"Hunting" Idle

[imp]

'94 5.0L HO engine, occasionally suddenly fails to return to idle, plenty of power, hunts up and down, 1200 down to 500, several times, then dies. Throttle raises speed instantly, off throttle problem returns. Have a brand new 5.0 in a corner of my shop, put it's Throttle Body complete, TPS and IAC new on the offending engine, seemed good, problem returned. No CEL.

Unplugging MAF has little effect on idle (when it's running normally), eng. revs normally upon opening throttle. If MAF faulty, why is only idle affected, runs like a raped ape otherwise?

Any ideas appreciated! imp

 

[CDW6212R]

Those are tough, and you've heard it all from Mustang people and forums for sure. The IAC is the primary thing to work on, but only if everything else is mostly stock. I would clean the heck out of the IAC, and try different units. It is not uncommon to find IAC's that seem to work much different than others, with no obvious reasons why. Different brands make some difference, try for Ford for the IAC. But some new Motorcraft IAC's have had an issue rarely, and a swap solves it. The IAC is about 75% probably the cause of all hunting idle issues. Most of them are just dirty IAC's though.

People will do lots of modifications, and then the problem bothers them, and they have to rule out many things.

I'd want to check the TB closely, the blade orientation at idle mainly. The idle screw should never be turned, unless it's to correct someone else messing with it. I'm not familiar with the procedure to set the idle screw, but that would be on my list if nothing else shows up as an issue. The TB blade can become work enough to allow random amounts of air past the shaft seals. So an old TB would be suspect.

Check the TPS for sure, know that the voltage curve is smooth and linear, obviously near idle for that symptom. There isn't much to adjust there, though some people try hard. If the voltage is near 1.0 at idle(idle screw stop(another reason to not touch the idle screw)), the computer is fine with it. I think the range wanted is about .95ish to 1.1 at most. Usually if it can't get to about .97-1.05, people swap the TPS instead of trying to make the old one work. Most of this I've read multiple times on the Corral or the SBFtech forums.