I have the idle surge when in gear when engine is at operating temp. I did the typical changes: New IAC, Diff Pressure EGR sensor, checked the relatively new Dif Press silicone hoses for damage or blockage, replaced the EGR and EGR solenoid. I cleaned the throttle body but it was relatively clean already. TPS shows no jumping on the scanner. I found no vacuum leaks under the hood. The problem does not always occur.

I noticed that with the AC off the idle goes smooth during the surge event times. When on AC mode it surges and when on vent mode idle goes smooth.

Could the Climate Control Panel (manual style panel even though a 2001 King Ranch) be creating a vacuum leak that could cause the idle surge?

5.4 with 366,000 miles.

 

I'd check your AC condenser but that's just me.

 

With the AC on, you are loading the engine. This could cause the surging. You probably have a clogged port in the throttle body, behind the butterfly. When loaded, the engine requires more air, that it may not be getting. I'd check the TB for crap behind the bfly. Use a pin and clean the small holes that get missed with cleaner.

With the miles you have on your engine, it could also be a valve or cylinder issue. Did you run a compression test on this motor? That would help you determine if your problem is serious or if you are chasing gremlins. I do it this way, because I'm old school. I also use a vacuum gauge to see if I have correct engine vacuum at idle or under throttle. Both can save you a ton of time and grief. Just throwing that out there, because of your vehicle mileage. Hope you find your problem.

 

I pulled the engine out at 340K when it spit a spark plug out and broke off the coil from the impact. I wanted a better helicoil fix than I could do from the top so I have anodized aluminum inserts installed (same thermal expansion as head) and a side hole/pin to trap the insert from moving. Also have at least 10 threads for plug engagement now and had it bored 30 over to clean it up. All new valve guides, bearings, timing chain, timing chain guides, replaced the AC compressor while the engine was out and the plastic intake since plastic and high mileage. New coils every 100K.
Sucks that it only acts up under load of drive and ac on. If I put it in park the problem goes away. I will follow your suggestion and pull the TB off then check all little holes, especially those affiliated with the IAC.

 

Disconnect the battery for 15 minutes or so, reconnect the battery and fire it up w/AC on. It's a way to do it in order to initiate a cycle of adaptive strategy per Ford Service. It may help, give it a try, see what happens.

 

Mine only surges when I'm low on refrigerant. About every 3 months or so. I keep saying I'm going to fix it but I don't. Maybe over the winter or next spring.