold. When the temp gauge is about halfway between stone cold and normal operating temperature and up to normal operating temperature the surging may or may not begin. That is what is maddening. It is typically at a traffic light in rush hour so I can't pull over to check it out. If I can keep it running the problem typically goes away (for a while) by the time I get to the next light.

The engine has about 160K miles on it.

I'd check the vacuum line to the PCV valve. The 90° elbow that steps the 5/8" intake manifold fitting down to a 3/8" line likes to split open. The fitting is on the upper rear of the intake manifold and the PCV valve is in the right (passenger side) valve cover. It has to be done largely by feel because the engine is far back against the firewall and partly under the windshield cowl. You might be able to hear it hissing when the engine is idling.

If you can rule out cold it adds another piece. Most likely happening as it goes into closed loop. That is where you start to take info from the O2 sensors and fuel starts to be metered more precisely. It could still be a mechanicsl issue because in closed loop the computer adds enough fuel to possibly mask issues may be present.

It sucks to hear but try to gather exact info as to when this is happening. Listen, look. Does it occur right after warm up or after a hard pull? In drive or only when in gear and the foot on the brake. When it is happening and you accelerate does it stutter or pull forward smoothly?

Troubleshooting is an art, and unfortunately doing it over the internet sucks.

I rented a smoker to test for the vacuum leaks. Turns out I didn't have just one... more like four. FYI, if you're like me and never used one, they are a huge time saver. I got them fixed and replaced the PCV. We'll see if that fixes the problem. I know it won't hurt. Thanks a bunch for the suggestions.

Change pcv valve and clean throttle body. Easy to do.

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We'll see if that fixes the problem.

Well? Did it?

Have you replaced the battery or had to jump it lately? It might need to relearn it's idle. It doesn't sound like its the case but...

My Expedition had issues after a dead battery, mostly transmission hunting. Following the operators manual instructions cleared it all up. It's free to do and takes about 5 minutes.

Sounds like the new Idle Air Control is fouled.